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diff --git a/crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-01.txt b/crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-01.txt deleted file mode 100644 index 78db9d78f3cb..000000000000 --- a/crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-01.txt +++ /dev/null @@ -1,6214 +0,0 @@ - -INTERNET-DRAFT Clifford Neuman - John Kohl - Theodore Ts'o - 21 November 1997 - -The Kerberos Network Authentication Service (V5) - -STATUS OF THIS MEMO - -This document is an Internet-Draft. Internet-Drafts are working documents of -the Internet Engineering Task Force (IETF), its areas, and its working -groups. Note that other groups may also distribute working documents as -Internet-Drafts. - -Internet-Drafts are draft documents valid for a maximum of six months and -may be updated, replaced, or obsoleted by other documents at any time. It is -inappropriate to use Internet-Drafts as reference material or to cite them -other than as 'work in progress.' - -To learn the current status of any Internet-Draft, please check the -'1id-abstracts.txt' listing contained in the Internet-Drafts Shadow -Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), -ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). - -The distribution of this memo is unlimited. It is filed as -draft-ietf-cat-kerberos-r-01.txt, and expires 21 May 1998. Please send -comments to: krb-protocol@MIT.EDU - -ABSTRACT - -This document provides an overview and specification of Version 5 of the -Kerberos protocol, and updates RFC1510 to clarify aspects of the protocol -and its intended use that require more detailed or clearer explanation than -was provided in RFC1510. This document is intended to provide a detailed -description of the protocol, suitable for implementation, together with -descriptions of the appropriate use of protocol messages and fields within -those messages. - -This document is not intended to describe Kerberos to the end user, system -administrator, or application developer. Higher level papers describing -Version 5 of the Kerberos system [NT94] and documenting version 4 [SNS88], -are available elsewhere. - -OVERVIEW - -This INTERNET-DRAFT describes the concepts and model upon which the Kerberos -network authentication system is based. It also specifies Version 5 of the -Kerberos protocol. - -The motivations, goals, assumptions, and rationale behind most design -decisions are treated cursorily; they are more fully described in a paper -available in IEEE communications [NT94] and earlier in the Kerberos portion -of the Athena Technical Plan [MNSS87]. The protocols have been a proposed - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -standard and are being considered for advancement for draft standard through -the IETF standard process. Comments are encouraged on the presentation, but -only minor refinements to the protocol as implemented or extensions that fit -within current protocol framework will be considered at this time. - -Requests for addition to an electronic mailing list for discussion of -Kerberos, kerberos@MIT.EDU, may be addressed to kerberos-request@MIT.EDU. -This mailing list is gatewayed onto the Usenet as the group -comp.protocols.kerberos. Requests for further information, including -documents and code availability, may be sent to info-kerberos@MIT.EDU. - -BACKGROUND - -The Kerberos model is based in part on Needham and Schroeder's trusted -third-party authentication protocol [NS78] and on modifications suggested by -Denning and Sacco [DS81]. The original design and implementation of Kerberos -Versions 1 through 4 was the work of two former Project Athena staff -members, Steve Miller of Digital Equipment Corporation and Clifford Neuman -(now at the Information Sciences Institute of the University of Southern -California), along with Jerome Saltzer, Technical Director of Project -Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members -of Project Athena have also contributed to the work on Kerberos. - -Version 5 of the Kerberos protocol (described in this document) has evolved -from Version 4 based on new requirements and desires for features not -available in Version 4. The design of Version 5 of the Kerberos protocol was -led by Clifford Neuman and John Kohl with much input from the community. The -development of the MIT reference implementation was led at MIT by John Kohl -and Theodore T'so, with help and contributed code from many others. -Reference implementations of both version 4 and version 5 of Kerberos are -publicly available and commercial implementations have been developed and -are widely used. - -Details on the differences between Kerberos Versions 4 and 5 can be found in -[KNT92]. - -1. Introduction - -Kerberos provides a means of verifying the identities of principals, (e.g. a -workstation user or a network server) on an open (unprotected) network. This -is accomplished without relying on assertions by the host operating system, -without basing trust on host addresses, without requiring physical security -of all the hosts on the network, and under the assumption that packets -traveling along the network can be read, modified, and inserted at will[1]. -Kerberos performs authentication under these conditions as a trusted -third-party authentication service by using conventional (shared secret key -[2] cryptography. Kerberos extensions have been proposed and implemented -that provide for the use of public key cryptography during certain phases of -the authentication protocol. These extensions provide for authentication of -users registered with public key certification authorities, and allow the -system to provide certain benefits of public key cryptography in situations -where they are needed. - -The basic Kerberos authentication process proceeds as follows: A client - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -sends a request to the authentication server (AS) requesting 'credentials' -for a given server. The AS responds with these credentials, encrypted in the -client's key. The credentials consist of 1) a 'ticket' for the server and 2) -a temporary encryption key (often called a "session key"). The client -transmits the ticket (which contains the client's identity and a copy of the -session key, all encrypted in the server's key) to the server. The session -key (now shared by the client and server) is used to authenticate the -client, and may optionally be used to authenticate the server. It may also -be used to encrypt further communication between the two parties or to -exchange a separate sub-session key to be used to encrypt further -communication. - -Implementation of the basic protocol consists of one or more authentication -servers running on physically secure hosts. The authentication servers -maintain a database of principals (i.e., users and servers) and their secret -keys. Code libraries provide encryption and implement the Kerberos protocol. -In order to add authentication to its transactions, a typical network -application adds one or two calls to the Kerberos library directly or -through the Generic Security Services Application Programming Interface, -GSSAPI, described in separate document. These calls result in the -transmission of the necessary messages to achieve authentication. - -The Kerberos protocol consists of several sub-protocols (or exchanges). -There are two basic methods by which a client can ask a Kerberos server for -credentials. In the first approach, the client sends a cleartext request for -a ticket for the desired server to the AS. The reply is sent encrypted in -the client's secret key. Usually this request is for a ticket-granting -ticket (TGT) which can later be used with the ticket-granting server (TGS). -In the second method, the client sends a request to the TGS. The client uses -the TGT to authenticate itself to the TGS in the same manner as if it were -contacting any other application server that requires Kerberos -authentication. The reply is encrypted in the session key from the TGT. -Though the protocol specification describes the AS and the TGS as separate -servers, they are implemented in practice as different protocol entry points -within a single Kerberos server. - -Once obtained, credentials may be used to verify the identity of the -principals in a transaction, to ensure the integrity of messages exchanged -between them, or to preserve privacy of the messages. The application is -free to choose whatever protection may be necessary. - -To verify the identities of the principals in a transaction, the client -transmits the ticket to the application server. Since the ticket is sent "in -the clear" (parts of it are encrypted, but this encryption doesn't thwart -replay) and might be intercepted and reused by an attacker, additional -information is sent to prove that the message originated with the principal -to whom the ticket was issued. This information (called the authenticator) -is encrypted in the session key, and includes a timestamp. The timestamp -proves that the message was recently generated and is not a replay. -Encrypting the authenticator in the session key proves that it was generated -by a party possessing the session key. Since no one except the requesting -principal and the server know the session key (it is never sent over the -network in the clear) this guarantees the identity of the client. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -The integrity of the messages exchanged between principals can also be -guaranteed using the session key (passed in the ticket and contained in the -credentials). This approach provides detection of both replay attacks and -message stream modification attacks. It is accomplished by generating and -transmitting a collision-proof checksum (elsewhere called a hash or digest -function) of the client's message, keyed with the session key. Privacy and -integrity of the messages exchanged between principals can be secured by -encrypting the data to be passed using the session key contained in the -ticket or the subsession key found in the authenticator. - -The authentication exchanges mentioned above require read-only access to the -Kerberos database. Sometimes, however, the entries in the database must be -modified, such as when adding new principals or changing a principal's key. -This is done using a protocol between a client and a third Kerberos server, -the Kerberos Administration Server (KADM). There is also a protocol for -maintaining multiple copies of the Kerberos database. Neither of these -protocols are described in this document. - -1.1. Cross-Realm Operation - -The Kerberos protocol is designed to operate across organizational -boundaries. A client in one organization can be authenticated to a server in -another. Each organization wishing to run a Kerberos server establishes its -own 'realm'. The name of the realm in which a client is registered is part -of the client's name, and can be used by the end-service to decide whether -to honor a request. - -By establishing 'inter-realm' keys, the administrators of two realms can -allow a client authenticated in the local realm to prove its identity to -servers in other realms[3]. The exchange of inter-realm keys (a separate key -may be used for each direction) registers the ticket-granting service of -each realm as a principal in the other realm. A client is then able to -obtain a ticket-granting ticket for the remote realm's ticket-granting -service from its local realm. When that ticket-granting ticket is used, the -remote ticket-granting service uses the inter-realm key (which usually -differs from its own normal TGS key) to decrypt the ticket-granting ticket, -and is thus certain that it was issued by the client's own TGS. Tickets -issued by the remote ticket-granting service will indicate to the -end-service that the client was authenticated from another realm. - -A realm is said to communicate with another realm if the two realms share an -inter-realm key, or if the local realm shares an inter-realm key with an -intermediate realm that communicates with the remote realm. An -authentication path is the sequence of intermediate realms that are -transited in communicating from one realm to another. - -Realms are typically organized hierarchically. Each realm shares a key with -its parent and a different key with each child. If an inter-realm key is not -directly shared by two realms, the hierarchical organization allows an -authentication path to be easily constructed. If a hierarchical organization -is not used, it may be necessary to consult a database in order to construct -an authentication path between realms. - -Although realms are typically hierarchical, intermediate realms may be - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -bypassed to achieve cross-realm authentication through alternate -authentication paths (these might be established to make communication -between two realms more efficient). It is important for the end-service to -know which realms were transited when deciding how much faith to place in -the authentication process. To facilitate this decision, a field in each -ticket contains the names of the realms that were involved in authenticating -the client. - -The application server is ultimately responsible for accepting or rejecting -authentication and should check the transited field. The application server -may choose to rely on the KDC for the application server's realm to check -the transited field. The application server's KDC will set the -TRANSITED-POLICY-CHECKED flag in this case. The KDC's for intermediate -realms may also check the transited field as they issue -ticket-granting-tickets for other realms, but they are encouraged not to do -so. A client may request that the KDC's not check the transited field by -setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not -required to honor this flag. - -1.2. Authorization - -As an authentication service, Kerberos provides a means of verifying the -identity of principals on a network. Authentication is usually useful -primarily as a first step in the process of authorization, determining -whether a client may use a service, which objects the client is allowed to -access, and the type of access allowed for each. Kerberos does not, by -itself, provide authorization. Possession of a client ticket for a service -provides only for authentication of the client to that service, and in the -absence of a separate authorization procedure, it should not be considered -by an application as authorizing the use of that service. - -Such separate authorization methods may be implemented as application -specific access control functions and may be based on files such as the -application server, or on separately issued authorization credentials such -as those based on proxies [Neu93] , or on other authorization services. - -Applications should not be modified to accept the issuance of a service -ticket by the Kerberos server (even by an modified Kerberos server) as -granting authority to use the service, since such applications may become -vulnerable to the bypass of this authorization check in an environment if -they interoperate with other KDCs or where other options for application -authentication (e.g. the PKTAPP proposal) are provided. - -1.3. Environmental assumptions - -Kerberos imposes a few assumptions on the environment in which it can -properly function: - - * 'Denial of service' attacks are not solved with Kerberos. There are - places in these protocols where an intruder can prevent an application - from participating in the proper authentication steps. Detection and - solution of such attacks (some of which can appear to be nnot-uncommon - 'normal' failure modes for the system) is usually best left to the - human administrators and users. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - * Principals must keep their secret keys secret. If an intruder somehow - steals a principal's key, it will be able to masquerade as that - principal or impersonate any server to the legitimate principal. - * 'Password guessing' attacks are not solved by Kerberos. If a user - chooses a poor password, it is possible for an attacker to successfully - mount an offline dictionary attack by repeatedly attempting to decrypt, - with successive entries from a dictionary, messages obtained which are - encrypted under a key derived from the user's password. - * Each host on the network must have a clock which is 'loosely - synchronized' to the time of the other hosts; this synchronization is - used to reduce the bookkeeping needs of application servers when they - do replay detection. The degree of "looseness" can be configured on a - per-server basis, but is typically on the order of 5 minutes. If the - clocks are synchronized over the network, the clock synchronization - protocol must itself be secured from network attackers. - * Principal identifiers are not recycled on a short-term basis. A typical - mode of access control will use access control lists (ACLs) to grant - permissions to particular principals. If a stale ACL entry remains for - a deleted principal and the principal identifier is reused, the new - principal will inherit rights specified in the stale ACL entry. By not - re-using principal identifiers, the danger of inadvertent access is - removed. - -1.4. Glossary of terms - -Below is a list of terms used throughout this document. - -Authentication - Verifying the claimed identity of a principal. -Authentication header - A record containing a Ticket and an Authenticator to be presented to a - server as part of the authentication process. -Authentication path - A sequence of intermediate realms transited in the authentication - process when communicating from one realm to another. -Authenticator - A record containing information that can be shown to have been recently - generated using the session key known only by the client and server. -Authorization - The process of determining whether a client may use a service, which - objects the client is allowed to access, and the type of access allowed - for each. -Capability - A token that grants the bearer permission to access an object or - service. In Kerberos, this might be a ticket whose use is restricted by - the contents of the authorization data field, but which lists no - network addresses, together with the session key necessary to use the - ticket. -Ciphertext - The output of an encryption function. Encryption transforms plaintext - into ciphertext. -Client - A process that makes use of a network service on behalf of a user. Note - that in some cases a Server may itself be a client of some other server - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - (e.g. a print server may be a client of a file server). -Credentials - A ticket plus the secret session key necessary to successfully use that - ticket in an authentication exchange. -KDC - Key Distribution Center, a network service that supplies tickets and - temporary session keys; or an instance of that service or the host on - which it runs. The KDC services both initial ticket and ticket-granting - ticket requests. The initial ticket portion is sometimes referred to as - the Authentication Server (or service). The ticket-granting ticket - portion is sometimes referred to as the ticket-granting server (or - service). -Kerberos - Aside from the 3-headed dog guarding Hades, the name given to Project - Athena's authentication service, the protocol used by that service, or - the code used to implement the authentication service. -Plaintext - The input to an encryption function or the output of a decryption - function. Decryption transforms ciphertext into plaintext. -Principal - A uniquely named client or server instance that participates in a - network communication. -Principal identifier - The name used to uniquely identify each different principal. -Seal - To encipher a record containing several fields in such a way that the - fields cannot be individually replaced without either knowledge of the - encryption key or leaving evidence of tampering. -Secret key - An encryption key shared by a principal and the KDC, distributed - outside the bounds of the system, with a long lifetime. In the case of - a human user's principal, the secret key is derived from a password. -Server - A particular Principal which provides a resource to network clients. - The server is sometimes refered to as the Application Server. -Service - A resource provided to network clients; often provided by more than one - server (for example, remote file service). -Session key - A temporary encryption key used between two principals, with a lifetime - limited to the duration of a single login "session". -Sub-session key - A temporary encryption key used between two principals, selected and - exchanged by the principals using the session key, and with a lifetime - limited to the duration of a single association. -Ticket - A record that helps a client authenticate itself to a server; it - contains the client's identity, a session key, a timestamp, and other - information, all sealed using the server's secret key. It only serves - to authenticate a client when presented along with a fresh - Authenticator. - -2. Ticket flag uses and requests - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -Each Kerberos ticket contains a set of flags which are used to indicate -various attributes of that ticket. Most flags may be requested by a client -when the ticket is obtained; some are automatically turned on and off by a -Kerberos server as required. The following sections explain what the various -flags mean, and gives examples of reasons to use such a flag. - -2.1. Initial and pre-authenticated tickets - -The INITIAL flag indicates that a ticket was issued using the AS protocol -and not issued based on a ticket-granting ticket. Application servers that -want to require the demonstrated knowledge of a client's secret key (e.g. a -password-changing program) can insist that this flag be set in any tickets -they accept, and thus be assured that the client's key was recently -presented to the application client. - -The PRE-AUTHENT and HW-AUTHENT flags provide addition information about the -initial authentication, regardless of whether the current ticket was issued -directly (in which case INITIAL will also be set) or issued on the basis of -a ticket-granting ticket (in which case the INITIAL flag is clear, but the -PRE-AUTHENT and HW-AUTHENT flags are carried forward from the -ticket-granting ticket). - -2.2. Invalid tickets - -The INVALID flag indicates that a ticket is invalid. Application servers -must reject tickets which have this flag set. A postdated ticket will -usually be issued in this form. Invalid tickets must be validated by the KDC -before use, by presenting them to the KDC in a TGS request with the VALIDATE -option specified. The KDC will only validate tickets after their starttime -has passed. The validation is required so that postdated tickets which have -been stolen before their starttime can be rendered permanently invalid -(through a hot-list mechanism) (see section 3.3.3.1). - -2.3. Renewable tickets - -Applications may desire to hold tickets which can be valid for long periods -of time. However, this can expose their credentials to potential theft for -equally long periods, and those stolen credentials would be valid until the -expiration time of the ticket(s). Simply using short-lived tickets and -obtaining new ones periodically would require the client to have long-term -access to its secret key, an even greater risk. Renewable tickets can be -used to mitigate the consequences of theft. Renewable tickets have two -"expiration times": the first is when the current instance of the ticket -expires, and the second is the latest permissible value for an individual -expiration time. An application client must periodically (i.e. before it -expires) present a renewable ticket to the KDC, with the RENEW option set in -the KDC request. The KDC will issue a new ticket with a new session key and -a later expiration time. All other fields of the ticket are left unmodified -by the renewal process. When the latest permissible expiration time arrives, -the ticket expires permanently. At each renewal, the KDC may consult a -hot-list to determine if the ticket had been reported stolen since its last -renewal; it will refuse to renew such stolen tickets, and thus the usable -lifetime of stolen tickets is reduced. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -The RENEWABLE flag in a ticket is normally only interpreted by the -ticket-granting service (discussed below in section 3.3). It can usually be -ignored by application servers. However, some particularly careful -application servers may wish to disallow renewable tickets. - -If a renewable ticket is not renewed by its expiration time, the KDC will -not renew the ticket. The RENEWABLE flag is reset by default, but a client -may request it be set by setting the RENEWABLE option in the KRB_AS_REQ -message. If it is set, then the renew-till field in the ticket contains the -time after which the ticket may not be renewed. - -2.4. Postdated tickets - -Applications may occasionally need to obtain tickets for use much later, -e.g. a batch submission system would need tickets to be valid at the time -the batch job is serviced. However, it is dangerous to hold valid tickets in -a batch queue, since they will be on-line longer and more prone to theft. -Postdated tickets provide a way to obtain these tickets from the KDC at job -submission time, but to leave them "dormant" until they are activated and -validated by a further request of the KDC. If a ticket theft were reported -in the interim, the KDC would refuse to validate the ticket, and the thief -would be foiled. - -The MAY-POSTDATE flag in a ticket is normally only interpreted by the -ticket-granting service. It can be ignored by application servers. This flag -must be set in a ticket-granting ticket in order to issue a postdated ticket -based on the presented ticket. It is reset by default; it may be requested -by a client by setting the ALLOW-POSTDATE option in the KRB_AS_REQ message. -This flag does not allow a client to obtain a postdated ticket-granting -ticket; postdated ticket-granting tickets can only by obtained by requesting -the postdating in the KRB_AS_REQ message. The life (endtime-starttime) of a -postdated ticket will be the remaining life of the ticket-granting ticket at -the time of the request, unless the RENEWABLE option is also set, in which -case it can be the full life (endtime-starttime) of the ticket-granting -ticket. The KDC may limit how far in the future a ticket may be postdated. - -The POSTDATED flag indicates that a ticket has been postdated. The -application server can check the authtime field in the ticket to see when -the original authentication occurred. Some services may choose to reject -postdated tickets, or they may only accept them within a certain period -after the original authentication. When the KDC issues a POSTDATED ticket, -it will also be marked as INVALID, so that the application client must -present the ticket to the KDC to be validated before use. - -2.5. Proxiable and proxy tickets - -At times it may be necessary for a principal to allow a service to perform -an operation on its behalf. The service must be able to take on the identity -of the client, but only for a particular purpose. A principal can allow a -service to take on the principal's identity for a particular purpose by -granting it a proxy. - -The process of granting a proxy using the proxy and proxiable flags is used -to provide credentials for use with specific services. Though conceptually - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -also a proxy, user's wishing to delegate their identity for ANY purpose must -use the ticket forwarding mechanism described in the next section to forward -a ticket granting ticket. - -The PROXIABLE flag in a ticket is normally only interpreted by the -ticket-granting service. It can be ignored by application servers. When set, -this flag tells the ticket-granting server that it is OK to issue a new -ticket (but not a ticket-granting ticket) with a different network address -based on this ticket. This flag is set if requested by the client on initial -authentication. By default, the client will request that it be set when -requesting a ticket granting ticket, and reset when requesting any other -ticket. - -This flag allows a client to pass a proxy to a server to perform a remote -request on its behalf, e.g. a print service client can give the print server -a proxy to access the client's files on a particular file server in order to -satisfy a print request. - -In order to complicate the use of stolen credentials, Kerberos tickets are -usually valid from only those network addresses specifically included in the -ticket[4]. When granting a proxy, the client must specify the new network -address from which the proxy is to be used, or indicate that the proxy is to -be issued for use from any address. - -The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket. -Application servers may check this flag and at their option they may require -additional authentication from the agent presenting the proxy in order to -provide an audit trail. - -2.6. Forwardable tickets - -Authentication forwarding is an instance of a proxy where the service is -granted complete use of the client's identity. An example where it might be -used is when a user logs in to a remote system and wants authentication to -work from that system as if the login were local. - -The FORWARDABLE flag in a ticket is normally only interpreted by the -ticket-granting service. It can be ignored by application servers. The -FORWARDABLE flag has an interpretation similar to that of the PROXIABLE -flag, except ticket-granting tickets may also be issued with different -network addresses. This flag is reset by default, but users may request that -it be set by setting the FORWARDABLE option in the AS request when they -request their initial ticket- granting ticket. - -This flag allows for authentication forwarding without requiring the user to -enter a password again. If the flag is not set, then authentication -forwarding is not permitted, but the same result can still be achieved if -the user engages in the AS exchange specifying the requested network -addresses and supplies a password. - -The FORWARDED flag is set by the TGS when a client presents a ticket with -the FORWARDABLE flag set and requests a forwarded ticket by specifying the -FORWARDED KDC option and supplying a set of addresses for the new ticket. It -is also set in all tickets issued based on tickets with the FORWARDED flag - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -set. Application servers may choose to process FORWARDED tickets differently -than non-FORWARDED tickets. - -2.7. Other KDC options - -There are two additional options which may be set in a client's request of -the KDC. The RENEWABLE-OK option indicates that the client will accept a -renewable ticket if a ticket with the requested life cannot otherwise be -provided. If a ticket with the requested life cannot be provided, then the -KDC may issue a renewable ticket with a renew-till equal to the the -requested endtime. The value of the renew-till field may still be adjusted -by site-determined limits or limits imposed by the individual principal or -server. - -The ENC-TKT-IN-SKEY option is honored only by the ticket-granting service. -It indicates that the ticket to be issued for the end server is to be -encrypted in the session key from the a additional second ticket-granting -ticket provided with the request. See section 3.3.3 for specific details. - -3. Message Exchanges - -The following sections describe the interactions between network clients and -servers and the messages involved in those exchanges. - -3.1. The Authentication Service Exchange - - Summary - Message direction Message type Section - 1. Client to Kerberos KRB_AS_REQ 5.4.1 - 2. Kerberos to client KRB_AS_REP or 5.4.2 - KRB_ERROR 5.9.1 - -The Authentication Service (AS) Exchange between the client and the Kerberos -Authentication Server is initiated by a client when it wishes to obtain -authentication credentials for a given server but currently holds no -credentials. In its basic form, the client's secret key is used for -encryption and decryption. This exchange is typically used at the initiation -of a login session to obtain credentials for a Ticket-Granting Server which -will subsequently be used to obtain credentials for other servers (see -section 3.3) without requiring further use of the client's secret key. This -exchange is also used to request credentials for services which must not be -mediated through the Ticket-Granting Service, but rather require a -principal's secret key, such as the password-changing service[5]. This -exchange does not by itself provide any assurance of the the identity of the -user[6]. - -The exchange consists of two messages: KRB_AS_REQ from the client to -Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these -messages are described in sections 5.4.1, 5.4.2, and 5.9.1. - -In the request, the client sends (in cleartext) its own identity and the -identity of the server for which it is requesting credentials. The response, -KRB_AS_REP, contains a ticket for the client to present to the server, and a -session key that will be shared by the client and the server. The session - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -key and additional information are encrypted in the client's secret key. The -KRB_AS_REP message contains information which can be used to detect replays, -and to associate it with the message to which it replies. Various errors can -occur; these are indicated by an error response (KRB_ERROR) instead of the -KRB_AS_REP response. The error message is not encrypted. The KRB_ERROR -message contains information which can be used to associate it with the -message to which it replies. The lack of encryption in the KRB_ERROR message -precludes the ability to detect replays, fabrications, or modifications of -such messages. - -Without preautentication, the authentication server does not know whether -the client is actually the principal named in the request. It simply sends a -reply without knowing or caring whether they are the same. This is -acceptable because nobody but the principal whose identity was given in the -request will be able to use the reply. Its critical information is encrypted -in that principal's key. The initial request supports an optional field that -can be used to pass additional information that might be needed for the -initial exchange. This field may be used for preauthentication as described -in section [hl<>]. - -3.1.1. Generation of KRB_AS_REQ message - -The client may specify a number of options in the initial request. Among -these options are whether pre-authentication is to be performed; whether the -requested ticket is to be renewable, proxiable, or forwardable; whether it -should be postdated or allow postdating of derivative tickets; and whether a -renewable ticket will be accepted in lieu of a non-renewable ticket if the -requested ticket expiration date cannot be satisfied by a non-renewable -ticket (due to configuration constraints; see section 4). See section A.1 -for pseudocode. - -The client prepares the KRB_AS_REQ message and sends it to the KDC. - -3.1.2. Receipt of KRB_AS_REQ message - -If all goes well, processing the KRB_AS_REQ message will result in the -creation of a ticket for the client to present to the server. The format for -the ticket is described in section 5.3.1. The contents of the ticket are -determined as follows. - -3.1.3. Generation of KRB_AS_REP message - -The authentication server looks up the client and server principals named in -the KRB_AS_REQ in its database, extracting their respective keys. If -required, the server pre-authenticates the request, and if the -pre-authentication check fails, an error message with the code -KDC_ERR_PREAUTH_FAILED is returned. If the server cannot accommodate the -requested encryption type, an error message with code KDC_ERR_ETYPE_NOSUPP -is returned. Otherwise it generates a 'random' session key[7]. - -If there are multiple encryption keys registered for a client in the -Kerberos database (or if the key registered supports multiple encryption -types; e.g. DES-CBC-CRC and DES-CBC-MD5), then the etype field from the AS -request is used by the KDC to select the encryption method to be used for - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -encrypting the response to the client. If there is more than one supported, -strong encryption type in the etype list, the first valid etype for which an -encryption key is available is used. The encryption method used to respond -to a TGS request is taken from the keytype of the session key found in the -ticket granting ticket. - -When the etype field is present in a KDC request, whether an AS or TGS -request, the KDC will attempt to assign the type of the random session key -from the list of methods in the etype field. The KDC will select the -appropriate type using the list of methods provided together with -information from the Kerberos database indicating acceptable encryption -methods for the application server. The KDC will not issue tickets with a -weak session key encryption type. - -If the requested start time is absent, indicates a time in the past, or is -within the window of acceptable clock skew for the KDC and the POSTDATE -option has not been specified, then the start time of the ticket is set to -the authentication server's current time. If it indicates a time in the -future beyond the acceptable clock skew, but the POSTDATED option has not -been specified then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise -the requested start time is checked against the policy of the local realm -(the administrator might decide to prohibit certain types or ranges of -postdated tickets), and if acceptable, the ticket's start time is set as -requested and the INVALID flag is set in the new ticket. The postdated -ticket must be validated before use by presenting it to the KDC after the -start time has been reached. - -The expiration time of the ticket will be set to the minimum of the -following: - - * The expiration time (endtime) requested in the KRB_AS_REQ message. - * The ticket's start time plus the maximum allowable lifetime associated - with the client principal (the authentication server's database - includes a maximum ticket lifetime field in each principal's record; - see section 4). - * The ticket's start time plus the maximum allowable lifetime associated - with the server principal. - * The ticket's start time plus the maximum lifetime set by the policy of - the local realm. - -If the requested expiration time minus the start time (as determined above) -is less than a site-determined minimum lifetime, an error message with code -KDC_ERR_NEVER_VALID is returned. If the requested expiration time for the -ticket exceeds what was determined as above, and if the 'RENEWABLE-OK' -option was requested, then the 'RENEWABLE' flag is set in the new ticket, -and the renew-till value is set as if the 'RENEWABLE' option were requested -(the field and option names are described fully in section 5.4.1). - -If the RENEWABLE option has been requested or if the RENEWABLE-OK option has -been set and a renewable ticket is to be issued, then the renew-till field -is set to the minimum of: - - * Its requested value. - * The start time of the ticket plus the minimum of the two maximum - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - renewable lifetimes associated with the principals' database entries. - * The start time of the ticket plus the maximum renewable lifetime set by - the policy of the local realm. - -The flags field of the new ticket will have the following options set if -they have been requested and if the policy of the local realm allows: -FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE. If the new -ticket is post-dated (the start time is in the future), its INVALID flag -will also be set. - -If all of the above succeed, the server formats a KRB_AS_REP message (see -section 5.4.2), copying the addresses in the request into the caddr of the -response, placing any required pre-authentication data into the padata of -the response, and encrypts the ciphertext part in the client's key using the -requested encryption method, and sends it to the client. See section A.2 for -pseudocode. - -3.1.4. Generation of KRB_ERROR message - -Several errors can occur, and the Authentication Server responds by -returning an error message, KRB_ERROR, to the client, with the error-code -and e-text fields set to appropriate values. The error message contents and -details are described in Section 5.9.1. - -3.1.5. Receipt of KRB_AS_REP message - -If the reply message type is KRB_AS_REP, then the client verifies that the -cname and crealm fields in the cleartext portion of the reply match what it -requested. If any padata fields are present, they may be used to derive the -proper secret key to decrypt the message. The client decrypts the encrypted -part of the response using its secret key, verifies that the nonce in the -encrypted part matches the nonce it supplied in its request (to detect -replays). It also verifies that the sname and srealm in the response match -those in the request (or are otherwise expected values), and that the host -address field is also correct. It then stores the ticket, session key, start -and expiration times, and other information for later use. The -key-expiration field from the encrypted part of the response may be checked -to notify the user of impending key expiration (the client program could -then suggest remedial action, such as a password change). See section A.3 -for pseudocode. - -Proper decryption of the KRB_AS_REP message is not sufficient to verify the -identity of the user; the user and an attacker could cooperate to generate a -KRB_AS_REP format message which decrypts properly but is not from the proper -KDC. If the host wishes to verify the identity of the user, it must require -the user to present application credentials which can be verified using a -securely-stored secret key for the host. If those credentials can be -verified, then the identity of the user can be assured. - -3.1.6. Receipt of KRB_ERROR message - -If the reply message type is KRB_ERROR, then the client interprets it as an -error and performs whatever application-specific tasks are necessary to -recover. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -3.2. The Client/Server Authentication Exchange - - Summary -Message direction Message type Section -Client to Application server KRB_AP_REQ 5.5.1 -[optional] Application server to client KRB_AP_REP or 5.5.2 - KRB_ERROR 5.9.1 - -The client/server authentication (CS) exchange is used by network -applications to authenticate the client to the server and vice versa. The -client must have already acquired credentials for the server using the AS or -TGS exchange. - -3.2.1. The KRB_AP_REQ message - -The KRB_AP_REQ contains authentication information which should be part of -the first message in an authenticated transaction. It contains a ticket, an -authenticator, and some additional bookkeeping information (see section -5.5.1 for the exact format). The ticket by itself is insufficient to -authenticate a client, since tickets are passed across the network in -cleartext[DS90], so the authenticator is used to prevent invalid replay of -tickets by proving to the server that the client knows the session key of -the ticket and thus is entitled to use the ticket. The KRB_AP_REQ message is -referred to elsewhere as the 'authentication header.' - -3.2.2. Generation of a KRB_AP_REQ message - -When a client wishes to initiate authentication to a server, it obtains -(either through a credentials cache, the AS exchange, or the TGS exchange) a -ticket and session key for the desired service. The client may re-use any -tickets it holds until they expire. To use a ticket the client constructs a -new Authenticator from the the system time, its name, and optionally an -application specific checksum, an initial sequence number to be used in -KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used in -negotiations for a session key unique to this particular session. -Authenticators may not be re-used and will be rejected if replayed to a -server[LGDSR87]. If a sequence number is to be included, it should be -randomly chosen so that even after many messages have been exchanged it is -not likely to collide with other sequence numbers in use. - -The client may indicate a requirement of mutual authentication or the use of -a session-key based ticket by setting the appropriate flag(s) in the -ap-options field of the message. - -The Authenticator is encrypted in the session key and combined with the -ticket to form the KRB_AP_REQ message which is then sent to the end server -along with any additional application-specific information. See section A.9 -for pseudocode. - -3.2.3. Receipt of KRB_AP_REQ message - -Authentication is based on the server's current time of day (clocks must be -loosely synchronized), the authenticator, and the ticket. Several errors are - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -possible. If an error occurs, the server is expected to reply to the client -with a KRB_ERROR message. This message may be encapsulated in the -application protocol if its 'raw' form is not acceptable to the protocol. -The format of error messages is described in section 5.9.1. - -The algorithm for verifying authentication information is as follows. If the -message type is not KRB_AP_REQ, the server returns the KRB_AP_ERR_MSG_TYPE -error. If the key version indicated by the Ticket in the KRB_AP_REQ is not -one the server can use (e.g., it indicates an old key, and the server no -longer possesses a copy of the old key), the KRB_AP_ERR_BADKEYVER error is -returned. If the USE-SESSION-KEY flag is set in the ap-options field, it -indicates to the server that the ticket is encrypted in the session key from -the server's ticket-granting ticket rather than its secret key[10]. Since it -is possible for the server to be registered in multiple realms, with -different keys in each, the srealm field in the unencrypted portion of the -ticket in the KRB_AP_REQ is used to specify which secret key the server -should use to decrypt that ticket. The KRB_AP_ERR_NOKEY error code is -returned if the server doesn't have the proper key to decipher the ticket. - -The ticket is decrypted using the version of the server's key specified by -the ticket. If the decryption routines detect a modification of the ticket -(each encryption system must provide safeguards to detect modified -ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY error is returned -(chances are good that different keys were used to encrypt and decrypt). - -The authenticator is decrypted using the session key extracted from the -decrypted ticket. If decryption shows it to have been modified, the -KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm of the client -from the ticket are compared against the same fields in the authenticator. -If they don't match, the KRB_AP_ERR_BADMATCH error is returned (they might -not match, for example, if the wrong session key was used to encrypt the -authenticator). The addresses in the ticket (if any) are then searched for -an address matching the operating-system reported address of the client. If -no match is found or the server insists on ticket addresses but none are -present in the ticket, the KRB_AP_ERR_BADADDR error is returned. - -If the local (server) time and the client time in the authenticator differ -by more than the allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW -error is returned. If the server name, along with the client name, time and -microsecond fields from the Authenticator match any recently-seen such -tuples, the KRB_AP_ERR_REPEAT error is returned[11]. The server must -remember any authenticator presented within the allowable clock skew, so -that a replay attempt is guaranteed to fail. If a server loses track of any -authenticator presented within the allowable clock skew, it must reject all -requests until the clock skew interval has passed. This assures that any -lost or re-played authenticators will fall outside the allowable clock skew -and can no longer be successfully replayed (If this is not done, an attacker -could conceivably record the ticket and authenticator sent over the network -to a server, then disable the client's host, pose as the disabled host, and -replay the ticket and authenticator to subvert the authentication.). If a -sequence number is provided in the authenticator, the server saves it for -later use in processing KRB_SAFE and/or KRB_PRIV messages. If a subkey is -present, the server either saves it for later use or uses it to help -generate its own choice for a subkey to be returned in a KRB_AP_REP message. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -The server computes the age of the ticket: local (server) time minus the -start time inside the Ticket. If the start time is later than the current -time by more than the allowable clock skew or if the INVALID flag is set in -the ticket, the KRB_AP_ERR_TKT_NYV error is returned. Otherwise, if the -current time is later than end time by more than the allowable clock skew, -the KRB_AP_ERR_TKT_EXPIRED error is returned. - -If all these checks succeed without an error, the server is assured that the -client possesses the credentials of the principal named in the ticket and -thus, the client has been authenticated to the server. See section A.10 for -pseudocode. - -Passing these checks provides only authentication of the named principal; it -does not imply authorization to use the named service. Applications must -make a separate authorization decisions based upon the authenticated name of -the user, the requested operation, local acces control information such as -that contained in a .k5login or .k5users file, and possibly a separate -distributed authorization service. - -3.2.4. Generation of a KRB_AP_REP message - -Typically, a client's request will include both the authentication -information and its initial request in the same message, and the server need -not explicitly reply to the KRB_AP_REQ. However, if mutual authentication -(not only authenticating the client to the server, but also the server to -the client) is being performed, the KRB_AP_REQ message will have -MUTUAL-REQUIRED set in its ap-options field, and a KRB_AP_REP message is -required in response. As with the error message, this message may be -encapsulated in the application protocol if its "raw" form is not acceptable -to the application's protocol. The timestamp and microsecond field used in -the reply must be the client's timestamp and microsecond field (as provided -in the authenticator)[12]. If a sequence number is to be included, it should -be randomly chosen as described above for the authenticator. A subkey may be -included if the server desires to negotiate a different subkey. The -KRB_AP_REP message is encrypted in the session key extracted from the -ticket. See section A.11 for pseudocode. - -3.2.5. Receipt of KRB_AP_REP message - -If a KRB_AP_REP message is returned, the client uses the session key from -the credentials obtained for the server[13] to decrypt the message, and -verifies that the timestamp and microsecond fields match those in the -Authenticator it sent to the server. If they match, then the client is -assured that the server is genuine. The sequence number and subkey (if -present) are retained for later use. See section A.12 for pseudocode. - -3.2.6. Using the encryption key - -After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and server -share an encryption key which can be used by the application. The 'true -session key' to be used for KRB_PRIV, KRB_SAFE, or other -application-specific uses may be chosen by the application based on the -subkeys in the KRB_AP_REP message and the authenticator[14]. In some cases, - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -the use of this session key will be implicit in the protocol; in others the -method of use must be chosen from several alternatives. We leave the -protocol negotiations of how to use the key (e.g. selecting an encryption or -checksum type) to the application programmer; the Kerberos protocol does not -constrain the implementation options, but an example of how this might be -done follows. - -One way that an application may choose to negotiate a key to be used for -subequent integrity and privacy protection is for the client to propose a -key in the subkey field of the authenticator. The server can then choose a -key using the proposed key from the client as input, returning the new -subkey in the subkey field of the application reply. This key could then be -used for subsequent communication. To make this example more concrete, if -the encryption method in use required a 56 bit key, and for whatever reason, -one of the parties was prevented from using a key with more than 40 unknown -bits, this method would allow the the party which is prevented from using -more than 40 bits to either propose (if the client) an initial key with a -known quantity for 16 of those bits, or to mask 16 of the bits (if the -server) with the known quantity. The application implementor is warned, -however, that this is only an example, and that an analysis of the -particular crytosystem to be used, and the reasons for limiting the key -length, must be made before deciding whether it is acceptable to mask bits -of the key. - -With both the one-way and mutual authentication exchanges, the peers should -take care not to send sensitive information to each other without proper -assurances. In particular, applications that require privacy or integrity -should use the KRB_AP_REP response from the server to client to assure both -client and server of their peer's identity. If an application protocol -requires privacy of its messages, it can use the KRB_PRIV message (section -3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity. - -3.3. The Ticket-Granting Service (TGS) Exchange - - Summary - Message direction Message type Section - 1. Client to Kerberos KRB_TGS_REQ 5.4.1 - 2. Kerberos to client KRB_TGS_REP or 5.4.2 - KRB_ERROR 5.9.1 - -The TGS exchange between a client and the Kerberos Ticket-Granting Server is -initiated by a client when it wishes to obtain authentication credentials -for a given server (which might be registered in a remote realm), when it -wishes to renew or validate an existing ticket, or when it wishes to obtain -a proxy ticket. In the first case, the client must already have acquired a -ticket for the Ticket-Granting Service using the AS exchange (the -ticket-granting ticket is usually obtained when a client initially -authenticates to the system, such as when a user logs in). The message -format for the TGS exchange is almost identical to that for the AS exchange. -The primary difference is that encryption and decryption in the TGS exchange -does not take place under the client's key. Instead, the session key from -the ticket-granting ticket or renewable ticket, or sub-session key from an -Authenticator is used. As is the case for all application servers, expired -tickets are not accepted by the TGS, so once a renewable or ticket-granting - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -ticket expires, the client must use a separate exchange to obtain valid -tickets. - -The TGS exchange consists of two messages: A request (KRB_TGS_REQ) from the -client to the Kerberos Ticket-Granting Server, and a reply (KRB_TGS_REP or -KRB_ERROR). The KRB_TGS_REQ message includes information authenticating the -client plus a request for credentials. The authentication information -consists of the authentication header (KRB_AP_REQ) which includes the -client's previously obtained ticket-granting, renewable, or invalid ticket. -In the ticket-granting ticket and proxy cases, the request may include one -or more of: a list of network addresses, a collection of typed authorization -data to be sealed in the ticket for authorization use by the application -server, or additional tickets (the use of which are described later). The -TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted in the -session key from the ticket-granting ticket or renewable ticket, or if -present, in the sub-session key from the Authenticator (part of the -authentication header). The KRB_ERROR message contains an error code and -text explaining what went wrong. The KRB_ERROR message is not encrypted. The -KRB_TGS_REP message contains information which can be used to detect -replays, and to associate it with the message to which it replies. The -KRB_ERROR message also contains information which can be used to associate -it with the message to which it replies, but the lack of encryption in the -KRB_ERROR message precludes the ability to detect replays or fabrications of -such messages. - -3.3.1. Generation of KRB_TGS_REQ message - -Before sending a request to the ticket-granting service, the client must -determine in which realm the application server is registered[15]. If the -client does not already possess a ticket-granting ticket for the appropriate -realm, then one must be obtained. This is first attempted by requesting a -ticket-granting ticket for the destination realm from a Kerberos server for -which the client does posess a ticket-granting ticket (using the KRB_TGS_REQ -message recursively). The Kerberos server may return a TGT for the desired -realm in which case one can proceed. Alternatively, the Kerberos server may -return a TGT for a realm which is 'closer' to the desired realm (further -along the standard hierarchical path), in which case this step must be -repeated with a Kerberos server in the realm specified in the returned TGT. -If neither are returned, then the request must be retried with a Kerberos -server for a realm higher in the hierarchy. This request will itself require -a ticket-granting ticket for the higher realm which must be obtained by -recursively applying these directions. - -Once the client obtains a ticket-granting ticket for the appropriate realm, -it determines which Kerberos servers serve that realm, and contacts one. The -list might be obtained through a configuration file or network service or it -may be generated from the name of the realm; as long as the secret keys -exchanged by realms are kept secret, only denial of service results from -using a false Kerberos server. - -As in the AS exchange, the client may specify a number of options in the -KRB_TGS_REQ message. The client prepares the KRB_TGS_REQ message, providing -an authentication header as an element of the padata field, and including -the same fields as used in the KRB_AS_REQ message along with several - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -optional fields: the enc-authorization-data field for application server use -and additional tickets required by some options. - -In preparing the authentication header, the client can select a sub-session -key under which the response from the Kerberos server will be encrypted[16]. -If the sub-session key is not specified, the session key from the -ticket-granting ticket will be used. If the enc-authorization-data is -present, it must be encrypted in the sub-session key, if present, from the -authenticator portion of the authentication header, or if not present, using -the session key from the ticket-granting ticket. - -Once prepared, the message is sent to a Kerberos server for the destination -realm. See section A.5 for pseudocode. - -3.3.2. Receipt of KRB_TGS_REQ message - -The KRB_TGS_REQ message is processed in a manner similar to the KRB_AS_REQ -message, but there are many additional checks to be performed. First, the -Kerberos server must determine which server the accompanying ticket is for -and it must select the appropriate key to decrypt it. For a normal -KRB_TGS_REQ message, it will be for the ticket granting service, and the -TGS's key will be used. If the TGT was issued by another realm, then the -appropriate inter-realm key must be used. If the accompanying ticket is not -a ticket granting ticket for the current realm, but is for an application -server in the current realm, the RENEW, VALIDATE, or PROXY options are -specified in the request, and the server for which a ticket is requested is -the server named in the accompanying ticket, then the KDC will decrypt the -ticket in the authentication header using the key of the server for which it -was issued. If no ticket can be found in the padata field, the -KDC_ERR_PADATA_TYPE_NOSUPP error is returned. - -Once the accompanying ticket has been decrypted, the user-supplied checksum -in the Authenticator must be verified against the contents of the request, -and the message rejected if the checksums do not match (with an error code -of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or not -collision-proof (with an error code of KRB_AP_ERR_INAPP_CKSUM). If the -checksum type is not supported, the KDC_ERR_SUMTYPE_NOSUPP error is -returned. If the authorization-data are present, they are decrypted using -the sub-session key from the Authenticator. - -If any of the decryptions indicate failed integrity checks, the -KRB_AP_ERR_BAD_INTEGRITY error is returned. - -3.3.3. Generation of KRB_TGS_REP message - -The KRB_TGS_REP message shares its format with the KRB_AS_REP (KRB_KDC_REP), -but with its type field set to KRB_TGS_REP. The detailed specification is in -section 5.4.2. - -The response will include a ticket for the requested server. The Kerberos -database is queried to retrieve the record for the requested server -(including the key with which the ticket will be encrypted). If the request -is for a ticket granting ticket for a remote realm, and if no key is shared -with the requested realm, then the Kerberos server will select the realm - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -"closest" to the requested realm with which it does share a key, and use -that realm instead. This is the only case where the response from the KDC -will be for a different server than that requested by the client. - -By default, the address field, the client's name and realm, the list of -transited realms, the time of initial authentication, the expiration time, -and the authorization data of the newly-issued ticket will be copied from -the ticket-granting ticket (TGT) or renewable ticket. If the transited field -needs to be updated, but the transited type is not supported, the -KDC_ERR_TRTYPE_NOSUPP error is returned. - -If the request specifies an endtime, then the endtime of the new ticket is -set to the minimum of (a) that request, (b) the endtime from the TGT, and -(c) the starttime of the TGT plus the minimum of the maximum life for the -application server and the maximum life for the local realm (the maximum -life for the requesting principal was already applied when the TGT was -issued). If the new ticket is to be a renewal, then the endtime above is -replaced by the minimum of (a) the value of the renew_till field of the -ticket and (b) the starttime for the new ticket plus the life -(endtime-starttime) of the old ticket. - -If the FORWARDED option has been requested, then the resulting ticket will -contain the addresses specified by the client. This option will only be -honored if the FORWARDABLE flag is set in the TGT. The PROXY option is -similar; the resulting ticket will contain the addresses specified by the -client. It will be honored only if the PROXIABLE flag in the TGT is set. The -PROXY option will not be honored on requests for additional ticket-granting -tickets. - -If the requested start time is absent, indicates a time in the past, or is -within the window of acceptable clock skew for the KDC and the POSTDATE -option has not been specified, then the start time of the ticket is set to -the authentication server's current time. If it indicates a time in the -future beyond the acceptable clock skew, but the POSTDATED option has not -been specified or the MAY-POSTDATE flag is not set in the TGT, then the -error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the ticket-granting -ticket has the MAY-POSTDATE flag set, then the resulting ticket will be -postdated and the requested starttime is checked against the policy of the -local realm. If acceptable, the ticket's start time is set as requested, and -the INVALID flag is set. The postdated ticket must be validated before use -by presenting it to the KDC after the starttime has been reached. However, -in no case may the starttime, endtime, or renew-till time of a newly-issued -postdated ticket extend beyond the renew-till time of the ticket-granting -ticket. - -If the ENC-TKT-IN-SKEY option has been specified and an additional ticket -has been included in the request, the KDC will decrypt the additional ticket -using the key for the server to which the additional ticket was issued and -verify that it is a ticket-granting ticket. If the name of the requested -server is missing from the request, the name of the client in the additional -ticket will be used. Otherwise the name of the requested server will be -compared to the name of the client in the additional ticket and if -different, the request will be rejected. If the request succeeds, the -session key from the additional ticket will be used to encrypt the new - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -ticket that is issued instead of using the key of the server for which the -new ticket will be used[17]. - -If the name of the server in the ticket that is presented to the KDC as part -of the authentication header is not that of the ticket-granting server -itself, the server is registered in the realm of the KDC, and the RENEW -option is requested, then the KDC will verify that the RENEWABLE flag is set -in the ticket, that the INVALID flag is not set in the ticket, and that the -renew_till time is still in the future. If the VALIDATE option is rqeuested, -the KDC will check that the starttime has passed and the INVALID flag is -set. If the PROXY option is requested, then the KDC will check that the -PROXIABLE flag is set in the ticket. If the tests succeed, and the ticket -passes the hotlist check described in the next paragraph, the KDC will issue -the appropriate new ticket. - -3.3.3.1. Checking for revoked tickets - -Whenever a request is made to the ticket-granting server, the presented -ticket(s) is(are) checked against a hot-list of tickets which have been -canceled. This hot-list might be implemented by storing a range of issue -timestamps for 'suspect tickets'; if a presented ticket had an authtime in -that range, it would be rejected. In this way, a stolen ticket-granting -ticket or renewable ticket cannot be used to gain additional tickets -(renewals or otherwise) once the theft has been reported. Any normal ticket -obtained before it was reported stolen will still be valid (because they -require no interaction with the KDC), but only until their normal expiration -time. - -The ciphertext part of the response in the KRB_TGS_REP message is encrypted -in the sub-session key from the Authenticator, if present, or the session -key key from the ticket-granting ticket. It is not encrypted using the -client's secret key. Furthermore, the client's key's expiration date and the -key version number fields are left out since these values are stored along -with the client's database record, and that record is not needed to satisfy -a request based on a ticket-granting ticket. See section A.6 for pseudocode. - -3.3.3.2. Encoding the transited field - -If the identity of the server in the TGT that is presented to the KDC as -part of the authentication header is that of the ticket-granting service, -but the TGT was issued from another realm, the KDC will look up the -inter-realm key shared with that realm and use that key to decrypt the -ticket. If the ticket is valid, then the KDC will honor the request, subject -to the constraints outlined above in the section describing the AS exchange. -The realm part of the client's identity will be taken from the -ticket-granting ticket. The name of the realm that issued the -ticket-granting ticket will be added to the transited field of the ticket to -be issued. This is accomplished by reading the transited field from the -ticket-granting ticket (which is treated as an unordered set of realm -names), adding the new realm to the set, then constructing and writing out -its encoded (shorthand) form (this may involve a rearrangement of the -existing encoding). - -Note that the ticket-granting service does not add the name of its own - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -realm. Instead, its responsibility is to add the name of the previous realm. -This prevents a malicious Kerberos server from intentionally leaving out its -own name (it could, however, omit other realms' names). - -The names of neither the local realm nor the principal's realm are to be -included in the transited field. They appear elsewhere in the ticket and -both are known to have taken part in authenticating the principal. Since the -endpoints are not included, both local and single-hop inter-realm -authentication result in a transited field that is empty. - -Because the name of each realm transited is added to this field, it might -potentially be very long. To decrease the length of this field, its contents -are encoded. The initially supported encoding is optimized for the normal -case of inter-realm communication: a hierarchical arrangement of realms -using either domain or X.500 style realm names. This encoding (called -DOMAIN-X500-COMPRESS) is now described. - -Realm names in the transited field are separated by a ",". The ",", "\", -trailing "."s, and leading spaces (" ") are special characters, and if they -are part of a realm name, they must be quoted in the transited field by -preced- ing them with a "\". - -A realm name ending with a "." is interpreted as being prepended to the -previous realm. For example, we can encode traversal of EDU, MIT.EDU, -ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as: - - "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.". - -Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points, that they -would not be included in this field, and we would have: - - "EDU,MIT.,WASHINGTON.EDU" - -A realm name beginning with a "/" is interpreted as being appended to the -previous realm[18]. If it is to stand by itself, then it should be preceded -by a space (" "). For example, we can encode traversal of /COM/HP/APOLLO, -/COM/HP, /COM, and /COM/DEC as: - - "/COM,/HP,/APOLLO, /COM/DEC". - -Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints, they -they would not be included in this field, and we would have: - - "/COM,/HP" - -A null subfield preceding or following a "," indicates that all realms -between the previous realm and the next realm have been traversed[19]. Thus, -"," means that all realms along the path between the client and the server -have been traversed. ",EDU, /COM," means that that all realms from the -client's realm up to EDU (in a domain style hierarchy) have been traversed, -and that everything from /COM down to the server's realm in an X.500 style -has also been traversed. This could occur if the EDU realm in one hierarchy -shares an inter-realm key directly with the /COM realm in another hierarchy. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -3.3.4. Receipt of KRB_TGS_REP message - -When the KRB_TGS_REP is received by the client, it is processed in the same -manner as the KRB_AS_REP processing described above. The primary difference -is that the ciphertext part of the response must be decrypted using the -session key from the ticket-granting ticket rather than the client's secret -key. See section A.7 for pseudocode. - -3.4. The KRB_SAFE Exchange - -The KRB_SAFE message may be used by clients requiring the ability to detect -modifications of messages they exchange. It achieves this by including a -keyed collision-proof checksum of the user data and some control -information. The checksum is keyed with an encryption key (usually the last -key negotiated via subkeys, or the session key if no negotiation has -occured). - -3.4.1. Generation of a KRB_SAFE message - -When an application wishes to send a KRB_SAFE message, it collects its data -and the appropriate control information and computes a checksum over them. -The checksum algorithm should be a keyed one-way hash function (such as the -RSA- MD5-DES checksum algorithm specified in section 6.4.5, or the DES MAC), -generated using the sub-session key if present, or the session key. -Different algorithms may be selected by changing the checksum type in the -message. Unkeyed or non-collision-proof checksums are not suitable for this -use. - -The control information for the KRB_SAFE message includes both a timestamp -and a sequence number. The designer of an application using the KRB_SAFE -message must choose at least one of the two mechanisms. This choice should -be based on the needs of the application protocol. - -Sequence numbers are useful when all messages sent will be received by one's -peer. Connection state is presently required to maintain the session key, so -maintaining the next sequence number should not present an additional -problem. - -If the application protocol is expected to tolerate lost messages without -them being resent, the use of the timestamp is the appropriate replay -detection mechanism. Using timestamps is also the appropriate mechanism for -multi-cast protocols where all of one's peers share a common sub-session -key, but some messages will be sent to a subset of one's peers. - -After computing the checksum, the client then transmits the information and -checksum to the recipient in the message format specified in section 5.6.1. - -3.4.2. Receipt of KRB_SAFE message - -When an application receives a KRB_SAFE message, it verifies it as follows. -If any error occurs, an error code is reported for use by the application. - -The message is first checked by verifying that the protocol version and type -fields match the current version and KRB_SAFE, respectively. A mismatch - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The -application verifies that the checksum used is a collision-proof keyed -checksum, and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is generated. The -recipient verifies that the operating system's report of the sender's -address matches the sender's address in the message, and (if a recipient -address is specified or the recipient requires an address) that one of the -recipient's addresses appears as the recipient's address in the message. A -failed match for either case generates a KRB_AP_ERR_BADADDR error. Then the -timestamp and usec and/or the sequence number fields are checked. If -timestamp and usec are expected and not present, or they are present but not -current, the KRB_AP_ERR_SKEW error is generated. If the server name, along -with the client name, time and microsecond fields from the Authenticator -match any recently-seen (sent or received[20] ) such tuples, the -KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence number is -included, or a sequence number is expected but not present, the -KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec or -a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated. -Finally, the checksum is computed over the data and control information, and -if it doesn't match the received checksum, a KRB_AP_ERR_MODIFIED error is -generated. - -If all the checks succeed, the application is assured that the message was -generated by its peer and was not modi- fied in transit. - -3.5. The KRB_PRIV Exchange - -The KRB_PRIV message may be used by clients requiring confidentiality and -the ability to detect modifications of exchanged messages. It achieves this -by encrypting the messages and adding control information. - -3.5.1. Generation of a KRB_PRIV message - -When an application wishes to send a KRB_PRIV message, it collects its data -and the appropriate control information (specified in section 5.7.1) and -encrypts them under an encryption key (usually the last key negotiated via -subkeys, or the session key if no negotiation has occured). As part of the -control information, the client must choose to use either a timestamp or a -sequence number (or both); see the discussion in section 3.4.1 for -guidelines on which to use. After the user data and control information are -encrypted, the client transmits the ciphertext and some 'envelope' -information to the recipient. - -3.5.2. Receipt of KRB_PRIV message - -When an application receives a KRB_PRIV message, it verifies it as follows. -If any error occurs, an error code is reported for use by the application. - -The message is first checked by verifying that the protocol version and type -fields match the current version and KRB_PRIV, respectively. A mismatch -generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The -application then decrypts the ciphertext and processes the resultant -plaintext. If decryption shows the data to have been modified, a -KRB_AP_ERR_BAD_INTEGRITY error is generated. The recipient verifies that the -operating system's report of the sender's address matches the sender's - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -address in the message, and (if a recipient address is specified or the -recipient requires an address) that one of the recipient's addresses appears -as the recipient's address in the message. A failed match for either case -generates a KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the -sequence number fields are checked. If timestamp and usec are expected and -not present, or they are present but not current, the KRB_AP_ERR_SKEW error -is generated. If the server name, along with the client name, time and -microsecond fields from the Authenticator match any recently-seen such -tuples, the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence -number is included, or a sequence number is expected but not present, the -KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec or -a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated. - -If all the checks succeed, the application can assume the message was -generated by its peer, and was securely transmitted (without intruders able -to see the unencrypted contents). - -3.6. The KRB_CRED Exchange - -The KRB_CRED message may be used by clients requiring the ability to send -Kerberos credentials from one host to another. It achieves this by sending -the tickets together with encrypted data containing the session keys and -other information associated with the tickets. - -3.6.1. Generation of a KRB_CRED message - -When an application wishes to send a KRB_CRED message it first (using the -KRB_TGS exchange) obtains credentials to be sent to the remote host. It then -constructs a KRB_CRED message using the ticket or tickets so obtained, -placing the session key needed to use each ticket in the key field of the -corresponding KrbCredInfo sequence of the encrypted part of the the KRB_CRED -message. - -Other information associated with each ticket and obtained during the -KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence in -the encrypted part of the KRB_CRED message. The current time and, if -specifically required by the application the nonce, s-address, and r-address -fields, are placed in the encrypted part of the KRB_CRED message which is -then encrypted under an encryption key previosuly exchanged in the KRB_AP -exchange (usually the last key negotiated via subkeys, or the session key if -no negotiation has occured). - -3.6.2. Receipt of KRB_CRED message - -When an application receives a KRB_CRED message, it verifies it. If any -error occurs, an error code is reported for use by the application. The -message is verified by checking that the protocol version and type fields -match the current version and KRB_CRED, respectively. A mismatch generates a -KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then -decrypts the ciphertext and processes the resultant plaintext. If decryption -shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is -generated. - -If present or required, the recipient verifies that the operating system's - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -report of the sender's address matches the sender's address in the message, -and that one of the recipient's addresses appears as the recipient's address -in the message. A failed match for either case generates a -KRB_AP_ERR_BADADDR error. The timestamp and usec fields (and the nonce field -if required) are checked next. If the timestamp and usec are not present, or -they are present but not current, the KRB_AP_ERR_SKEW error is generated. - -If all the checks succeed, the application stores each of the new tickets in -its ticket cache together with the session key and other information in the -corresponding KrbCredInfo sequence from the encrypted part of the KRB_CRED -message. - -4. The Kerberos Database - -The Kerberos server must have access to a database contain- ing the -principal identifiers and secret keys of principals to be authenticated[21]. - -4.1. Database contents - -A database entry should contain at least the following fields: - -Field Value - -name Principal's identifier -key Principal's secret key -p_kvno Principal's key version -max_life Maximum lifetime for Tickets -max_renewable_life Maximum total lifetime for renewable Tickets - -The name field is an encoding of the principal's identifier. The key field -contains an encryption key. This key is the principal's secret key. (The key -can be encrypted before storage under a Kerberos "master key" to protect it -in case the database is compromised but the master key is not. In that case, -an extra field must be added to indicate the master key version used, see -below.) The p_kvno field is the key version number of the principal's secret -key. The max_life field contains the maximum allowable lifetime (endtime - -starttime) for any Ticket issued for this principal. The max_renewable_life -field contains the maximum allowable total lifetime for any renewable Ticket -issued for this principal. (See section 3.1 for a description of how these -lifetimes are used in determining the lifetime of a given Ticket.) - -A server may provide KDC service to several realms, as long as the database -representation provides a mechanism to distinguish between principal records -with identifiers which differ only in the realm name. - -When an application server's key changes, if the change is routine (i.e. not -the result of disclosure of the old key), the old key should be retained by -the server until all tickets that had been issued using that key have -expired. Because of this, it is possible for several keys to be active for a -single principal. Ciphertext encrypted in a principal's key is always tagged -with the version of the key that was used for encryption, to help the -recipient find the proper key for decryption. - -When more than one key is active for a particular principal, the principal - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -will have more than one record in the Kerberos database. The keys and key -version numbers will differ between the records (the rest of the fields may -or may not be the same). Whenever Kerberos issues a ticket, or responds to a -request for initial authentication, the most recent key (known by the -Kerberos server) will be used for encryption. This is the key with the -highest key version number. - -4.2. Additional fields - -Project Athena's KDC implementation uses additional fields in its database: - -Field Value - -K_kvno Kerberos' key version -expiration Expiration date for entry -attributes Bit field of attributes -mod_date Timestamp of last modification -mod_name Modifying principal's identifier - -The K_kvno field indicates the key version of the Kerberos master key under -which the principal's secret key is encrypted. - -After an entry's expiration date has passed, the KDC will return an error to -any client attempting to gain tickets as or for the principal. (A database -may want to maintain two expiration dates: one for the principal, and one -for the principal's current key. This allows password aging to work -independently of the principal's expiration date. However, due to the -limited space in the responses, the KDC must combine the key expiration and -principal expiration date into a single value called 'key_exp', which is -used as a hint to the user to take administrative action.) - -The attributes field is a bitfield used to govern the operations involving -the principal. This field might be useful in conjunction with user -registration procedures, for site-specific policy implementations (Project -Athena currently uses it for their user registration process controlled by -the system-wide database service, Moira [LGDSR87]), to identify whether a -principal can play the role of a client or server or both, to note whether a -server is appropriate trusted to recieve credentials delegated by a client, -or to identify the 'string to key' conversion algorithm used for a -principal's key[22]. Other bits are used to indicate that certain ticket -options should not be allowed in tickets encrypted under a principal's key -(one bit each): Disallow issuing postdated tickets, disallow issuing -forwardable tickets, disallow issuing tickets based on TGT authentication, -disallow issuing renewable tickets, disallow issuing proxiable tickets, and -disallow issuing tickets for which the principal is the server. - -The mod_date field contains the time of last modification of the entry, and -the mod_name field contains the name of the principal which last modified -the entry. - -4.3. Frequently Changing Fields - -Some KDC implementations may wish to maintain the last time that a request -was made by a particular principal. Information that might be maintained - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -includes the time of the last request, the time of the last request for a -ticket-granting ticket, the time of the last use of a ticket-granting -ticket, or other times. This information can then be returned to the user in -the last-req field (see section 5.2). - -Other frequently changing information that can be maintained is the latest -expiration time for any tickets that have been issued using each key. This -field would be used to indicate how long old keys must remain valid to allow -the continued use of outstanding tickets. - -4.4. Site Constants - -The KDC implementation should have the following configurable constants or -options, to allow an administrator to make and enforce policy decisions: - - * The minimum supported lifetime (used to determine whether the - KDC_ERR_NEVER_VALID error should be returned). This constant should - reflect reasonable expectations of round-trip time to the KDC, - encryption/decryption time, and processing time by the client and - target server, and it should allow for a minimum 'useful' lifetime. - * The maximum allowable total (renewable) lifetime of a ticket - (renew_till - starttime). - * The maximum allowable lifetime of a ticket (endtime - starttime). - * Whether to allow the issue of tickets with empty address fields - (including the ability to specify that such tickets may only be issued - if the request specifies some authorization_data). - * Whether proxiable, forwardable, renewable or post-datable tickets are - to be issued. - -5. Message Specifications - -The following sections describe the exact contents and encoding of protocol -messages and objects. The ASN.1 base definitions are presented in the first -subsection. The remaining subsections specify the protocol objects (tickets -and authenticators) and messages. Specification of encryption and checksum -techniques, and the fields related to them, appear in section 6. - -5.1. ASN.1 Distinguished Encoding Representation - -All uses of ASN.1 in Kerberos shall use the Distinguished Encoding -Representation of the data elements as described in the X.509 specification, -section 8.7 [X509-88]. - -5.2. ASN.1 Base Definitions - -The following ASN.1 base definitions are used in the rest of this section. -Note that since the underscore character (_) is not permitted in ASN.1 -names, the hyphen (-) is used in its place for the purposes of ASN.1 names. - -Realm ::= GeneralString -PrincipalName ::= SEQUENCE { - name-type[0] INTEGER, - name-string[1] SEQUENCE OF GeneralString -} - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -Kerberos realms are encoded as GeneralStrings. Realms shall not contain a -character with the code 0 (the ASCII NUL). Most realms will usually consist -of several components separated by periods (.), in the style of Internet -Domain Names, or separated by slashes (/) in the style of X.500 names. -Acceptable forms for realm names are specified in section 7. A PrincipalName -is a typed sequence of components consisting of the following sub-fields: - -name-type - This field specifies the type of name that follows. Pre-defined values - for this field are specified in section 7.2. The name-type should be - treated as a hint. Ignoring the name type, no two names can be the same - (i.e. at least one of the components, or the realm, must be different). - This constraint may be eliminated in the future. -name-string - This field encodes a sequence of components that form a name, each - component encoded as a GeneralString. Taken together, a PrincipalName - and a Realm form a principal identifier. Most PrincipalNames will have - only a few components (typically one or two). - -KerberosTime ::= GeneralizedTime - -- Specifying UTC time zone (Z) - -The timestamps used in Kerberos are encoded as GeneralizedTimes. An encoding -shall specify the UTC time zone (Z) and shall not include any fractional -portions of the seconds. It further shall not include any separators. -Example: The only valid format for UTC time 6 minutes, 27 seconds after 9 pm -on 6 November 1985 is 19851106210627Z. - -HostAddress ::= SEQUENCE { - addr-type[0] INTEGER, - address[1] OCTET STRING -} - -HostAddresses ::= SEQUENCE OF HostAddress - -The host adddress encodings consists of two fields: - -addr-type - This field specifies the type of address that follows. Pre-defined - values for this field are specified in section 8.1. -address - This field encodes a single address of type addr-type. - -The two forms differ slightly. HostAddress contains exactly one address; -HostAddresses contains a sequence of possibly many addresses. - -AuthorizationData ::= SEQUENCE OF SEQUENCE { - ad-type[0] INTEGER, - ad-data[1] OCTET STRING -} - -ad-data - This field contains authorization data to be interpreted according to - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - the value of the corresponding ad-type field. -ad-type - This field specifies the format for the ad-data subfield. All negative - values are reserved for local use. Non-negative values are reserved for - registered use. - -Each sequence of type and data is refered to as an authorization element. -Elements may be application specific, however, there is a common set of -recursive elements that should be understood by all implementations. These -elements contain other elements embedded within them, and the interpretation -of the encapsulating element determines which of the embedded elements must -be interpreted, and which may be ignored. Definitions for these common -elements may be found in Appendix B. - -TicketExtensions ::= SEQUENCE OF SEQUENCE { - te-type[0] INTEGER, - te-data[1] OCTET STRING -} - - - -te-data - This field contains opaque data that must be caried with the ticket to - support extensions to the Kerberos protocol including but not limited - to some forms of inter-realm key exchange and plaintext authorization - data. See appendix C for some common uses of this field. -te-type - This field specifies the format for the te-data subfield. All negative - values are reserved for local use. Non-negative values are reserved for - registered use. - -APOptions ::= BIT STRING { - reserved(0), - use-session-key(1), - mutual-required(2) -} - -TicketFlags ::= BIT STRING { - reserved(0), - forwardable(1), - forwarded(2), - proxiable(3), - proxy(4), - may-postdate(5), - postdated(6), - invalid(7), - renewable(8), - initial(9), - pre-authent(10), - hw-authent(11), - transited-policy-checked(12), - ok-as-delegate(13) -} - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -KDCOptions ::= BIT STRING { - reserved(0), - forwardable(1), - forwarded(2), - proxiable(3), - proxy(4), - allow-postdate(5), - postdated(6), - unused7(7), - renewable(8), - unused9(9), - unused10(10), - unused11(11), - unused12(12), - unused13(13), - disable-transited-check(26), - renewable-ok(27), - enc-tkt-in-skey(28), - renew(30), - validate(31) -} - -ASN.1 Bit strings have a length and a value. When used in Kerberos for the -APOptions, TicketFlags, and KDCOptions, the length of the bit string on -generated values should be the smallest multiple of 32 bits needed to -include the highest order bit that is set (1), but in no case less than 32 -bits. Implementations should accept values of bit strings of any length and -treat the value of flags cooresponding to bits beyond the end of the bit -string as if the bit were reset (0). Comparisonof bit strings of different -length should treat the smaller string as if it were padded with zeros -beyond the high order bits to the length of the longer string[23]. - -LastReq ::= SEQUENCE OF SEQUENCE { - lr-type[0] INTEGER, - lr-value[1] KerberosTime -} - -lr-type - This field indicates how the following lr-value field is to be - interpreted. Negative values indicate that the information pertains - only to the responding server. Non-negative values pertain to all - servers for the realm. If the lr-type field is zero (0), then no - information is conveyed by the lr-value subfield. If the absolute value - of the lr-type field is one (1), then the lr-value subfield is the time - of last initial request for a TGT. If it is two (2), then the lr-value - subfield is the time of last initial request. If it is three (3), then - the lr-value subfield is the time of issue for the newest - ticket-granting ticket used. If it is four (4), then the lr-value - subfield is the time of the last renewal. If it is five (5), then the - lr-value subfield is the time of last request (of any type). -lr-value - This field contains the time of the last request. the time must be - interpreted according to the contents of the accompanying lr-type - subfield. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -See section 6 for the definitions of Checksum, ChecksumType, EncryptedData, -EncryptionKey, EncryptionType, and KeyType. - -5.3. Tickets and Authenticators - -This section describes the format and encryption parameters for tickets and -authenticators. When a ticket or authenticator is included in a protocol -message it is treated as an opaque object. - -5.3.1. Tickets - -A ticket is a record that helps a client authenticate to a service. A Ticket -contains the following information: - -Ticket ::= [APPLICATION 1] SEQUENCE { - tkt-vno[0] INTEGER, - realm[1] Realm, - sname[2] PrincipalName, - enc-part[3] EncryptedData, - extensions[4] TicketExtensions OPTIONAL -} - --- Encrypted part of ticket -EncTicketPart ::= [APPLICATION 3] SEQUENCE { - flags[0] TicketFlags, - key[1] EncryptionKey, - crealm[2] Realm, - cname[3] PrincipalName, - transited[4] TransitedEncoding, - authtime[5] KerberosTime, - starttime[6] KerberosTime OPTIONAL, - endtime[7] KerberosTime, - renew-till[8] KerberosTime OPTIONAL, - caddr[9] HostAddresses OPTIONAL, - authorization-data[10] AuthorizationData OPTIONAL -} --- encoded Transited field -TransitedEncoding ::= SEQUENCE { - tr-type[0] INTEGER, -- must be registered - contents[1] OCTET STRING -} - -The encoding of EncTicketPart is encrypted in the key shared by Kerberos and -the end server (the server's secret key). See section 6 for the format of -the ciphertext. - -tkt-vno - This field specifies the version number for the ticket format. This - document describes version number 5. -realm - This field specifies the realm that issued a ticket. It also serves to - identify the realm part of the server's principal identifier. Since a - Kerberos server can only issue tickets for servers within its realm, - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - the two will always be identical. -sname - This field specifies the name part of the server's identity. -enc-part - This field holds the encrypted encoding of the EncTicketPart sequence. -extensions - This optional field contains a sequence of extentions that may be used - to carry information that must be carried with the ticket to support - several extensions, including but not limited to plaintext - authorization data, tokens for exchanging inter-realm keys, and other - information that must be associated with a ticket for use by the - application server. See Appendix C for definitions of some common - extensions. - - Note that some older versions of Kerberos did not support this field. - Because this is an optional field it will not break older clients, but - older clients might strip this field from the ticket before sending it - to the application server. This limits the usefulness of this ticket - field to environments where the ticket will not be parsed and - reconstructed by these older Kerberos clients. - - If it is known that the client will strip this field from the ticket, - as an interim measure the KDC may append this field to the end of the - enc-part of the ticket and append a traler indicating the lenght of the - appended extensions field. (this paragraph is open for discussion, - including the form of the traler). -flags - This field indicates which of various options were used or requested - when the ticket was issued. It is a bit-field, where the selected - options are indicated by the bit being set (1), and the unselected - options and reserved fields being reset (0). Bit 0 is the most - significant bit. The encoding of the bits is specified in section 5.2. - The flags are described in more detail above in section 2. The meanings - of the flags are: - - Bit(s) Name Description - - 0 RESERVED - Reserved for future expansion of this - field. - - 1 FORWARDABLE - The FORWARDABLE flag is normally only - interpreted by the TGS, and can be - ignored by end servers. When set, this - flag tells the ticket-granting server - that it is OK to issue a new ticket- - granting ticket with a different network - address based on the presented ticket. - - 2 FORWARDED - When set, this flag indicates that the - ticket has either been forwarded or was - issued based on authentication involving - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - a forwarded ticket-granting ticket. - - 3 PROXIABLE - The PROXIABLE flag is normally only - interpreted by the TGS, and can be - ignored by end servers. The PROXIABLE - flag has an interpretation identical to - that of the FORWARDABLE flag, except - that the PROXIABLE flag tells the - ticket-granting server that only non- - ticket-granting tickets may be issued - with different network addresses. - - 4 PROXY - When set, this flag indicates that a - ticket is a proxy. - - 5 MAY-POSTDATE - The MAY-POSTDATE flag is normally only - interpreted by the TGS, and can be - ignored by end servers. This flag tells - the ticket-granting server that a post- - dated ticket may be issued based on this - ticket-granting ticket. - - 6 POSTDATED - This flag indicates that this ticket has - been postdated. The end-service can - check the authtime field to see when the - original authentication occurred. - - 7 INVALID - This flag indicates that a ticket is - invalid, and it must be validated by the - KDC before use. Application servers - must reject tickets which have this flag - set. - - 8 RENEWABLE - The RENEWABLE flag is normally only - interpreted by the TGS, and can usually - be ignored by end servers (some particu- - larly careful servers may wish to disal- - low renewable tickets). A renewable - ticket can be used to obtain a replace- - ment ticket that expires at a later - date. - - 9 INITIAL - This flag indicates that this ticket was - issued using the AS protocol, and not - issued based on a ticket-granting - ticket. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - 10 PRE-AUTHENT - This flag indicates that during initial - authentication, the client was authenti- - cated by the KDC before a ticket was - issued. The strength of the pre- - authentication method is not indicated, - but is acceptable to the KDC. - - 11 HW-AUTHENT - This flag indicates that the protocol - employed for initial authentication - required the use of hardware expected to - be possessed solely by the named client. - The hardware authentication method is - selected by the KDC and the strength of - the method is not indicated. - - 12 TRANSITED This flag indicates that the KDC for the - POLICY-CHECKED realm has checked the transited field - against a realm defined policy for - trusted certifiers. If this flag is - reset (0), then the application server - must check the transited field itself, - and if unable to do so it must reject - the authentication. If the flag is set - (1) then the application server may skip - its own validation of the transited - field, relying on the validation - performed by the KDC. At its option the - application server may still apply its - own validation based on a separate - policy for acceptance. - - 13 OK-AS-DELEGATE This flag indicates that the server (not - the client) specified in the ticket has - been determined by policy of the realm - to be a suitable recipient of - delegation. A client can use the - presence of this flag to help it make a - decision whether to delegate credentials - (either grant a proxy or a forwarded - ticket granting ticket) to this server. - The client is free to ignore the value - of this flag. When setting this flag, - an administrator should consider the - Security and placement of the server on - which the service will run, as well as - whether the service requires the use of - delegated credentials. - - 14 ANONYMOUS - This flag indicates that the principal - named in the ticket is a generic princi- - pal for the realm and does not identify - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - the individual using the ticket. The - purpose of the ticket is only to - securely distribute a session key, and - not to identify the user. Subsequent - requests using the same ticket and ses- - sion may be considered as originating - from the same user, but requests with - the same username but a different ticket - are likely to originate from different - users. - - 15-31 RESERVED - Reserved for future use. - -key - This field exists in the ticket and the KDC response and is used to - pass the session key from Kerberos to the application server and the - client. The field's encoding is described in section 6.2. -crealm - This field contains the name of the realm in which the client is - registered and in which initial authentication took place. -cname - This field contains the name part of the client's principal identifier. -transited - This field lists the names of the Kerberos realms that took part in - authenticating the user to whom this ticket was issued. It does not - specify the order in which the realms were transited. See section - 3.3.3.2 for details on how this field encodes the traversed realms. -authtime - This field indicates the time of initial authentication for the named - principal. It is the time of issue for the original ticket on which - this ticket is based. It is included in the ticket to provide - additional information to the end service, and to provide the necessary - information for implementation of a `hot list' service at the KDC. An - end service that is particularly paranoid could refuse to accept - tickets for which the initial authentication occurred "too far" in the - past. This field is also returned as part of the response from the KDC. - When returned as part of the response to initial authentication - (KRB_AS_REP), this is the current time on the Ker- beros server[24]. -starttime - This field in the ticket specifies the time after which the ticket is - valid. Together with endtime, this field specifies the life of the - ticket. If it is absent from the ticket, its value should be treated as - that of the authtime field. -endtime - This field contains the time after which the ticket will not be honored - (its expiration time). Note that individual services may place their - own limits on the life of a ticket and may reject tickets which have - not yet expired. As such, this is really an upper bound on the - expiration time for the ticket. -renew-till - This field is only present in tickets that have the RENEWABLE flag set - in the flags field. It indicates the maximum endtime that may be - included in a renewal. It can be thought of as the absolute expiration - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - time for the ticket, including all renewals. -caddr - This field in a ticket contains zero (if omitted) or more (if present) - host addresses. These are the addresses from which the ticket can be - used. If there are no addresses, the ticket can be used from any - location. The decision by the KDC to issue or by the end server to - accept zero-address tickets is a policy decision and is left to the - Kerberos and end-service administrators; they may refuse to issue or - accept such tickets. The suggested and default policy, however, is that - such tickets will only be issued or accepted when additional - information that can be used to restrict the use of the ticket is - included in the authorization_data field. Such a ticket is a - capability. - - Network addresses are included in the ticket to make it harder for an - attacker to use stolen credentials. Because the session key is not sent - over the network in cleartext, credentials can't be stolen simply by - listening to the network; an attacker has to gain access to the session - key (perhaps through operating system security breaches or a careless - user's unattended session) to make use of stolen tickets. - - It is important to note that the network address from which a - connection is received cannot be reliably determined. Even if it could - be, an attacker who has compromised the client's worksta- tion could - use the credentials from there. Including the network addresses only - makes it more difficult, not impossible, for an attacker to walk off - with stolen credentials and then use them from a "safe" location. -authorization-data - The authorization-data field is used to pass authorization data from - the principal on whose behalf a ticket was issued to the application - service. If no authorization data is included, this field will be left - out. Experience has shown that the name of this field is confusing, and - that a better name for this field would be restrictions. Unfortunately, - it is not possible to change the name of this field at this time. - - This field contains restrictions on any authority obtained on the basis - of authentication using the ticket. It is possible for any principal in - posession of credentials to add entries to the authorization data field - since these entries further restrict what can be done with the ticket. - Such additions can be made by specifying the additional entries when a - new ticket is obtained during the TGS exchange, or they may be added - during chained delegation using the authorization data field of the - authenticator. - - Because entries may be added to this field by the holder of - credentials, it is not allowable for the presence of an entry in the - authorization data field of a ticket to amplify the priveleges one - would obtain from using a ticket. - - The data in this field may be specific to the end service; the field - will contain the names of service specific objects, and the rights to - those objects. The format for this field is described in section 5.2. - Although Kerberos is not concerned with the format of the contents of - the sub-fields, it does carry type information (ad-type). - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - - By using the authorization_data field, a principal is able to issue a - proxy that is valid for a specific purpose. For example, a client - wishing to print a file can obtain a file server proxy to be passed to - the print server. By specifying the name of the file in the - authorization_data field, the file server knows that the print server - can only use the client's rights when accessing the particular file to - be printed. - - A separate service providing authorization or certifying group - membership may be built using the authorization-data field. In this - case, the entity granting authorization (not the authorized entity), - obtains a ticket in its own name (e.g. the ticket is issued in the name - of a privelege server), and this entity adds restrictions on its own - authority and delegates the restricted authority through a proxy to the - client. The client would then present this authorization credential to - the application server separately from the authentication exchange. - - Similarly, if one specifies the authorization-data field of a proxy and - leaves the host addresses blank, the resulting ticket and session key - can be treated as a capability. See [Neu93] for some suggested uses of - this field. - - The authorization-data field is optional and does not have to be - included in a ticket. - -5.3.2. Authenticators - -An authenticator is a record sent with a ticket to a server to certify the -client's knowledge of the encryption key in the ticket, to help the server -detect replays, and to help choose a "true session key" to use with the -particular session. The encoding is encrypted in the ticket's session key -shared by the client and the server: - --- Unencrypted authenticator -Authenticator ::= [APPLICATION 2] SEQUENCE { - authenticator-vno[0] INTEGER, - crealm[1] Realm, - cname[2] PrincipalName, - cksum[3] Checksum OPTIONAL, - cusec[4] INTEGER, - ctime[5] KerberosTime, - subkey[6] EncryptionKey OPTIONAL, - seq-number[7] INTEGER OPTIONAL, - authorization-data[8] AuthorizationData OPTIONAL -} - - -authenticator-vno - This field specifies the version number for the format of the - authenticator. This document specifies version 5. -crealm and cname - These fields are the same as those described for the ticket in section - 5.3.1. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -cksum - This field contains a checksum of the the applica- tion data that - accompanies the KRB_AP_REQ. -cusec - This field contains the microsecond part of the client's timestamp. Its - value (before encryption) ranges from 0 to 999999. It often appears - along with ctime. The two fields are used together to specify a - reasonably accurate timestamp. -ctime - This field contains the current time on the client's host. -subkey - This field contains the client's choice for an encryption key which is - to be used to protect this specific application session. Unless an - application specifies otherwise, if this field is left out the session - key from the ticket will be used. -seq-number - This optional field includes the initial sequence number to be used by - the KRB_PRIV or KRB_SAFE messages when sequence numbers are used to - detect replays (It may also be used by application specific messages). - When included in the authenticator this field specifies the initial - sequence number for messages from the client to the server. When - included in the AP-REP message, the initial sequence number is that for - messages from the server to the client. When used in KRB_PRIV or - KRB_SAFE messages, it is incremented by one after each message is sent. - - For sequence numbers to adequately support the detection of replays - they should be non-repeating, even across connection boundaries. The - initial sequence number should be random and uniformly distributed - across the full space of possible sequence numbers, so that it cannot - be guessed by an attacker and so that it and the successive sequence - numbers do not repeat other sequences. -authorization-data - This field is the same as described for the ticket in section 5.3.1. It - is optional and will only appear when additional restrictions are to be - placed on the use of a ticket, beyond those carried in the ticket - itself. - -5.4. Specifications for the AS and TGS exchanges - -This section specifies the format of the messages used in the exchange -between the client and the Kerberos server. The format of possible error -messages appears in section 5.9.1. - -5.4.1. KRB_KDC_REQ definition - -The KRB_KDC_REQ message has no type of its own. Instead, its type is one of -KRB_AS_REQ or KRB_TGS_REQ depending on whether the request is for an initial -ticket or an additional ticket. In either case, the message is sent from the -client to the Authentication Server to request credentials for a service. - -The message fields are: - -AS-REQ ::= [APPLICATION 10] KDC-REQ -TGS-REQ ::= [APPLICATION 12] KDC-REQ - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -KDC-REQ ::= SEQUENCE { - pvno[1] INTEGER, - msg-type[2] INTEGER, - padata[3] SEQUENCE OF PA-DATA OPTIONAL, - req-body[4] KDC-REQ-BODY -} - -PA-DATA ::= SEQUENCE { - padata-type[1] INTEGER, - padata-value[2] OCTET STRING, - -- might be encoded AP-REQ -} - -KDC-REQ-BODY ::= SEQUENCE { - kdc-options[0] KDCOptions, - cname[1] PrincipalName OPTIONAL, - -- Used only in AS-REQ - realm[2] Realm, -- Server's realm - -- Also client's in AS-REQ - sname[3] PrincipalName OPTIONAL, - from[4] KerberosTime OPTIONAL, - till[5] KerberosTime OPTIONAL, - rtime[6] KerberosTime OPTIONAL, - nonce[7] INTEGER, - etype[8] SEQUENCE OF INTEGER, - -- EncryptionType, - -- in preference order - addresses[9] HostAddresses OPTIONAL, - enc-authorization-data[10] EncryptedData OPTIONAL, - -- Encrypted AuthorizationData - -- encoding - additional-tickets[11] SEQUENCE OF Ticket OPTIONAL -} - -The fields in this message are: - -pvno - This field is included in each message, and specifies the protocol - version number. This document specifies protocol version 5. -msg-type - This field indicates the type of a protocol message. It will almost - always be the same as the application identifier associated with a - message. It is included to make the identifier more readily accessible - to the application. For the KDC-REQ message, this type will be - KRB_AS_REQ or KRB_TGS_REQ. -padata - The padata (pre-authentication data) field contains a sequence of - authentication information which may be needed before credentials can - be issued or decrypted. In the case of requests for additional tickets - (KRB_TGS_REQ), this field will include an element with padata-type of - PA-TGS-REQ and data of an authentication header (ticket-granting ticket - and authenticator). The checksum in the authenticator (which must be - collision-proof) is to be computed over the KDC-REQ-BODY encoding. In - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - most requests for initial authentication (KRB_AS_REQ) and most replies - (KDC-REP), the padata field will be left out. - - This field may also contain information needed by certain extensions to - the Kerberos protocol. For example, it might be used to initially - verify the identity of a client before any response is returned. This - is accomplished with a padata field with padata-type equal to - PA-ENC-TIMESTAMP and padata-value defined as follows: - - padata-type ::= PA-ENC-TIMESTAMP - padata-value ::= EncryptedData -- PA-ENC-TS-ENC - - PA-ENC-TS-ENC ::= SEQUENCE { - patimestamp[0] KerberosTime, -- client's time - pausec[1] INTEGER OPTIONAL - } - - with patimestamp containing the client's time and pausec containing the - microseconds which may be omitted if a client will not generate more - than one request per second. The ciphertext (padata-value) consists of - the PA-ENC-TS-ENC sequence, encrypted using the client's secret key. - - [use-specified-kvno item is here for discussion and may be removed] It - may also be used by the client to specify the version of a key that is - being used for accompanying preauthentication, and/or which should be - used to encrypt the reply from the KDC. - - PA-USE-SPECIFIED-KVNO ::= Integer - - The KDC should only accept and abide by the value of the - use-specified-kvno preauthentication data field when the specified key - is still valid and until use of a new key is confirmed. This situation - is likely to occur primarily during the period during which an updated - key is propagating to other KDC's in a realm. - - The padata field can also contain information needed to help the KDC or - the client select the key needed for generating or decrypting the - response. This form of the padata is useful for supporting the use of - certain token cards with Kerberos. The details of such extensions are - specified in separate documents. See [Pat92] for additional uses of - this field. -padata-type - The padata-type element of the padata field indicates the way that the - padata-value element is to be interpreted. Negative values of - padata-type are reserved for unregistered use; non-negative values are - used for a registered interpretation of the element type. -req-body - This field is a placeholder delimiting the extent of the remaining - fields. If a checksum is to be calculated over the request, it is - calculated over an encoding of the KDC-REQ-BODY sequence which is - enclosed within the req-body field. -kdc-options - This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to the - KDC and indicates the flags that the client wants set on the tickets as - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - well as other information that is to modify the behavior of the KDC. - Where appropriate, the name of an option may be the same as the flag - that is set by that option. Although in most case, the bit in the - options field will be the same as that in the flags field, this is not - guaranteed, so it is not acceptable to simply copy the options field to - the flags field. There are various checks that must be made before - honoring an option anyway. - - The kdc_options field is a bit-field, where the selected options are - indicated by the bit being set (1), and the unselected options and - reserved fields being reset (0). The encoding of the bits is specified - in section 5.2. The options are described in more detail above in - section 2. The meanings of the options are: - - Bit(s) Name Description - 0 RESERVED - Reserved for future expansion of this - field. - - 1 FORWARDABLE - The FORWARDABLE option indicates that - the ticket to be issued is to have its - forwardable flag set. It may only be - set on the initial request, or in a sub- - sequent request if the ticket-granting - ticket on which it is based is also for- - wardable. - - 2 FORWARDED - The FORWARDED option is only specified - in a request to the ticket-granting - server and will only be honored if the - ticket-granting ticket in the request - has its FORWARDABLE bit set. This - option indicates that this is a request - for forwarding. The address(es) of the - host from which the resulting ticket is - to be valid are included in the - addresses field of the request. - - 3 PROXIABLE - The PROXIABLE option indicates that the - ticket to be issued is to have its prox- - iable flag set. It may only be set on - the initial request, or in a subsequent - request if the ticket-granting ticket on - which it is based is also proxiable. - - 4 PROXY - The PROXY option indicates that this is - a request for a proxy. This option will - only be honored if the ticket-granting - ticket in the request has its PROXIABLE - bit set. The address(es) of the host - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - from which the resulting ticket is to be - valid are included in the addresses - field of the request. - - 5 ALLOW-POSTDATE - The ALLOW-POSTDATE option indicates that - the ticket to be issued is to have its - MAY-POSTDATE flag set. It may only be - set on the initial request, or in a sub- - sequent request if the ticket-granting - ticket on which it is based also has its - MAY-POSTDATE flag set. - - 6 POSTDATED - The POSTDATED option indicates that this - is a request for a postdated ticket. - This option will only be honored if the - ticket-granting ticket on which it is - based has its MAY-POSTDATE flag set. - The resulting ticket will also have its - INVALID flag set, and that flag may be - reset by a subsequent request to the KDC - after the starttime in the ticket has - been reached. - - 7 UNUSED - This option is presently unused. - - 8 RENEWABLE - The RENEWABLE option indicates that the - ticket to be issued is to have its - RENEWABLE flag set. It may only be set - on the initial request, or when the - ticket-granting ticket on which the - request is based is also renewable. If - this option is requested, then the rtime - field in the request contains the - desired absolute expiration time for the - ticket. - - 9-13 UNUSED - These options are presently unused. - - 14 REQUEST-ANONYMOUS - The REQUEST-ANONYMOUS option indicates - that the ticket to be issued is not to - identify the user to which it was - issued. Instead, the principal identif- - ier is to be generic, as specified by - the policy of the realm (e.g. usually - anonymous@realm). The purpose of the - ticket is only to securely distribute a - session key, and not to identify the - user. The ANONYMOUS flag on the ticket - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - to be returned should be set. If the - local realms policy does not permit - anonymous credentials, the request is to - be rejected. - - 15-25 RESERVED - Reserved for future use. - - 26 DISABLE-TRANSITED-CHECK - By default the KDC will check the - transited field of a ticket-granting- - ticket against the policy of the local - realm before it will issue derivative - tickets based on the ticket granting - ticket. If this flag is set in the - request, checking of the transited field - is disabled. Tickets issued without the - performance of this check will be noted - by the reset (0) value of the - TRANSITED-POLICY-CHECKED flag, - indicating to the application server - that the tranisted field must be checked - locally. KDC's are encouraged but not - required to honor the - DISABLE-TRANSITED-CHECK option. - - 27 RENEWABLE-OK - The RENEWABLE-OK option indicates that a - renewable ticket will be acceptable if a - ticket with the requested life cannot - otherwise be provided. If a ticket with - the requested life cannot be provided, - then a renewable ticket may be issued - with a renew-till equal to the the - requested endtime. The value of the - renew-till field may still be limited by - local limits, or limits selected by the - individual principal or server. - - 28 ENC-TKT-IN-SKEY - This option is used only by the ticket- - granting service. The ENC-TKT-IN-SKEY - option indicates that the ticket for the - end server is to be encrypted in the - session key from the additional ticket- - granting ticket provided. - - 29 RESERVED - Reserved for future use. - - 30 RENEW - This option is used only by the ticket- - granting service. The RENEW option - indicates that the present request is - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - for a renewal. The ticket provided is - encrypted in the secret key for the - server on which it is valid. This - option will only be honored if the - ticket to be renewed has its RENEWABLE - flag set and if the time in its renew- - till field has not passed. The ticket - to be renewed is passed in the padata - field as part of the authentication - header. - - 31 VALIDATE - This option is used only by the ticket- - granting service. The VALIDATE option - indicates that the request is to vali- - date a postdated ticket. It will only - be honored if the ticket presented is - postdated, presently has its INVALID - flag set, and would be otherwise usable - at this time. A ticket cannot be vali- - dated before its starttime. The ticket - presented for validation is encrypted in - the key of the server for which it is - valid and is passed in the padata field - as part of the authentication header. - -cname and sname - These fields are the same as those described for the ticket in section - 5.3.1. sname may only be absent when the ENC-TKT-IN-SKEY option is - specified. If absent, the name of the server is taken from the name of - the client in the ticket passed as additional-tickets. -enc-authorization-data - The enc-authorization-data, if present (and it can only be present in - the TGS_REQ form), is an encoding of the desired authorization-data - encrypted under the sub-session key if present in the Authenticator, or - alternatively from the session key in the ticket-granting ticket, both - from the padata field in the KRB_AP_REQ. -realm - This field specifies the realm part of the server's principal - identifier. In the AS exchange, this is also the realm part of the - client's principal identifier. -from - This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket - requests when the requested ticket is to be postdated. It specifies the - desired start time for the requested ticket. If this field is omitted - then the KDC should use the current time instead. -till - This field contains the expiration date requested by the client in a - ticket request. It is optional and if omitted the requested ticket is - to have the maximum endtime permitted according to KDC policy for the - parties to the authentication exchange as limited by expiration date of - the ticket granting ticket or other preauthentication credentials. -rtime - This field is the requested renew-till time sent from a client to the - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - KDC in a ticket request. It is optional. -nonce - This field is part of the KDC request and response. It it intended to - hold a random number generated by the client. If the same number is - included in the encrypted response from the KDC, it provides evidence - that the response is fresh and has not been replayed by an attacker. - Nonces must never be re-used. Ideally, it should be generated randomly, - but if the correct time is known, it may suffice[25]. -etype - This field specifies the desired encryption algorithm to be used in the - response. -addresses - This field is included in the initial request for tickets, and - optionally included in requests for additional tickets from the - ticket-granting server. It specifies the addresses from which the - requested ticket is to be valid. Normally it includes the addresses for - the client's host. If a proxy is requested, this field will contain - other addresses. The contents of this field are usually copied by the - KDC into the caddr field of the resulting ticket. -additional-tickets - Additional tickets may be optionally included in a request to the - ticket-granting server. If the ENC-TKT-IN-SKEY option has been - specified, then the session key from the additional ticket will be used - in place of the server's key to encrypt the new ticket. If more than - one option which requires additional tickets has been specified, then - the additional tickets are used in the order specified by the ordering - of the options bits (see kdc-options, above). - -The application code will be either ten (10) or twelve (12) depending on -whether the request is for an initial ticket (AS-REQ) or for an additional -ticket (TGS-REQ). - -The optional fields (addresses, authorization-data and additional-tickets) -are only included if necessary to perform the operation specified in the -kdc-options field. - -It should be noted that in KRB_TGS_REQ, the protocol version number appears -twice and two different message types appear: the KRB_TGS_REQ message -contains these fields as does the authentication header (KRB_AP_REQ) that is -passed in the padata field. - -5.4.2. KRB_KDC_REP definition - -The KRB_KDC_REP message format is used for the reply from the KDC for either -an initial (AS) request or a subsequent (TGS) request. There is no message -type for KRB_KDC_REP. Instead, the type will be either KRB_AS_REP or -KRB_TGS_REP. The key used to encrypt the ciphertext part of the reply -depends on the message type. For KRB_AS_REP, the ciphertext is encrypted in -the client's secret key, and the client's key version number is included in -the key version number for the encrypted data. For KRB_TGS_REP, the -ciphertext is encrypted in the sub-session key from the Authenticator, or if -absent, the session key from the ticket-granting ticket used in the request. -In that case, no version number will be present in the EncryptedData -sequence. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -The KRB_KDC_REP message contains the following fields: - -AS-REP ::= [APPLICATION 11] KDC-REP -TGS-REP ::= [APPLICATION 13] KDC-REP - -KDC-REP ::= SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - padata[2] SEQUENCE OF PA-DATA OPTIONAL, - crealm[3] Realm, - cname[4] PrincipalName, - ticket[5] Ticket, - enc-part[6] EncryptedData -} - -EncASRepPart ::= [APPLICATION 25[27]] EncKDCRepPart -EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart - -EncKDCRepPart ::= SEQUENCE { - key[0] EncryptionKey, - last-req[1] LastReq, - nonce[2] INTEGER, - key-expiration[3] KerberosTime OPTIONAL, - flags[4] TicketFlags, - authtime[5] KerberosTime, - starttime[6] KerberosTime OPTIONAL, - endtime[7] KerberosTime, - renew-till[8] KerberosTime OPTIONAL, - srealm[9] Realm, - sname[10] PrincipalName, - caddr[11] HostAddresses OPTIONAL -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is either - KRB_AS_REP or KRB_TGS_REP. -padata - This field is described in detail in section 5.4.1. One possible use - for this field is to encode an alternate "mix-in" string to be used - with a string-to-key algorithm (such as is described in section 6.3.2). - This ability is useful to ease transitions if a realm name needs to - change (e.g. when a company is acquired); in such a case all existing - password-derived entries in the KDC database would be flagged as - needing a special mix-in string until the next password change. -crealm, cname, srealm and sname - These fields are the same as those described for the ticket in section - 5.3.1. -ticket - The newly-issued ticket, from section 5.3.1. -enc-part - This field is a place holder for the ciphertext and related information - that forms the encrypted part of a message. The description of the - encrypted part of the message follows each appearance of this field. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - The encrypted part is encoded as described in section 6.1. -key - This field is the same as described for the ticket in section 5.3.1. -last-req - This field is returned by the KDC and specifies the time(s) of the last - request by a principal. Depending on what information is available, - this might be the last time that a request for a ticket-granting ticket - was made, or the last time that a request based on a ticket-granting - ticket was successful. It also might cover all servers for a realm, or - just the particular server. Some implementations may display this - information to the user to aid in discovering unauthorized use of one's - identity. It is similar in spirit to the last login time displayed when - logging into timesharing systems. -nonce - This field is described above in section 5.4.1. -key-expiration - The key-expiration field is part of the response from the KDC and - specifies the time that the client's secret key is due to expire. The - expiration might be the result of password aging or an account - expiration. This field will usually be left out of the TGS reply since - the response to the TGS request is encrypted in a session key and no - client information need be retrieved from the KDC database. It is up to - the application client (usually the login program) to take appropriate - action (such as notifying the user) if the expiration time is imminent. -flags, authtime, starttime, endtime, renew-till and caddr - These fields are duplicates of those found in the encrypted portion of - the attached ticket (see section 5.3.1), provided so the client may - verify they match the intended request and to assist in proper ticket - caching. If the message is of type KRB_TGS_REP, the caddr field will - only be filled in if the request was for a proxy or forwarded ticket, - or if the user is substituting a subset of the addresses from the - ticket granting ticket. If the client-requested addresses are not - present or not used, then the addresses contained in the ticket will be - the same as those included in the ticket-granting ticket. - -5.5. Client/Server (CS) message specifications - -This section specifies the format of the messages used for the -authentication of the client to the application server. - -5.5.1. KRB_AP_REQ definition - -The KRB_AP_REQ message contains the Kerberos protocol version number, the -message type KRB_AP_REQ, an options field to indicate any options in use, -and the ticket and authenticator themselves. The KRB_AP_REQ message is often -referred to as the 'authentication header'. - -AP-REQ ::= [APPLICATION 14] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - ap-options[2] APOptions, - ticket[3] Ticket, - authenticator[4] EncryptedData -} - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -APOptions ::= BIT STRING { - reserved(0), - use-session-key(1), - mutual-required(2) -} - - - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_AP_REQ. -ap-options - This field appears in the application request (KRB_AP_REQ) and affects - the way the request is processed. It is a bit-field, where the selected - options are indicated by the bit being set (1), and the unselected - options and reserved fields being reset (0). The encoding of the bits - is specified in section 5.2. The meanings of the options are: - - Bit(s) Name Description - 0 RESERVED - Reserved for future expansion of this - field. - - 1 USE-SESSION-KEY - The USE-SESSION-KEY option indicates - that the ticket the client is presenting - to a server is encrypted in the session - key from the server's ticket-granting - ticket. When this option is not speci- - fied, the ticket is encrypted in the - server's secret key. - - 2 MUTUAL-REQUIRED - The MUTUAL-REQUIRED option tells the - server that the client requires mutual - authentication, and that it must respond - with a KRB_AP_REP message. - - 3-31 RESERVED - Reserved for future use. -ticket - This field is a ticket authenticating the client to the server. -authenticator - This contains the authenticator, which includes the client's choice of - a subkey. Its encoding is described in section 5.3.2. - -5.5.2. KRB_AP_REP definition - -The KRB_AP_REP message contains the Kerberos protocol version number, the -message type, and an encrypted time- stamp. The message is sent in in -response to an application request (KRB_AP_REQ) where the mutual -authentication option has been selected in the ap-options field. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -AP-REP ::= [APPLICATION 15] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - enc-part[2] EncryptedData -} - -EncAPRepPart ::= [APPLICATION 27[29]] SEQUENCE { - ctime[0] KerberosTime, - cusec[1] INTEGER, - subkey[2] EncryptionKey OPTIONAL, - seq-number[3] INTEGER OPTIONAL -} - -The encoded EncAPRepPart is encrypted in the shared session key of the -ticket. The optional subkey field can be used in an application-arranged -negotiation to choose a per association session key. - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_AP_REP. -enc-part - This field is described above in section 5.4.2. -ctime - This field contains the current time on the client's host. -cusec - This field contains the microsecond part of the client's timestamp. -subkey - This field contains an encryption key which is to be used to protect - this specific application session. See section 3.2.6 for specifics on - how this field is used to negotiate a key. Unless an application - specifies otherwise, if this field is left out, the sub-session key - from the authenticator, or if also left out, the session key from the - ticket will be used. - -5.5.3. Error message reply - -If an error occurs while processing the application request, the KRB_ERROR -message will be sent in response. See section 5.9.1 for the format of the -error message. The cname and crealm fields may be left out if the server -cannot determine their appropriate values from the corresponding KRB_AP_REQ -message. If the authenticator was decipherable, the ctime and cusec fields -will contain the values from it. - -5.6. KRB_SAFE message specification - -This section specifies the format of a message that can be used by either -side (client or server) of an application to send a tamper-proof message to -its peer. It presumes that a session key has previously been exchanged (for -example, by using the KRB_AP_REQ/KRB_AP_REP messages). - -5.6.1. KRB_SAFE definition - -The KRB_SAFE message contains user data along with a collision-proof -checksum keyed with the last encryption key negotiated via subkeys, or the - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -session key if no negotiation has occured. The message fields are: - -KRB-SAFE ::= [APPLICATION 20] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - safe-body[2] KRB-SAFE-BODY, - cksum[3] Checksum -} - -KRB-SAFE-BODY ::= SEQUENCE { - user-data[0] OCTET STRING, - timestamp[1] KerberosTime OPTIONAL, - usec[2] INTEGER OPTIONAL, - seq-number[3] INTEGER OPTIONAL, - s-address[4] HostAddress OPTIONAL, - r-address[5] HostAddress OPTIONAL -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_SAFE. -safe-body - This field is a placeholder for the body of the KRB-SAFE message. It is - to be encoded separately and then have the checksum computed over it, - for use in the cksum field. -cksum - This field contains the checksum of the application data. Checksum - details are described in section 6.4. The checksum is computed over the - encoding of the KRB-SAFE-BODY sequence. -user-data - This field is part of the KRB_SAFE and KRB_PRIV messages and contain - the application specific data that is being passed from the sender to - the recipient. -timestamp - This field is part of the KRB_SAFE and KRB_PRIV messages. Its contents - are the current time as known by the sender of the message. By checking - the timestamp, the recipient of the message is able to make sure that - it was recently generated, and is not a replay. -usec - This field is part of the KRB_SAFE and KRB_PRIV headers. It contains - the microsecond part of the timestamp. -seq-number - This field is described above in section 5.3.2. -s-address - This field specifies the address in use by the sender of the message. -r-address - This field specifies the address in use by the recipient of the - message. It may be omitted for some uses (such as broadcast protocols), - but the recipient may arbitrarily reject such messages. This field - along with s-address can be used to help detect messages which have - been incorrectly or maliciously delivered to the wrong recipient. - -5.7. KRB_PRIV message specification - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -This section specifies the format of a message that can be used by either -side (client or server) of an application to securely and privately send a -message to its peer. It presumes that a session key has previously been -exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages). - -5.7.1. KRB_PRIV definition - -The KRB_PRIV message contains user data encrypted in the Session Key. The -message fields are: - -KRB-PRIV ::= [APPLICATION 21] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - enc-part[3] EncryptedData -} - -EncKrbPrivPart ::= [APPLICATION 28[31]] SEQUENCE { - user-data[0] OCTET STRING, - timestamp[1] KerberosTime OPTIONAL, - usec[2] INTEGER OPTIONAL, - seq-number[3] INTEGER OPTIONAL, - s-address[4] HostAddress OPTIONAL, -- sender's addr - r-address[5] HostAddress OPTIONAL -- recip's addr -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_PRIV. -enc-part - This field holds an encoding of the EncKrbPrivPart sequence encrypted - under the session key[32]. This encrypted encoding is used for the - enc-part field of the KRB-PRIV message. See section 6 for the format of - the ciphertext. -user-data, timestamp, usec, s-address and r-address - These fields are described above in section 5.6.1. -seq-number - This field is described above in section 5.3.2. - -5.8. KRB_CRED message specification - -This section specifies the format of a message that can be used to send -Kerberos credentials from one principal to another. It is presented here to -encourage a common mechanism to be used by applications when forwarding -tickets or providing proxies to subordinate servers. It presumes that a -session key has already been exchanged perhaps by using the -KRB_AP_REQ/KRB_AP_REP messages. - -5.8.1. KRB_CRED definition - -The KRB_CRED message contains a sequence of tickets to be sent and -information needed to use the tickets, including the session key from each. -The information needed to use the tickets is encrypted under an encryption -key previously exchanged or transferred alongside the KRB_CRED message. The -message fields are: - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -KRB-CRED ::= [APPLICATION 22] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, -- KRB_CRED - tickets[2] SEQUENCE OF Ticket, - enc-part[3] EncryptedData -} - -EncKrbCredPart ::= [APPLICATION 29] SEQUENCE { - ticket-info[0] SEQUENCE OF KrbCredInfo, - nonce[1] INTEGER OPTIONAL, - timestamp[2] KerberosTime OPTIONAL, - usec[3] INTEGER OPTIONAL, - s-address[4] HostAddress OPTIONAL, - r-address[5] HostAddress OPTIONAL -} - -KrbCredInfo ::= SEQUENCE { - key[0] EncryptionKey, - prealm[1] Realm OPTIONAL, - pname[2] PrincipalName OPTIONAL, - flags[3] TicketFlags OPTIONAL, - authtime[4] KerberosTime OPTIONAL, - starttime[5] KerberosTime OPTIONAL, - endtime[6] KerberosTime OPTIONAL - renew-till[7] KerberosTime OPTIONAL, - srealm[8] Realm OPTIONAL, - sname[9] PrincipalName OPTIONAL, - caddr[10] HostAddresses OPTIONAL -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_CRED. -tickets - These are the tickets obtained from the KDC specifically for use by the - intended recipient. Successive tickets are paired with the - corresponding KrbCredInfo sequence from the enc-part of the KRB-CRED - message. -enc-part - This field holds an encoding of the EncKrbCredPart sequence encrypted - under the session key shared between the sender and the intended - recipient. This encrypted encoding is used for the enc-part field of - the KRB-CRED message. See section 6 for the format of the ciphertext. -nonce - If practical, an application may require the inclusion of a nonce - generated by the recipient of the message. If the same value is - included as the nonce in the message, it provides evidence that the - message is fresh and has not been replayed by an attacker. A nonce must - never be re-used; it should be generated randomly by the recipient of - the message and provided to the sender of the message in an application - specific manner. -timestamp and usec - These fields specify the time that the KRB-CRED message was generated. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - The time is used to provide assurance that the message is fresh. -s-address and r-address - These fields are described above in section 5.6.1. They are used - optionally to provide additional assurance of the integrity of the - KRB-CRED message. -key - This field exists in the corresponding ticket passed by the KRB-CRED - message and is used to pass the session key from the sender to the - intended recipient. The field's encoding is described in section 6.2. - -The following fields are optional. If present, they can be associated with -the credentials in the remote ticket file. If left out, then it is assumed -that the recipient of the credentials already knows their value. - -prealm and pname - The name and realm of the delegated principal identity. -flags, authtime, starttime, endtime, renew-till, srealm, sname, and caddr - These fields contain the values of the correspond- ing fields from the - ticket found in the ticket field. Descriptions of the fields are - identical to the descriptions in the KDC-REP message. - -5.9. Error message specification - -This section specifies the format for the KRB_ERROR message. The fields -included in the message are intended to return as much information as -possible about an error. It is not expected that all the information -required by the fields will be available for all types of errors. If the -appropriate information is not available when the message is composed, the -corresponding field will be left out of the message. - -Note that since the KRB_ERROR message is not protected by any encryption, it -is quite possible for an intruder to synthesize or modify such a message. In -particular, this means that the client should not use any fields in this -message for security-critical purposes, such as setting a system clock or -generating a fresh authenticator. The message can be useful, however, for -advising a user on the reason for some failure. - -5.9.1. KRB_ERROR definition - -The KRB_ERROR message consists of the following fields: - -KRB-ERROR ::= [APPLICATION 30] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - ctime[2] KerberosTime OPTIONAL, - cusec[3] INTEGER OPTIONAL, - stime[4] KerberosTime, - susec[5] INTEGER, - error-code[6] INTEGER, - crealm[7] Realm OPTIONAL, - cname[8] PrincipalName OPTIONAL, - realm[9] Realm, -- Correct realm - sname[10] PrincipalName, -- Correct name - e-text[11] GeneralString OPTIONAL, - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - e-data[12] OCTET STRING OPTIONAL, - e-cksum[13] Checksum OPTIONAL, - e-typed-data[14] SEQUENCE of ETypedData OPTIONAL -} - -ETypedData ::= SEQUENCE { - e-data-type [1] INTEGER, - e-data-value [2] OCTET STRING, -} - - - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_ERROR. -ctime - This field is described above in section 5.4.1. -cusec - This field is described above in section 5.5.2. -stime - This field contains the current time on the server. It is of type - KerberosTime. -susec - This field contains the microsecond part of the server's timestamp. Its - value ranges from 0 to 999999. It appears along with stime. The two - fields are used in conjunction to specify a reasonably accurate - timestamp. -error-code - This field contains the error code returned by Kerberos or the server - when a request fails. To interpret the value of this field see the list - of error codes in section 8. Implementations are encouraged to provide - for national language support in the display of error messages. -crealm, cname, srealm and sname - These fields are described above in section 5.3.1. -e-text - This field contains additional text to help explain the error code - associated with the failed request (for example, it might include a - principal name which was unknown). -e-data - This field contains additional data about the error for use by the - application to help it recover from or handle the error. If the - errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will - contain an encoding of a sequence of padata fields, each corresponding - to an acceptable pre-authentication method and optionally containing - data for the method: - - METHOD-DATA ::= SEQUENCE of PA-DATA - - If the error-code is KRB_AP_ERR_METHOD, then the e-data field will - contain an encoding of the following sequence: - - METHOD-DATA ::= SEQUENCE { - method-type[0] INTEGER, - method-data[1] OCTET STRING OPTIONAL - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - } - - method-type will indicate the required alternate method; method-data - will contain any required additional information. -e-cksum - This field contains an optional checksum for the KRB-ERROR message. The - checksum is calculated over the Kerberos ASN.1 encoding of the - KRB-ERROR message with the checksum absent. The checksum is then added - to the KRB-ERROR structure and the message is re-encoded. The Checksum - should be calculated using the session key from the ticket granting - ticket or service ticket, where available. If the error is in response - to a TGS or AP request, the checksum should be calculated uing the the - session key from the client's ticket. If the error is in response to an - AS request, then the checksum should be calulated using the client's - secret key ONLY if there has been suitable preauthentication to prove - knowledge of the secret key by the client[33]. If a checksum can not be - computed because the key to be used is not available, no checksum will - be included. -e-typed-data - [This field for discussion, may be deleted from final spec] This field - contains optional data that may be used to help the client recover from - the indicated error. [This could contain the METHOD-DATA specified - since I don't think anyone actually uses it yet. It could also contain - the PA-DATA sequence for the preauth required error if we had a clear - way to transition to the use of this field from the use of the untype - e-data field.] For example, this field may specify the key version of - the key used to verify preauthentication: - - e-data-type := 20 -- Key version number - e-data-value := Integer -- Key version number used to verify - preauthentication - -6. Encryption and Checksum Specifications - -The Kerberos protocols described in this document are designed to use stream -encryption ciphers, which can be simulated using commonly available block -encryption ciphers, such as the Data Encryption Standard, [DES77] in -conjunction with block chaining and checksum methods [DESM80]. Encryption is -used to prove the identities of the network entities participating in -message exchanges. The Key Distribution Center for each realm is trusted by -all principals registered in that realm to store a secret key in confidence. -Proof of knowledge of this secret key is used to verify the authenticity of -a principal. - -The KDC uses the principal's secret key (in the AS exchange) or a shared -session key (in the TGS exchange) to encrypt responses to ticket requests; -the ability to obtain the secret key or session key implies the knowledge of -the appropriate keys and the identity of the KDC. The ability of a principal -to decrypt the KDC response and present a Ticket and a properly formed -Authenticator (generated with the session key from the KDC response) to a -service verifies the identity of the principal; likewise the ability of the -service to extract the session key from the Ticket and prove its knowledge -thereof in a response verifies the identity of the service. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -The Kerberos protocols generally assume that the encryption used is secure -from cryptanalysis; however, in some cases, the order of fields in the -encrypted portions of messages are arranged to minimize the effects of -poorly chosen keys. It is still important to choose good keys. If keys are -derived from user-typed passwords, those passwords need to be well chosen to -make brute force attacks more difficult. Poorly chosen keys still make easy -targets for intruders. - -The following sections specify the encryption and checksum mechanisms -currently defined for Kerberos. The encodings, chaining, and padding -requirements for each are described. For encryption methods, it is often -desirable to place random information (often referred to as a confounder) at -the start of the message. The requirements for a confounder are specified -with each encryption mechanism. - -Some encryption systems use a block-chaining method to improve the the -security characteristics of the ciphertext. However, these chaining methods -often don't provide an integrity check upon decryption. Such systems (such -as DES in CBC mode) must be augmented with a checksum of the plain-text -which can be verified at decryption and used to detect any tampering or -damage. Such checksums should be good at detecting burst errors in the -input. If any damage is detected, the decryption routine is expected to -return an error indicating the failure of an integrity check. Each -encryption type is expected to provide and verify an appropriate checksum. -The specification of each encryption method sets out its checksum -requirements. - -Finally, where a key is to be derived from a user's password, an algorithm -for converting the password to a key of the appropriate type is included. It -is desirable for the string to key function to be one-way, and for the -mapping to be different in different realms. This is important because users -who are registered in more than one realm will often use the same password -in each, and it is desirable that an attacker compromising the Kerberos -server in one realm not obtain or derive the user's key in another. - -For an discussion of the integrity characteristics of the candidate -encryption and checksum methods considered for Kerberos, the the reader is -referred to [SG92]. - -6.1. Encryption Specifications - -The following ASN.1 definition describes all encrypted messages. The -enc-part field which appears in the unencrypted part of messages in section -5 is a sequence consisting of an encryption type, an optional key version -number, and the ciphertext. - -EncryptedData ::= SEQUENCE { - etype[0] INTEGER, -- EncryptionType - kvno[1] INTEGER OPTIONAL, - cipher[2] OCTET STRING -- ciphertext -} - - - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -etype - This field identifies which encryption algorithm was used to encipher - the cipher. Detailed specifications for selected encryption types - appear later in this section. -kvno - This field contains the version number of the key under which data is - encrypted. It is only present in messages encrypted under long lasting - keys, such as principals' secret keys. -cipher - This field contains the enciphered text, encoded as an OCTET STRING. - -The cipher field is generated by applying the specified encryption algorithm -to data composed of the message and algorithm-specific inputs. Encryption -mechanisms defined for use with Kerberos must take sufficient measures to -guarantee the integrity of the plaintext, and we recommend they also take -measures to protect against precomputed dictionary attacks. If the -encryption algorithm is not itself capable of doing so, the protections can -often be enhanced by adding a checksum and a confounder. - -The suggested format for the data to be encrypted includes a confounder, a -checksum, the encoded plaintext, and any necessary padding. The msg-seq -field contains the part of the protocol message described in section 5 which -is to be encrypted. The confounder, checksum, and padding are all untagged -and untyped, and their length is exactly sufficient to hold the appropriate -item. The type and length is implicit and specified by the particular -encryption type being used (etype). The format for the data to be encrypted -is described in the following diagram: - - +-----------+----------+-------------+-----+ - |confounder | check | msg-seq | pad | - +-----------+----------+-------------+-----+ - -The format cannot be described in ASN.1, but for those who prefer an -ASN.1-like notation: - -CipherText ::= ENCRYPTED SEQUENCE { - confounder[0] UNTAGGED[35] OCTET STRING(conf_length) OPTIONAL, - check[1] UNTAGGED OCTET STRING(checksum_length) OPTIONAL, - msg-seq[2] MsgSequence, - pad UNTAGGED OCTET STRING(pad_length) OPTIONAL -} - -One generates a random confounder of the appropriate length, placing it in -confounder; zeroes out check; calculates the appropriate checksum over -confounder, check, and msg-seq, placing the result in check; adds the -necessary padding; then encrypts using the specified encryption type and the -appropriate key. - -Unless otherwise specified, a definition of an encryption algorithm that -specifies a checksum, a length for the confounder field, or an octet -boundary for padding uses this ciphertext format[36]. Those fields which are -not specified will be omitted. - -In the interest of allowing all implementations using a particular - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -encryption type to communicate with all others using that type, the -specification of an encryption type defines any checksum that is needed as -part of the encryption process. If an alternative checksum is to be used, a -new encryption type must be defined. - -Some cryptosystems require additional information beyond the key and the -data to be encrypted. For example, DES, when used in cipher-block-chaining -mode, requires an initialization vector. If required, the description for -each encryption type must specify the source of such additional information. -6.2. Encryption Keys - -The sequence below shows the encoding of an encryption key: - - EncryptionKey ::= SEQUENCE { - keytype[0] INTEGER, - keyvalue[1] OCTET STRING - } - -keytype - This field specifies the type of encryption key that follows in the - keyvalue field. It will almost always correspond to the encryption - algorithm used to generate the EncryptedData, though more than one - algorithm may use the same type of key (the mapping is many to one). - This might happen, for example, if the encryption algorithm uses an - alternate checksum algorithm for an integrity check, or a different - chaining mechanism. -keyvalue - This field contains the key itself, encoded as an octet string. - -All negative values for the encryption key type are reserved for local use. -All non-negative values are reserved for officially assigned type fields and -interpreta- tions. - -6.3. Encryption Systems - -6.3.1. The NULL Encryption System (null) - -If no encryption is in use, the encryption system is said to be the NULL -encryption system. In the NULL encryption system there is no checksum, -confounder or padding. The ciphertext is simply the plaintext. The NULL Key -is used by the null encryption system and is zero octets in length, with -keytype zero (0). - -6.3.2. DES in CBC mode with a CRC-32 checksum (des-cbc-crc) - -The des-cbc-crc encryption mode encrypts information under the Data -Encryption Standard [DES77] using the cipher block chaining mode [DESM80]. A -CRC-32 checksum (described in ISO 3309 [ISO3309]) is applied to the -confounder and message sequence (msg-seq) and placed in the cksum field. DES -blocks are 8 bytes. As a result, the data to be encrypted (the concatenation -of confounder, checksum, and message) must be padded to an 8 byte boundary -before encryption. The details of the encryption of this data are identical -to those for the des-cbc-md5 encryption mode. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -Note that, since the CRC-32 checksum is not collision-proof, an attacker -could use a probabilistic chosen-plaintext attack to generate a valid -message even if a confounder is used [SG92]. The use of collision-proof -checksums is recommended for environments where such attacks represent a -significant threat. The use of the CRC-32 as the checksum for ticket or -authenticator is no longer mandated as an interoperability requirement for -Kerberos Version 5 Specification 1 (See section 9.1 for specific details). - -6.3.3. DES in CBC mode with an MD4 checksum (des-cbc-md4) - -The des-cbc-md4 encryption mode encrypts information under the Data -Encryption Standard [DES77] using the cipher block chaining mode [DESM80]. -An MD4 checksum (described in [MD492]) is applied to the confounder and -message sequence (msg-seq) and placed in the cksum field. DES blocks are 8 -bytes. As a result, the data to be encrypted (the concatenation of -confounder, checksum, and message) must be padded to an 8 byte boundary -before encryption. The details of the encryption of this data are identical -to those for the des-cbc-md5 encryption mode. - -6.3.4. DES in CBC mode with an MD5 checksum (des-cbc-md5) - -The des-cbc-md5 encryption mode encrypts information under the Data -Encryption Standard [DES77] using the cipher block chaining mode [DESM80]. -An MD5 checksum (described in [MD5-92].) is applied to the confounder and -message sequence (msg-seq) and placed in the cksum field. DES blocks are 8 -bytes. As a result, the data to be encrypted (the concatenation of -confounder, checksum, and message) must be padded to an 8 byte boundary -before encryption. - -Plaintext and DES ciphtertext are encoded as blocks of 8 octets which are -concatenated to make the 64-bit inputs for the DES algorithms. The first -octet supplies the 8 most significant bits (with the octet's MSbit used as -the DES input block's MSbit, etc.), the second octet the next 8 bits, ..., -and the eighth octet supplies the 8 least significant bits. - -Encryption under DES using cipher block chaining requires an additional -input in the form of an initialization vector. Unless otherwise specified, -zero should be used as the initialization vector. Kerberos' use of DES -requires an 8 octet confounder. - -The DES specifications identify some 'weak' and 'semi-weak' keys; those keys -shall not be used for encrypting messages for use in Kerberos. Additionally, -because of the way that keys are derived for the encryption of checksums, -keys shall not be used that yield 'weak' or 'semi-weak' keys when -eXclusive-ORed with the hexadecimal constant F0F0F0F0F0F0F0F0. - -A DES key is 8 octets of data, with keytype one (1). This consists of 56 -bits of key, and 8 parity bits (one per octet). The key is encoded as a -series of 8 octets written in MSB-first order. The bits within the key are -also encoded in MSB order. For example, if the encryption key is -(B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) where -B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the parity -bits, the first octet of the key would be B1,B2,...,B7,P1 (with B1 as the -MSbit). [See the FIPS 81 introduction for reference.] - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -String to key transformation - -To generate a DES key from a text string (password), the text string -normally must have the realm and each component of the principal's name -appended[37], then padded with ASCII nulls to an 8 byte boundary. This -string is then fan-folded and eXclusive-ORed with itself to form an 8 byte -DES key. The parity is corrected on the key, and it is used to generate a -DES CBC checksum on the initial string (with the realm and name appended). -Next, parity is corrected on the CBC checksum. If the result matches a -'weak' or 'semi-weak' key as described in the DES specification, it is -eXclusive-ORed with the constant 00000000000000F0. Finally, the result is -returned as the key. Pseudocode follows: - - string_to_key(string,realm,name) { - odd = 1; - s = string + realm; - for(each component in name) { - s = s + component; - } - tempkey = NULL; - pad(s); /* with nulls to 8 byte boundary */ - for(8byteblock in s) { - if(odd == 0) { - odd = 1; - reverse(8byteblock) - } - else odd = 0; - tempkey = tempkey XOR 8byteblock; - } - fixparity(tempkey); - key = DES-CBC-check(s,tempkey); - fixparity(key); - if(is_weak_key_key(key)) - key = key XOR 0xF0; - return(key); - } - -6.3.5. Triple DES EDE in outer CBC mode with an SHA1 check-sum -(des3-cbc-sha1) - -The des3-cbc-sha1 encryption encodes information using three Data Encryption -Standard transformations with three DES keys. The first key is used to -perform a DES ECB encryption on an eight-octet data block using the first -DES key, followed by a DES ECB decryption of the result using the second DES -key, and a DES ECB encryption of the result using the third DES key. Because -DES blocks are 8 bytes, the data to be encrypted (the concatenation of -confounder, checksum, and message) must first be padded to an 8 byte -boundary before encryption. To support the outer CBC mode, the input is -padded to an eight-octet boundary. The first 8 octets of the data to be -encrypted (the confounder) is exclusive-ored with an initialization vector -of zero and then ECB encrypted using triple DES as described above. -Subsequent blocks of 8 octets are exclusive-ored with the ciphertext -produced by the encryption on the previous block before ECB encryption. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -An HMAC-SHA1 checksum (described in [KBC96].) is applied to the confounder -and message sequence (msg-seq) and placed in the cksum field. - -Plaintext are encoded as blocks of 8 octets which are concatenated to make -the 64-bit inputs for the DES algorithms. The first octet supplies the 8 -most significant bits (with the octet's MSbit used as the DES input block's -MSbit, etc.), the second octet the next 8 bits, ..., and the eighth octet -supplies the 8 least significant bits. - -Encryption under Triple DES using cipher block chaining requires an -additional input in the form of an initialization vector. Unless otherwise -specified, zero should be used as the initialization vector. Kerberos' use -of DES requires an 8 octet confounder. - -The DES specifications identify some 'weak' and 'semi-weak' keys; those keys -shall not be used for encrypting messages for use in Kerberos. Additionally, -because of the way that keys are derived for the encryption of checksums, -keys shall not be used that yield 'weak' or 'semi-weak' keys when -eXclusive-ORed with the hexadecimal constant F0F0F0F0F0F0F0F0. - -A Triple DES key is 24 octets of data, with keytype seven (7). This consists -of 168 bits of key, and 24 parity bits (one per octet). The key is encoded -as a series of 24 octets written in MSB-first order, with the first 8 octets -treated as the first DES key, the second 8 octets as the second key, and the -third 8 octets the third DES key. The bits within each key are also encoded -in MSB order. For example, if the encryption key is -(B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) where -B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the parity -bits, the first octet of the key would be B1,B2,...,B7,P1 (with B1 as the -MSbit). [See the FIPS 81 introduction for reference.] - -Key derivation for specified operations (Horowitz) - -[Discussion is needed for this section, especially since it does not simply -derive key generation, but also specifies encryption using triple DES in a -manner that is different than the basic template that was specified for -single DES and similar systems] - -In the Kerberos protocol cryptographic keys are used in a number of places. -In order to minimize the effect of compromising a key, it is desirable to -use a different key in each of these places. Key derivation [Horowitz96] can -be used to construct different keys for each operation from the keys -transported on the network or derived from the password specified by the -user. - -For each place where a key is used in Kerberos, a ``key usage'' is specified -for that purpose. The key, key usage, and encryption/checksum type together -describe the transformation from plaintext to ciphertext. For backwards -compatibility, this key derivation is only specified here for encryption -methods based on triple DES. Encryption methods specified for use by -Kerberos in the future should specify the key derivation function to be -used. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -Kerberos requires that the ciphertext component of EncryptedData be -tamper-resistant as well as confidential. This implies encryption and -integrity functions, which must each use their own separate keys. So, for -each key usage, two keys must be generated, one for encryption (Ke), and one -for integrity (Ki): - - Ke = DK(protocol key, key usage | 0xAA) - Ki = DK(protocol key, key usage | 0x55) - -where the key usage is represented as a 32 bit integer in network byte -order. The ciphertest must be generated from the plaintext as follows: - - ciphertext = E(Ke, confounder | length | plaintext | padding) | - H(Ki, confounder | length | plaintext | padding) - -The confounder and padding are specific to the encryption algorithm E. - -When generating a checksum only, there is no need for a confounder or -padding. Again, a new key (Kc) must be used. Checksums must be generated -from the plaintext as follows: - - Kc = DK(protocol key, key usage | 0x99) - MAC = H(Kc, length | plaintext) - - -Note that each enctype is described by an encryption algorithm E and a keyed -hash algorithm H, and each checksum type is described by a keyed hash -algorithm H. HMAC, with an appropriate hash, is recommended for use as H. - -The key usage value will be taken from the following list of places where -keys are used in the Kerberos protocol, with key usage values and Kerberos -specification section numbers: - - 1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the - client key (section 5.4.1) - 2. AS-REP Ticket and TGS-REP Ticket (includes tgs session key or - application session key), encrypted with the service key - (section 5.4.2) - 3. AS-REP encrypted part (includes tgs session key or application - session key), encrypted with the client key (section 5.4.2) - - 4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs - session key (section 5.4.1) - 5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs - authenticator subkey (section 5.4.1) - 6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed - with the tgs session key (sections 5.3.2, 5.4.1) - 7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes tgs - authenticator subkey), encrypted with the tgs session key - (section 5.3.2) - 8. TGS-REP encrypted part (includes application session key), - encrypted with the tgs session key (section 5.4.2) - 9. TGS-REP encrypted part (includes application session key), - encrypted with the tgs authenticator subkey (section 5.4.2) - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - - 10. AP-REQ Authenticator cksum, keyed with the application session - key (section 5.3.2) - 11. AP-REQ Authenticator (includes application authenticator - subkey), encrypted with the application session key (section - 5.3.2) - 12. AP-REP encrypted part (includes application session subkey), - encrypted with the application session key (section 5.5.2) - - 13. KRB-PRIV encrypted part, encrypted with a key chosen by the - application (section 5.7.1) - 14. KRB-CRED encrypted part, encrypted with a key chosen by the - application (section 5.6.1) - 15. KRB-SAFE cksum, keyed with a key chosen by the application - (section 5.8.1) - - 16. Data which is defined in some specification outside of - Kerberos to be encrypted using Kerberos encryption type. - 17. Data which is defined in some specification outside of - Kerberos to be checksummed using Kerberos checksum type. - - 18. KRB-ERROR checksum (e-cksum in section 5.9.1) - 19. AD-KDCIssued checksum (ad-checksum in appendix B.1) - 20. Checksum for Mandatory Ticket Extensions (appendix B.6) - 21. Checksum in Authorization Data in Ticket Extensions (appendix B.7) - -String to key transformation - -To generate a DES key from a text string (password), the text string -normally must have the realm and each component of the principal's name -appended[38]. - -The input string (with any salt data appended to it) is n-folded into a 24 -octet (192 bit) string. To n-fold a number X, replicate the input value to a -length that is the least common multiple of n and the length of X. Before -each repetition, the input X is rotated to the right by 13 bit positions. -The successive n-bit chunks are added together using 1's-complement addition -(addition with end-around carry) to yield a n-bit result. (This -transformation was proposed by Richard Basch) - -Each successive set of 8 octets is taken as a DES key, and its parity is -adjusted in the same manner as previously described. If any of the three -sets of 8 octets match a 'weak' or 'semi-weak key as described in the DES -specification, that chunk is eXclusive-ORed with the hexadecimal constant -00000000000000F0. The resulting DES keys are then used in sequence to -perform a Triple-DES CBC encryption of the n-folded input string (appended -with any salt data), using a zero initial vector. Parity, weak, and -semi-weak keys are once again corrected and the result is returned as the 24 -octet key. - -Pseudocode follows: - - string_to_key(string,realm,name) { - s = string + realm; - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - for(each component in name) { - s = s + component; - } - tkey[24] = fold(s); - fixparity(tkey); - if(isweak(tkey[0-7])) tkey[0-7] = tkey[0-7] XOR 0xF0; - if(isweak(tkey[8-15])) tkey[8-15] = tkey[8-15] XOR 0xF0; - if(is_weak(tkey[16-23])) tkey[16-23] = tkey[16-23] XOR 0xF0; - key[24] = 3DES-CBC(data=fold(s),key=tkey,iv=0); - fixparity(key); - if(is_weak(key[0-7])) key[0-7] = key[0-7] XOR 0xF0; - if(is_weak(key[8-15])) key[8-15] = key[8-15] XOR 0xF0; - if(is_weak(key[16-23])) key[16-23] = key[16-23] XOR 0xF0; - return(key); - } - -6.4. Checksums - -The following is the ASN.1 definition used for a checksum: - - Checksum ::= SEQUENCE { - cksumtype[0] INTEGER, - checksum[1] OCTET STRING - } - -cksumtype - This field indicates the algorithm used to generate the accompanying - checksum. -checksum - This field contains the checksum itself, encoded as an octet string. - -Detailed specification of selected checksum types appear later in this -section. Negative values for the checksum type are reserved for local use. -All non-negative values are reserved for officially assigned type fields and -interpretations. - -Checksums used by Kerberos can be classified by two properties: whether they -are collision-proof, and whether they are keyed. It is infeasible to find -two plaintexts which generate the same checksum value for a collision-proof -checksum. A key is required to perturb or initialize the algorithm in a -keyed checksum. To prevent message-stream modification by an active -attacker, unkeyed checksums should only be used when the checksum and -message will be subsequently encrypted (e.g. the checksums defined as part -of the encryption algorithms covered earlier in this section). - -Collision-proof checksums can be made tamper-proof if the checksum value is -encrypted before inclusion in a message. In such cases, the composition of -the checksum and the encryption algorithm must be considered a separate -checksum algorithm (e.g. RSA-MD5 encrypted using DES is a new checksum -algorithm of type RSA-MD5-DES). For most keyed checksums, as well as for the -encrypted forms of unkeyed collision-proof checksums, Kerberos prepends a -confounder before the checksum is calculated. - -6.4.1. The CRC-32 Checksum (crc32) - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -The CRC-32 checksum calculates a checksum based on a cyclic redundancy check -as described in ISO 3309 [ISO3309]. The resulting checksum is four (4) -octets in length. The CRC-32 is neither keyed nor collision-proof. The use -of this checksum is not recommended. An attacker using a probabilistic -chosen-plaintext attack as described in [SG92] might be able to generate an -alternative message that satisfies the checksum. The use of collision-proof -checksums is recommended for environments where such attacks represent a -significant threat. - -6.4.2. The RSA MD4 Checksum (rsa-md4) - -The RSA-MD4 checksum calculates a checksum using the RSA MD4 algorithm -[MD4-92]. The algorithm takes as input an input message of arbitrary length -and produces as output a 128-bit (16 octet) checksum. RSA-MD4 is believed to -be collision-proof. - -6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4-des) - -The RSA-MD4-DES checksum calculates a keyed collision-proof checksum by -prepending an 8 octet confounder before the text, applying the RSA MD4 -checksum algorithm, and encrypting the confounder and the checksum using DES -in cipher-block-chaining (CBC) mode using a variant of the key, where the -variant is computed by eXclusive-ORing the key with the constant -F0F0F0F0F0F0F0F0[39]. The initialization vector should be zero. The -resulting checksum is 24 octets long (8 octets of which are redundant). This -checksum is tamper-proof and believed to be collision-proof. - -The DES specifications identify some weak keys' and 'semi-weak keys'; those -keys shall not be used for generating RSA-MD4 checksums for use in Kerberos. - -The format for the checksum is described in the follow- ing diagram: - -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ -| des-cbc(confounder + rsa-md4(confounder+msg),key=var(key),iv=0) | -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - -The format cannot be described in ASN.1, but for those who prefer an -ASN.1-like notation: - -rsa-md4-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE { - confounder[0] UNTAGGED OCTET STRING(8), - check[1] UNTAGGED OCTET STRING(16) -} - -6.4.4. The RSA MD5 Checksum (rsa-md5) - -The RSA-MD5 checksum calculates a checksum using the RSA MD5 algorithm. -[MD5-92]. The algorithm takes as input an input message of arbitrary length -and produces as output a 128-bit (16 octet) checksum. RSA-MD5 is believed to -be collision-proof. - -6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5-des) - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -The RSA-MD5-DES checksum calculates a keyed collision-proof checksum by -prepending an 8 octet confounder before the text, applying the RSA MD5 -checksum algorithm, and encrypting the confounder and the checksum using DES -in cipher-block-chaining (CBC) mode using a variant of the key, where the -variant is computed by eXclusive-ORing the key with the hexadecimal constant -F0F0F0F0F0F0F0F0. The initialization vector should be zero. The resulting -checksum is 24 octets long (8 octets of which are redundant). This checksum -is tamper-proof and believed to be collision-proof. - -The DES specifications identify some 'weak keys' and 'semi-weak keys'; those -keys shall not be used for encrypting RSA-MD5 checksums for use in Kerberos. - -The format for the checksum is described in the following diagram: - -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ -| des-cbc(confounder + rsa-md5(confounder+msg),key=var(key),iv=0) | -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - -The format cannot be described in ASN.1, but for those who prefer an -ASN.1-like notation: - -rsa-md5-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE { - confounder[0] UNTAGGED OCTET STRING(8), - check[1] UNTAGGED OCTET STRING(16) -} - -6.4.6. DES cipher-block chained checksum (des-mac) - -The DES-MAC checksum is computed by prepending an 8 octet confounder to the -plaintext, performing a DES CBC-mode encryption on the result using the key -and an initialization vector of zero, taking the last block of the -ciphertext, prepending the same confounder and encrypting the pair using DES -in cipher-block-chaining (CBC) mode using a a variant of the key, where the -variant is computed by eXclusive-ORing the key with the hexadecimal constant -F0F0F0F0F0F0F0F0. The initialization vector should be zero. The resulting -checksum is 128 bits (16 octets) long, 64 bits of which are redundant. This -checksum is tamper-proof and collision-proof. - -The format for the checksum is described in the following diagram: - -+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+ -| des-cbc(confounder + des-mac(conf+msg,iv=0,key),key=var(key),iv=0) | -+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+ - -The format cannot be described in ASN.1, but for those who prefer an -ASN.1-like notation: - -des-mac-checksum ::= ENCRYPTED UNTAGGED SEQUENCE { - confounder[0] UNTAGGED OCTET STRING(8), - check[1] UNTAGGED OCTET STRING(8) -} - -The DES specifications identify some 'weak' and 'semi-weak' keys; those keys -shall not be used for generating DES-MAC checksums for use in Kerberos, nor - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -shall a key be used whose variant is 'weak' or 'semi-weak'. - -6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative (rsa-md4-des-k) - -The RSA-MD4-DES-K checksum calculates a keyed collision-proof checksum by -applying the RSA MD4 checksum algorithm and encrypting the results using DES -in cipher-block-chaining (CBC) mode using a DES key as both key and -initialization vector. The resulting checksum is 16 octets long. This -checksum is tamper-proof and believed to be collision-proof. Note that this -checksum type is the old method for encoding the RSA-MD4-DES checksum and it -is no longer recommended. - -6.4.8. DES cipher-block chained checksum alternative (des-mac-k) - -The DES-MAC-K checksum is computed by performing a DES CBC-mode encryption -of the plaintext, and using the last block of the ciphertext as the checksum -value. It is keyed with an encryption key and an initialization vector; any -uses which do not specify an additional initialization vector will use the -key as both key and initialization vector. The resulting checksum is 64 bits -(8 octets) long. This checksum is tamper-proof and collision-proof. Note -that this checksum type is the old method for encoding the DES-MAC checksum -and it is no longer recommended. The DES specifications identify some 'weak -keys' and 'semi-weak keys'; those keys shall not be used for generating -DES-MAC checksums for use in Kerberos. - -7. Naming Constraints - -7.1. Realm Names - -Although realm names are encoded as GeneralStrings and although a realm can -technically select any name it chooses, interoperability across realm -boundaries requires agreement on how realm names are to be assigned, and -what information they imply. - -To enforce these conventions, each realm must conform to the conventions -itself, and it must require that any realms with which inter-realm keys are -shared also conform to the conventions and require the same from its -neighbors. - -Kerberos realm names are case sensitive. Realm names that differ only in the -case of the characters are not equivalent. There are presently four styles -of realm names: domain, X500, other, and reserved. Examples of each style -follow: - - domain: ATHENA.MIT.EDU (example) - X500: C=US/O=OSF (example) - other: NAMETYPE:rest/of.name=without-restrictions (example) - reserved: reserved, but will not conflict with above - -Domain names must look like domain names: they consist of components -separated by periods (.) and they contain neither colons (:) nor slashes -(/). Domain names must be converted to upper case when used as realm names. - -X.500 names contain an equal (=) and cannot contain a colon (:) before the - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -equal. The realm names for X.500 names will be string representations of the -names with components separated by slashes. Leading and trailing slashes -will not be included. - -Names that fall into the other category must begin with a prefix that -contains no equal (=) or period (.) and the prefix must be followed by a -colon (:) and the rest of the name. All prefixes must be assigned before -they may be used. Presently none are assigned. - -The reserved category includes strings which do not fall into the first -three categories. All names in this category are reserved. It is unlikely -that names will be assigned to this category unless there is a very strong -argument for not using the 'other' category. - -These rules guarantee that there will be no conflicts between the various -name styles. The following additional constraints apply to the assignment of -realm names in the domain and X.500 categories: the name of a realm for the -domain or X.500 formats must either be used by the organization owning (to -whom it was assigned) an Internet domain name or X.500 name, or in the case -that no such names are registered, authority to use a realm name may be -derived from the authority of the parent realm. For example, if there is no -domain name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can -authorize the creation of a realm with that name. - -This is acceptable because the organization to which the parent is assigned -is presumably the organization authorized to assign names to its children in -the X.500 and domain name systems as well. If the parent assigns a realm -name without also registering it in the domain name or X.500 hierarchy, it -is the parent's responsibility to make sure that there will not in the -future exists a name identical to the realm name of the child unless it is -assigned to the same entity as the realm name. - -7.2. Principal Names - -As was the case for realm names, conventions are needed to ensure that all -agree on what information is implied by a principal name. The name-type -field that is part of the principal name indicates the kind of information -implied by the name. The name-type should be treated as a hint. Ignoring the -name type, no two names can be the same (i.e. at least one of the -components, or the realm, must be different). The following name types are -defined: - - name-type value meaning - - NT-UNKNOWN 0 Name type not known - NT-PRINCIPAL 1 General principal name (e.g. username, or DCE principal) - NT-SRV-INST 2 Service and other unique instance (krbtgt) - NT-SRV-HST 3 Service with host name as instance (telnet, rcommands) - NT-SRV-XHST 4 Service with slash-separated host name components - NT-UID 5 Unique ID - NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 1779] - -When a name implies no information other than its uniqueness at a particular -time the name type PRINCIPAL should be used. The principal name type should - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -be used for users, and it might also be used for a unique server. If the -name is a unique machine generated ID that is guaranteed never to be -reassigned then the name type of UID should be used (note that it is -generally a bad idea to reassign names of any type since stale entries might -remain in access control lists). - -If the first component of a name identifies a service and the remaining -components identify an instance of the service in a server specified manner, -then the name type of SRV-INST should be used. An example of this name type -is the Kerberos ticket-granting service whose name has a first component of -krbtgt and a second component identifying the realm for which the ticket is -valid. - -If instance is a single component following the service name and the -instance identifies the host on which the server is running, then the name -type SRV-HST should be used. This type is typically used for Internet -services such as telnet and the Berkeley R commands. If the separate -components of the host name appear as successive components following the -name of the service, then the name type SRV-XHST should be used. This type -might be used to identify servers on hosts with X.500 names where the slash -(/) might otherwise be ambiguous. - -A name type of NT-X500-PRINCIPAL should be used when a name from an X.509 -certificiate is translated into a Kerberos name. The encoding of the X.509 -name as a Kerberos principal shall conform to the encoding rules specified -in RFC 1779. - -A name type of UNKNOWN should be used when the form of the name is not -known. When comparing names, a name of type UNKNOWN will match principals -authenticated with names of any type. A principal authenticated with a name -of type UNKNOWN, however, will only match other names of type UNKNOWN. - -Names of any type with an initial component of 'krbtgt' are reserved for the -Kerberos ticket granting service. See section 8.2.3 for the form of such -names. - -7.2.1. Name of server principals - -The principal identifier for a server on a host will generally be composed -of two parts: (1) the realm of the KDC with which the server is registered, -and (2) a two-component name of type NT-SRV-HST if the host name is an -Internet domain name or a multi-component name of type NT-SRV-XHST if the -name of the host is of a form such as X.500 that allows slash (/) -separators. The first component of the two- or multi-component name will -identify the service and the latter components will identify the host. Where -the name of the host is not case sensitive (for example, with Internet -domain names) the name of the host must be lower case. If specified by the -application protocol for services such as telnet and the Berkeley R commands -which run with system privileges, the first component may be the string -'host' instead of a service specific identifier. When a host has an official -name and one or more aliases, the official name of the host must be used -when constructing the name of the server principal. - -8. Constants and other defined values - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -8.1. Host address types - -All negative values for the host address type are reserved for local use. -All non-negative values are reserved for officially assigned type fields and -interpretations. - -The values of the types for the following addresses are chosen to match the -defined address family constants in the Berkeley Standard Distributions of -Unix. They can be found in with symbolic names AF_xxx (where xxx is an -abbreviation of the address family name). - -Internet (IPv4) Addresses - -Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in MSB -order. The type of IPv4 addresses is two (2). - -Internet (IPv6) Addresses - -IPv6 addresses are 128-bit (16-octet) quantities, encoded in MSB order. The -type of IPv6 addresses is twenty-four (24). [RFC1883] [RFC1884]. The -following addresses (see [RFC1884]) MUST not appear in any Kerberos packet: - - * the Unspecified Address - * the Loopback Address - * Link-Local addresses - -IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2. - -CHAOSnet addresses - -CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB order. -The type of CHAOSnet addresses is five (5). - -ISO addresses - -ISO addresses are variable-length. The type of ISO addresses is seven (7). - -Xerox Network Services (XNS) addresses - -XNS addresses are 48-bit (6-octet) quantities, encoded in MSB order. The -type of XNS addresses is six (6). - -AppleTalk Datagram Delivery Protocol (DDP) addresses - -AppleTalk DDP addresses consist of an 8-bit node number and a 16-bit network -number. The first octet of the address is the node number; the remaining two -octets encode the network number in MSB order. The type of AppleTalk DDP -addresses is sixteen (16). - -DECnet Phase IV addresses - -DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. The -type of DECnet Phase IV addresses is twelve (12). - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -8.2. KDC messages - -8.2.1. UDP/IP transport - -When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request using UDP -IP transport, the client shall send a UDP datagram containing only an -encoding of the request to port 88 (decimal) at the KDC's IP address; the -KDC will respond with a reply datagram containing only an encoding of the -reply message (either a KRB_ERROR or a KRB_KDC_REP) to the sending port at -the sender's IP address. Kerberos servers supporting IP transport must -accept UDP requests on port 88 (decimal). The response to a request made -through UDP/IP transport must also use UDP/IP transport. - -8.2.2. TCP/IP transport - -Kerberos servers (KDC's) must accept TCP requests on port 88 (decimal). When -the KRB_KDC_REQ message is sent to the KDC over a TCP stream, a new -connection will be established for each authentication exchange (request and -response). The KRB_KDC_REP or KRB_ERROR message will be returned to the -client on the same TCP stream that was established for the request. The -connection will be broken after the reply has been received (or upon -time-out). Care must be taken in managing TCP/IP connections with the KDC to -prevent denial of service attacks based on the number of TCP/IP connections -with the KDC that remain open. If multiple exchanges with the KDC are needed -for certain forms of preauthentication, multiple TCP connections will be -required. The response to a request made through TCP/IP transport must also -use TCP/IP transport. - -The first four octets of the TCP stream used to transmit the request request -will encode in network byte order the length of the request (KRB_KDC_REQ), -and the length will be followed by the request itself. The response will -similarly be preceeded by a 4 octet encoding in network byte order of the -length of the KRB_KDC_REP or the KRB_ERROR message and will be followed by -the KRB_KDC_REP or the KRB_ERROR response. - -8.2.3. OSI transport - -During authentication of an OSI client to an OSI server, the mutual -authentication of an OSI server to an OSI client, the transfer of -credentials from an OSI client to an OSI server, or during exchange of -private or integrity checked messages, Kerberos protocol messages may be -treated as opaque objects and the type of the authentication mechanism will -be: - -OBJECT IDENTIFIER ::= {iso (1), org(3), dod(6),internet(1), security(5),kerberosv5(2)} - -Depending on the situation, the opaque object will be an authentication -header (KRB_AP_REQ), an authentication reply (KRB_AP_REP), a safe message -(KRB_SAFE), a private message (KRB_PRIV), or a credentials message -(KRB_CRED). The opaque data contains an application code as specified in the -ASN.1 description for each message. The application code may be used by -Kerberos to determine the message type. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -8.2.3. Name of the TGS - -The principal identifier of the ticket-granting service shall be composed of -three parts: (1) the realm of the KDC issuing the TGS ticket (2) a two-part -name of type NT-SRV-INST, with the first part "krbtgt" and the second part -the name of the realm which will accept the ticket-granting ticket. For -example, a ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be -used to get tickets from the ATHENA.MIT.EDU KDC has a principal identifier -of "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A -ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be used to get -tickets from the MIT.EDU realm has a principal identifier of -"ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU") (name). - -8.3. Protocol constants and associated values - -The following tables list constants used in the protocol and defines their -meanings. - -Encryption type etype value block size minimum pad size confounder size -NULL 0 1 0 0 -des-cbc-crc 1 8 4 8 -des-cbc-md4 2 8 0 8 -des-cbc-md5 3 8 0 8 - 4 -des3-cbc-md5 5 8 0 8 - 6 -des3-cbc-sha1 7 8 0 8 -sign-dsa-generate 8 (pkinit) -encrypt-rsa-priv 9 (pkinit) -encrypt-rsa-pub 10 (pkinit) -rsa-pub-md5 11 (pkinit) -rsa-pub-sha1 12 (pkinit) -ENCTYPE_PK_CROSS 48 (reserved for pkcross) - 0x8003 - -Checksum type sumtype value checksum size -CRC32 1 4 -rsa-md4 2 16 -rsa-md4-des 3 24 -des-mac 4 16 -des-mac-k 5 8 -rsa-md4-des-k 6 16 -rsa-md5 7 16 -rsa-md5-des 8 24 -rsa-md5-des3 9 24 -hmac-sha1-des3 10 20 (I had this as 10, is it 12) - -padata type padata-type value - -PA-TGS-REQ 1 -PA-ENC-TIMESTAMP 2 -PA-PW-SALT 3 - 4 -PA-ENC-UNIX-TIME 5 - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -PA-SANDIA-SECUREID 6 -PA-SESAME 7 -PA-OSF-DCE 8 -PA-CYBERSAFE-SECUREID 9 -PA-AFS3-SALT 10 -PA-ETYPE-INFO 11 -SAM-CHALLENGE 12 (sam/otp) -SAM-RESPONSE 13 (sam/otp) -PA-PK-AS-REQ 14 (pkinit) -PA-PK-AS-REP 15 (pkinit) -PA-PK-AS-SIGN 16 (pkinit) -PA-PK-KEY-REQ 17 (pkinit) -PA-PK-KEY-REP 18 (pkinit) -PA-USE-SPECIFIED-KVNO 20 - -authorization data type ad-type value -AD-KDC-ISSUED 1 -AD-INTENDED-FOR-SERVER 2 -AD-INTENDED-FOR-APPLICATION-CLASS 3 -AD-IF-RELEVANT 4 -AD-OR 5 -AD-MANDATORY-TICKET-EXTENSIONS 6 -AD-IN-TICKET-EXTENSIONS 7 -reserved values 8-63 -OSF-DCE 64 -SESAME 65 - -Ticket Extension Types - -TE-TYPE-NULL 0 Null ticket extension -TE-TYPE-EXTERNAL-ADATA 1 Integrity protected authorization data - 2 TE-TYPE-PKCROSS-KDC (I have reservations) -TE-TYPE-PKCROSS-CLIENT 3 PKCROSS cross realm key ticket -TE-TYPE-CYBERSAFE-EXT 4 Assigned to CyberSafe Corp - 5 TE-TYPE-DEST-HOST (I have reservations) - -alternate authentication type method-type value -reserved values 0-63 -ATT-CHALLENGE-RESPONSE 64 - -transited encoding type tr-type value -DOMAIN-X500-COMPRESS 1 -reserved values all others - -Label Value Meaning or MIT code - -pvno 5 current Kerberos protocol version number - -message types - -KRB_AS_REQ 10 Request for initial authentication -KRB_AS_REP 11 Response to KRB_AS_REQ request -KRB_TGS_REQ 12 Request for authentication based on TGT -KRB_TGS_REP 13 Response to KRB_TGS_REQ request - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -KRB_AP_REQ 14 application request to server -KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL -KRB_SAFE 20 Safe (checksummed) application message -KRB_PRIV 21 Private (encrypted) application message -KRB_CRED 22 Private (encrypted) message to forward credentials -KRB_ERROR 30 Error response - -name types - -KRB_NT_UNKNOWN 0 Name type not known -KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or for users -KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt) -KRB_NT_SRV_HST 3 Service with host name as instance (telnet, rcommands) -KRB_NT_SRV_XHST 4 Service with host as remaining components -KRB_NT_UID 5 Unique ID -KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distingished name [RFC 1779] - -error codes - -KDC_ERR_NONE 0 No error -KDC_ERR_NAME_EXP 1 Client's entry in database has expired -KDC_ERR_SERVICE_EXP 2 Server's entry in database has expired -KDC_ERR_BAD_PVNO 3 Requested protocol version number not - supported -KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old master key -KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old master key -KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos database -KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos database -KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in database -KDC_ERR_NULL_KEY 9 The client or server has a null key -KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating -KDC_ERR_NEVER_VALID 11 Requested start time is later than end time -KDC_ERR_POLICY 12 KDC policy rejects request -KDC_ERR_BADOPTION 13 KDC cannot accommodate requested option -KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption type -KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum type -KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type -KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited type -KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been revoked -KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been revoked -KDC_ERR_TGT_REVOKED 20 TGT has been revoked -KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again later -KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again later -KDC_ERR_KEY_EXPIRED 23 Password has expired - change password - to reset -KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was invalid -KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-authenticationrequired [40] -KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket don't match -KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for user2user only -KDC_ERR_PATH_NOT_ACCPETED 28 KDC Policy rejects transited path -KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field failed -KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired -KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid -KRB_AP_ERR_REPEAT 34 Request is a replay -KRB_AP_ERR_NOT_US 35 The ticket isn't for us -KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't match - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -KRB_AP_ERR_SKEW 37 Clock skew too great -KRB_AP_ERR_BADADDR 38 Incorrect net address -KRB_AP_ERR_BADVERSION 39 Protocol version mismatch -KRB_AP_ERR_MSG_TYPE 40 Invalid msg type -KRB_AP_ERR_MODIFIED 41 Message stream modified -KRB_AP_ERR_BADORDER 42 Message out of order -KRB_AP_ERR_BADKEYVER 44 Specified version of key is not available -KRB_AP_ERR_NOKEY 45 Service key not available -KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed -KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction -KRB_AP_ERR_METHOD 48 Alternative authentication method required -KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in message -KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in message -KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited path -KRB_ERR_GENERIC 60 Generic error (description in e-text) -KRB_ERR_FIELD_TOOLONG 61 Field is too long for this implementation -KDC_ERROR_CLIENT_NOT_TRUSTED 62 (pkinit) -KDC_ERROR_KDC_NOT_TRUSTED 63 (pkinit) -KDC_ERROR_INVALID_SIG 64 (pkinit) -KDC_ERR_KEY_TOO_WEAK 65 (pkinit) -KDC_ERR_CERTIFICATE_MISMATCH 66 (pkinit) - -9. Interoperability requirements - -Version 5 of the Kerberos protocol supports a myriad of options. Among these -are multiple encryption and checksum types, alternative encoding schemes for -the transited field, optional mechanisms for pre-authentication, the -handling of tickets with no addresses, options for mutual authentication, -user to user authentication, support for proxies, forwarding, postdating, -and renewing tickets, the format of realm names, and the handling of -authorization data. - -In order to ensure the interoperability of realms, it is necessary to define -a minimal configuration which must be supported by all implementations. This -minimal configuration is subject to change as technology does. For example, -if at some later date it is discovered that one of the required encryption -or checksum algorithms is not secure, it will be replaced. - -9.1. Specification 2 - -This section defines the second specification of these options. -Implementations which are configured in this way can be said to support -Kerberos Version 5 Specification 2 (5.1). Specification 1 (depricated) may -be found in RFC1510. - -Transport - -TCP/IP and UDP/IP transport must be supported by KDCs claiming conformance -to specification 2. Kerberos clients claiming conformance to specification 2 -must support UDP/IP transport for messages with the KDC and may support -TCP/IP transport. - -Encryption and checksum methods - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -The following encryption and checksum mechanisms must be supported. -Implementations may support other mechanisms as well, but the additional -mechanisms may only be used when communicating with principals known to also -support them: This list is to be determined. - -Encryption: DES-CBC-MD5 -Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5 - -Realm Names - -All implementations must understand hierarchical realms in both the Internet -Domain and the X.500 style. When a ticket granting ticket for an unknown -realm is requested, the KDC must be able to determine the names of the -intermediate realms between the KDCs realm and the requested realm. - -Transited field encoding - -DOMAIN-X500-COMPRESS (described in section 3.3.3.2) must be supported. -Alternative encodings may be supported, but they may be used only when that -encoding is supported by ALL intermediate realms. - -Pre-authentication methods - -The TGS-REQ method must be supported. The TGS-REQ method is not used on the -initial request. The PA-ENC-TIMESTAMP method must be supported by clients -but whether it is enabled by default may be determined on a realm by realm -basis. If not used in the initial request and the error -KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENC-TIMESTAMP as an -acceptable method, the client should retry the initial request using the -PA-ENC-TIMESTAMP preauthentication method. Servers need not support the -PA-ENC-TIMESTAMP method, but if not supported the server should ignore the -presence of PA-ENC-TIMESTAMP pre-authentication in a request. - -Mutual authentication - -Mutual authentication (via the KRB_AP_REP message) must be supported. - -Ticket addresses and flags - -All KDC's must pass on tickets that carry no addresses (i.e. if a TGT -contains no addresses, the KDC will return derivative tickets), but each -realm may set its own policy for issuing such tickets, and each application -server will set its own policy with respect to accepting them. - -Proxies and forwarded tickets must be supported. Individual realms and -application servers can set their own policy on when such tickets will be -accepted. - -All implementations must recognize renewable and postdated tickets, but need -not actually implement them. If these options are not supported, the -starttime and endtime in the ticket shall specify a ticket's entire useful -life. When a postdated ticket is decoded by a server, all implementations -shall make the presence of the postdated flag visible to the calling server. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -User-to-user authentication - -Support for user to user authentication (via the ENC-TKT-IN-SKEY KDC option) -must be provided by implementations, but individual realms may decide as a -matter of policy to reject such requests on a per-principal or realm-wide -basis. - -Authorization data - -Implementations must pass all authorization data subfields from -ticket-granting tickets to any derivative tickets unless directed to -suppress a subfield as part of the definition of that registered subfield -type (it is never incorrect to pass on a subfield, and no registered -subfield types presently specify suppression at the KDC). - -Implementations must make the contents of any authorization data subfields -available to the server when a ticket is used. Implementations are not -required to allow clients to specify the contents of the authorization data -fields. - -9.2. Recommended KDC values - -Following is a list of recommended values for a KDC implementation, based on -the list of suggested configuration constants (see section 4.4). - -minimum lifetime 5 minutes -maximum renewable lifetime 1 week -maximum ticket lifetime 1 day -empty addresses only when suitable restrictions appear - in authorization data -proxiable, etc. Allowed. - -10. REFERENCES - -[NT94] B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti- - cation Service for Computer Networks," IEEE Communica- - tions Magazine, Vol. 32(9), pp. 33-38 (September 1994). - -[MNSS87] S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. - Saltzer, Section E.2.1: Kerberos Authentication and - Authorization System, M.I.T. Project Athena, Cambridge, - Massachusetts (December 21, 1987). - -[SNS88] J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Ker- - beros: An Authentication Service for Open Network Sys- - tems," pp. 191-202 in Usenix Conference Proceedings, - Dallas, Texas (February, 1988). - -[NS78] Roger M. Needham and Michael D. Schroeder, "Using - Encryption for Authentication in Large Networks of Com- - puters," Communications of the ACM, Vol. 21(12), - pp. 993-999 (December, 1978). - -[DS81] Dorothy E. Denning and Giovanni Maria Sacco, "Time- - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - stamps in Key Distribution Protocols," Communications - of the ACM, Vol. 24(8), pp. 533-536 (August 1981). - -[KNT92] John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, - "The Evolution of the Kerberos Authentication Service," - in an IEEE Computer Society Text soon to be published - (June 1992). - -[Neu93] B. Clifford Neuman, "Proxy-Based Authorization and - Accounting for Distributed Systems," in Proceedings of - the 13th International Conference on Distributed Com- - puting Systems, Pittsburgh, PA (May, 1993). - -[DS90] Don Davis and Ralph Swick, "Workstation Services and - Kerberos Authentication at Project Athena," Technical - Memorandum TM-424, MIT Laboratory for Computer Science - (February 1990). - -[LGDSR87] P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som- - merfeld, and K. Raeburn, Section E.1: Service Manage- - ment System, M.I.T. Project Athena, Cambridge, Mas- - sachusetts (1987). - -[X509-88] CCITT, Recommendation X.509: The Directory Authentica- - tion Framework, December 1988. - -[Pat92]. J. Pato, Using Pre-Authentication to Avoid Password - Guessing Attacks, Open Software Foundation DCE Request - for Comments 26 (December 1992). - -[DES77] National Bureau of Standards, U.S. Department of Com- - merce, "Data Encryption Standard," Federal Information - Processing Standards Publication 46, Washington, DC - (1977). - -[DESM80] National Bureau of Standards, U.S. Department of Com- - merce, "DES Modes of Operation," Federal Information - Processing Standards Publication 81, Springfield, VA - (December 1980). - -[SG92] Stuart G. Stubblebine and Virgil D. Gligor, "On Message - Integrity in Cryptographic Protocols," in Proceedings - of the IEEE Symposium on Research in Security and - Privacy, Oakland, California (May 1992). - -[IS3309] International Organization for Standardization, "ISO - Information Processing Systems - Data Communication - - High-Level Data Link Control Procedure - Frame Struc- - ture," IS 3309 (October 1984). 3rd Edition. - -[MD4-92] R. Rivest, "The MD4 Message Digest Algorithm," RFC - 1320, MIT Laboratory for Computer Science (April - 1992). - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -[MD5-92] R. Rivest, "The MD5 Message Digest Algorithm," RFC - 1321, MIT Laboratory for Computer Science (April - 1992). - -[KBC96] H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed- - Hashing for Message Authentication," Working Draft - draft-ietf-ipsec-hmac-md5-01.txt, (August 1996). - -A. Pseudo-code for protocol processing - -This appendix provides pseudo-code describing how the messages are to be -constructed and interpreted by clients and servers. - -A.1. KRB_AS_REQ generation - - request.pvno := protocol version; /* pvno = 5 */ - request.msg-type := message type; /* type = KRB_AS_REQ */ - - if(pa_enc_timestamp_required) then - request.padata.padata-type = PA-ENC-TIMESTAMP; - get system_time; - padata-body.patimestamp,pausec = system_time; - encrypt padata-body into request.padata.padata-value - using client.key; /* derived from password */ - endif - - body.kdc-options := users's preferences; - body.cname := user's name; - body.realm := user's realm; - body.sname := service's name; /* usually "krbtgt", "localrealm" */ - if (body.kdc-options.POSTDATED is set) then - body.from := requested starting time; - else - omit body.from; - endif - body.till := requested end time; - if (body.kdc-options.RENEWABLE is set) then - body.rtime := requested final renewal time; - endif - body.nonce := random_nonce(); - body.etype := requested etypes; - if (user supplied addresses) then - body.addresses := user's addresses; - else - omit body.addresses; - endif - omit body.enc-authorization-data; - request.req-body := body; - - kerberos := lookup(name of local kerberos server (or servers)); - send(packet,kerberos); - - wait(for response); - if (timed_out) then - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - retry or use alternate server; - endif - -A.2. KRB_AS_REQ verification and KRB_AS_REP generation - - decode message into req; - - client := lookup(req.cname,req.realm); - server := lookup(req.sname,req.realm); - - get system_time; - kdc_time := system_time.seconds; - - if (!client) then - /* no client in Database */ - error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN); - endif - if (!server) then - /* no server in Database */ - error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN); - endif - - if(client.pa_enc_timestamp_required and - pa_enc_timestamp not present) then - error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)); - endif - - if(pa_enc_timestamp present) then - decrypt req.padata-value into decrypted_enc_timestamp - using client.key; - using auth_hdr.authenticator.subkey; - if (decrypt_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - if(decrypted_enc_timestamp is not within allowable skew) then - error_out(KDC_ERR_PREAUTH_FAILED); - endif - if(decrypted_enc_timestamp and usec is replay) - error_out(KDC_ERR_PREAUTH_FAILED); - endif - add decrypted_enc_timestamp and usec to replay cache; - endif - - use_etype := first supported etype in req.etypes; - - if (no support for req.etypes) then - error_out(KDC_ERR_ETYPE_NOSUPP); - endif - - new_tkt.vno := ticket version; /* = 5 */ - new_tkt.sname := req.sname; - new_tkt.srealm := req.srealm; - reset all flags in new_tkt.flags; - - /* It should be noted that local policy may affect the */ - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - /* processing of any of these flags. For example, some */ - /* realms may refuse to issue renewable tickets */ - - if (req.kdc-options.FORWARDABLE is set) then - set new_tkt.flags.FORWARDABLE; - endif - if (req.kdc-options.PROXIABLE is set) then - set new_tkt.flags.PROXIABLE; - endif - - if (req.kdc-options.ALLOW-POSTDATE is set) then - set new_tkt.flags.MAY-POSTDATE; - endif - if ((req.kdc-options.RENEW is set) or - (req.kdc-options.VALIDATE is set) or - (req.kdc-options.PROXY is set) or - (req.kdc-options.FORWARDED is set) or - (req.kdc-options.ENC-TKT-IN-SKEY is set)) then - error_out(KDC_ERR_BADOPTION); - endif - - new_tkt.session := random_session_key(); - new_tkt.cname := req.cname; - new_tkt.crealm := req.crealm; - new_tkt.transited := empty_transited_field(); - - new_tkt.authtime := kdc_time; - - if (req.kdc-options.POSTDATED is set) then - if (against_postdate_policy(req.from)) then - error_out(KDC_ERR_POLICY); - endif - set new_tkt.flags.POSTDATED; - set new_tkt.flags.INVALID; - new_tkt.starttime := req.from; - else - omit new_tkt.starttime; /* treated as authtime when omitted */ - endif - if (req.till = 0) then - till := infinity; - else - till := req.till; - endif - - new_tkt.endtime := min(till, - new_tkt.starttime+client.max_life, - new_tkt.starttime+server.max_life, - new_tkt.starttime+max_life_for_realm); - - if ((req.kdc-options.RENEWABLE-OK is set) and - (new_tkt.endtime < req.till)) then - /* we set the RENEWABLE option for later processing */ - set req.kdc-options.RENEWABLE; - req.rtime := req.till; - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - endif - - if (req.rtime = 0) then - rtime := infinity; - else - rtime := req.rtime; - endif - - if (req.kdc-options.RENEWABLE is set) then - set new_tkt.flags.RENEWABLE; - new_tkt.renew-till := min(rtime, - new_tkt.starttime+client.max_rlife, - new_tkt.starttime+server.max_rlife, - new_tkt.starttime+max_rlife_for_realm); - else - omit new_tkt.renew-till; /* only present if RENEWABLE */ - endif - - if (req.addresses) then - new_tkt.caddr := req.addresses; - else - omit new_tkt.caddr; - endif - - new_tkt.authorization_data := empty_authorization_data(); - - encode to-be-encrypted part of ticket into OCTET STRING; - new_tkt.enc-part := encrypt OCTET STRING - using etype_for_key(server.key), server.key, server.p_kvno; - - /* Start processing the response */ - - resp.pvno := 5; - resp.msg-type := KRB_AS_REP; - resp.cname := req.cname; - resp.crealm := req.realm; - resp.ticket := new_tkt; - - resp.key := new_tkt.session; - resp.last-req := fetch_last_request_info(client); - resp.nonce := req.nonce; - resp.key-expiration := client.expiration; - resp.flags := new_tkt.flags; - - resp.authtime := new_tkt.authtime; - resp.starttime := new_tkt.starttime; - resp.endtime := new_tkt.endtime; - - if (new_tkt.flags.RENEWABLE) then - resp.renew-till := new_tkt.renew-till; - endif - - resp.realm := new_tkt.realm; - resp.sname := new_tkt.sname; - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - - resp.caddr := new_tkt.caddr; - - encode body of reply into OCTET STRING; - - resp.enc-part := encrypt OCTET STRING - using use_etype, client.key, client.p_kvno; - send(resp); - -A.3. KRB_AS_REP verification - - decode response into resp; - - if (resp.msg-type = KRB_ERROR) then - if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)) then - set pa_enc_timestamp_required; - goto KRB_AS_REQ; - endif - process_error(resp); - return; - endif - - /* On error, discard the response, and zero the session key */ - /* from the response immediately */ - - key = get_decryption_key(resp.enc-part.kvno, resp.enc-part.etype, - resp.padata); - unencrypted part of resp := decode of decrypt of resp.enc-part - using resp.enc-part.etype and key; - zero(key); - - if (common_as_rep_tgs_rep_checks fail) then - destroy resp.key; - return error; - endif - - if near(resp.princ_exp) then - print(warning message); - endif - save_for_later(ticket,session,client,server,times,flags); - -A.4. KRB_AS_REP and KRB_TGS_REP common checks - - if (decryption_error() or - (req.cname != resp.cname) or - (req.realm != resp.crealm) or - (req.sname != resp.sname) or - (req.realm != resp.realm) or - (req.nonce != resp.nonce) or - (req.addresses != resp.caddr)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - /* make sure no flags are set that shouldn't be, and that all that */ - /* should be are set */ - if (!check_flags_for_compatability(req.kdc-options,resp.flags)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - - if ((req.from = 0) and - (resp.starttime is not within allowable skew)) then - destroy resp.key; - return KRB_AP_ERR_SKEW; - endif - if ((req.from != 0) and (req.from != resp.starttime)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - if ((req.till != 0) and (resp.endtime > req.till)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - - if ((req.kdc-options.RENEWABLE is set) and - (req.rtime != 0) and (resp.renew-till > req.rtime)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - if ((req.kdc-options.RENEWABLE-OK is set) and - (resp.flags.RENEWABLE) and - (req.till != 0) and - (resp.renew-till > req.till)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - -A.5. KRB_TGS_REQ generation - - /* Note that make_application_request might have to recursivly */ - /* call this routine to get the appropriate ticket-granting ticket */ - - request.pvno := protocol version; /* pvno = 5 */ - request.msg-type := message type; /* type = KRB_TGS_REQ */ - - body.kdc-options := users's preferences; - /* If the TGT is not for the realm of the end-server */ - /* then the sname will be for a TGT for the end-realm */ - /* and the realm of the requested ticket (body.realm) */ - /* will be that of the TGS to which the TGT we are */ - /* sending applies */ - body.sname := service's name; - body.realm := service's realm; - - if (body.kdc-options.POSTDATED is set) then - body.from := requested starting time; - else - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - omit body.from; - endif - body.till := requested end time; - if (body.kdc-options.RENEWABLE is set) then - body.rtime := requested final renewal time; - endif - body.nonce := random_nonce(); - body.etype := requested etypes; - if (user supplied addresses) then - body.addresses := user's addresses; - else - omit body.addresses; - endif - - body.enc-authorization-data := user-supplied data; - if (body.kdc-options.ENC-TKT-IN-SKEY) then - body.additional-tickets_ticket := second TGT; - endif - - request.req-body := body; - check := generate_checksum (req.body,checksumtype); - - request.padata[0].padata-type := PA-TGS-REQ; - request.padata[0].padata-value := create a KRB_AP_REQ using - the TGT and checksum - - /* add in any other padata as required/supplied */ - - kerberos := lookup(name of local kerberose server (or servers)); - send(packet,kerberos); - - wait(for response); - if (timed_out) then - retry or use alternate server; - endif - -A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation - - /* note that reading the application request requires first - determining the server for which a ticket was issued, and choosing the - correct key for decryption. The name of the server appears in the - plaintext part of the ticket. */ - - if (no KRB_AP_REQ in req.padata) then - error_out(KDC_ERR_PADATA_TYPE_NOSUPP); - endif - verify KRB_AP_REQ in req.padata; - - /* Note that the realm in which the Kerberos server is operating is - determined by the instance from the ticket-granting ticket. The realm - in the ticket-granting ticket is the realm under which the ticket - granting ticket was issued. It is possible for a single Kerberos - server to support more than one realm. */ - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - auth_hdr := KRB_AP_REQ; - tgt := auth_hdr.ticket; - - if (tgt.sname is not a TGT for local realm and is not req.sname) then - error_out(KRB_AP_ERR_NOT_US); - - realm := realm_tgt_is_for(tgt); - - decode remainder of request; - - if (auth_hdr.authenticator.cksum is missing) then - error_out(KRB_AP_ERR_INAPP_CKSUM); - endif - - if (auth_hdr.authenticator.cksum type is not supported) then - error_out(KDC_ERR_SUMTYPE_NOSUPP); - endif - if (auth_hdr.authenticator.cksum is not both collision-proof and keyed) then - error_out(KRB_AP_ERR_INAPP_CKSUM); - endif - - set computed_checksum := checksum(req); - if (computed_checksum != auth_hdr.authenticatory.cksum) then - error_out(KRB_AP_ERR_MODIFIED); - endif - - server := lookup(req.sname,realm); - - if (!server) then - if (is_foreign_tgt_name(req.sname)) then - server := best_intermediate_tgs(req.sname); - else - /* no server in Database */ - error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN); - endif - endif - - session := generate_random_session_key(); - - use_etype := first supported etype in req.etypes; - - if (no support for req.etypes) then - error_out(KDC_ERR_ETYPE_NOSUPP); - endif - - new_tkt.vno := ticket version; /* = 5 */ - new_tkt.sname := req.sname; - new_tkt.srealm := realm; - reset all flags in new_tkt.flags; - - /* It should be noted that local policy may affect the */ - /* processing of any of these flags. For example, some */ - /* realms may refuse to issue renewable tickets */ - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - new_tkt.caddr := tgt.caddr; - resp.caddr := NULL; /* We only include this if they change */ - if (req.kdc-options.FORWARDABLE is set) then - if (tgt.flags.FORWARDABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.FORWARDABLE; - endif - if (req.kdc-options.FORWARDED is set) then - if (tgt.flags.FORWARDABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.FORWARDED; - new_tkt.caddr := req.addresses; - resp.caddr := req.addresses; - endif - if (tgt.flags.FORWARDED is set) then - set new_tkt.flags.FORWARDED; - endif - - if (req.kdc-options.PROXIABLE is set) then - if (tgt.flags.PROXIABLE is reset) - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.PROXIABLE; - endif - if (req.kdc-options.PROXY is set) then - if (tgt.flags.PROXIABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.PROXY; - new_tkt.caddr := req.addresses; - resp.caddr := req.addresses; - endif - - if (req.kdc-options.ALLOW-POSTDATE is set) then - if (tgt.flags.MAY-POSTDATE is reset) - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.MAY-POSTDATE; - endif - if (req.kdc-options.POSTDATED is set) then - if (tgt.flags.MAY-POSTDATE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.POSTDATED; - set new_tkt.flags.INVALID; - if (against_postdate_policy(req.from)) then - error_out(KDC_ERR_POLICY); - endif - new_tkt.starttime := req.from; - endif - - if (req.kdc-options.VALIDATE is set) then - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - if (tgt.flags.INVALID is reset) then - error_out(KDC_ERR_POLICY); - endif - if (tgt.starttime > kdc_time) then - error_out(KRB_AP_ERR_NYV); - endif - if (check_hot_list(tgt)) then - error_out(KRB_AP_ERR_REPEAT); - endif - tkt := tgt; - reset new_tkt.flags.INVALID; - endif - - if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW, - and those already processed) is set) then - error_out(KDC_ERR_BADOPTION); - endif - - new_tkt.authtime := tgt.authtime; - - if (req.kdc-options.RENEW is set) then - /* Note that if the endtime has already passed, the ticket would */ - /* have been rejected in the initial authentication stage, so */ - /* there is no need to check again here */ - if (tgt.flags.RENEWABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - if (tgt.renew-till < kdc_time) then - error_out(KRB_AP_ERR_TKT_EXPIRED); - endif - tkt := tgt; - new_tkt.starttime := kdc_time; - old_life := tgt.endttime - tgt.starttime; - new_tkt.endtime := min(tgt.renew-till, - new_tkt.starttime + old_life); - else - new_tkt.starttime := kdc_time; - if (req.till = 0) then - till := infinity; - else - till := req.till; - endif - new_tkt.endtime := min(till, - new_tkt.starttime+client.max_life, - new_tkt.starttime+server.max_life, - new_tkt.starttime+max_life_for_realm, - tgt.endtime); - - if ((req.kdc-options.RENEWABLE-OK is set) and - (new_tkt.endtime < req.till) and - (tgt.flags.RENEWABLE is set) then - /* we set the RENEWABLE option for later processing */ - set req.kdc-options.RENEWABLE; - req.rtime := min(req.till, tgt.renew-till); - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - endif - endif - - if (req.rtime = 0) then - rtime := infinity; - else - rtime := req.rtime; - endif - - if ((req.kdc-options.RENEWABLE is set) and - (tgt.flags.RENEWABLE is set)) then - set new_tkt.flags.RENEWABLE; - new_tkt.renew-till := min(rtime, - new_tkt.starttime+client.max_rlife, - new_tkt.starttime+server.max_rlife, - new_tkt.starttime+max_rlife_for_realm, - tgt.renew-till); - else - new_tkt.renew-till := OMIT; /* leave the renew-till field out */ - endif - if (req.enc-authorization-data is present) then - decrypt req.enc-authorization-data into decrypted_authorization_data - using auth_hdr.authenticator.subkey; - if (decrypt_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - endif - new_tkt.authorization_data := req.auth_hdr.ticket.authorization_data + - decrypted_authorization_data; - - new_tkt.key := session; - new_tkt.crealm := tgt.crealm; - new_tkt.cname := req.auth_hdr.ticket.cname; - - if (realm_tgt_is_for(tgt) := tgt.realm) then - /* tgt issued by local realm */ - new_tkt.transited := tgt.transited; - else - /* was issued for this realm by some other realm */ - if (tgt.transited.tr-type not supported) then - error_out(KDC_ERR_TRTYPE_NOSUPP); - endif - new_tkt.transited := compress_transited(tgt.transited + tgt.realm) - /* Don't check tranited field if TGT for foreign realm, - * or requested not to check */ - if (is_not_foreign_tgt_name(new_tkt.server) - && req.kdc-options.DISABLE-TRANSITED-CHECK not set) then - /* Check it, so end-server does not have to - * but don't fail, end-server may still accept it */ - if (check_transited_field(new_tkt.transited) == OK) - set new_tkt.flags.TRANSITED-POLICY-CHECKED; - endif - endif - endif - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - - encode encrypted part of new_tkt into OCTET STRING; - if (req.kdc-options.ENC-TKT-IN-SKEY is set) then - if (server not specified) then - server = req.second_ticket.client; - endif - if ((req.second_ticket is not a TGT) or - (req.second_ticket.client != server)) then - error_out(KDC_ERR_POLICY); - endif - - new_tkt.enc-part := encrypt OCTET STRING using - using etype_for_key(second-ticket.key), second-ticket.key; - else - new_tkt.enc-part := encrypt OCTET STRING - using etype_for_key(server.key), server.key, server.p_kvno; - endif - - resp.pvno := 5; - resp.msg-type := KRB_TGS_REP; - resp.crealm := tgt.crealm; - resp.cname := tgt.cname; - resp.ticket := new_tkt; - - resp.key := session; - resp.nonce := req.nonce; - resp.last-req := fetch_last_request_info(client); - resp.flags := new_tkt.flags; - - resp.authtime := new_tkt.authtime; - resp.starttime := new_tkt.starttime; - resp.endtime := new_tkt.endtime; - - omit resp.key-expiration; - - resp.sname := new_tkt.sname; - resp.realm := new_tkt.realm; - - if (new_tkt.flags.RENEWABLE) then - resp.renew-till := new_tkt.renew-till; - endif - - encode body of reply into OCTET STRING; - - if (req.padata.authenticator.subkey) - resp.enc-part := encrypt OCTET STRING using use_etype, - req.padata.authenticator.subkey; - else resp.enc-part := encrypt OCTET STRING using use_etype, tgt.key; - - send(resp); - -A.7. KRB_TGS_REP verification - - decode response into resp; - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - - if (resp.msg-type = KRB_ERROR) then - process_error(resp); - return; - endif - - /* On error, discard the response, and zero the session key from - the response immediately */ - - if (req.padata.authenticator.subkey) - unencrypted part of resp := decode of decrypt of resp.enc-part - using resp.enc-part.etype and subkey; - else unencrypted part of resp := decode of decrypt of resp.enc-part - using resp.enc-part.etype and tgt's session key; - if (common_as_rep_tgs_rep_checks fail) then - destroy resp.key; - return error; - endif - - check authorization_data as necessary; - save_for_later(ticket,session,client,server,times,flags); - -A.8. Authenticator generation - - body.authenticator-vno := authenticator vno; /* = 5 */ - body.cname, body.crealm := client name; - if (supplying checksum) then - body.cksum := checksum; - endif - get system_time; - body.ctime, body.cusec := system_time; - if (selecting sub-session key) then - select sub-session key; - body.subkey := sub-session key; - endif - if (using sequence numbers) then - select initial sequence number; - body.seq-number := initial sequence; - endif - -A.9. KRB_AP_REQ generation - - obtain ticket and session_key from cache; - - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_AP_REQ */ - - if (desired(MUTUAL_AUTHENTICATION)) then - set packet.ap-options.MUTUAL-REQUIRED; - else - reset packet.ap-options.MUTUAL-REQUIRED; - endif - if (using session key for ticket) then - set packet.ap-options.USE-SESSION-KEY; - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - else - reset packet.ap-options.USE-SESSION-KEY; - endif - packet.ticket := ticket; /* ticket */ - generate authenticator; - encode authenticator into OCTET STRING; - encrypt OCTET STRING into packet.authenticator using session_key; - -A.10. KRB_AP_REQ verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_AP_REQ) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - if (packet.ticket.tkt_vno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.ap_options.USE-SESSION-KEY is set) then - retrieve session key from ticket-granting ticket for - packet.ticket.{sname,srealm,enc-part.etype}; - else - retrieve service key for - packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno}; - endif - if (no_key_available) then - if (cannot_find_specified_skvno) then - error_out(KRB_AP_ERR_BADKEYVER); - else - error_out(KRB_AP_ERR_NOKEY); - endif - endif - decrypt packet.ticket.enc-part into decr_ticket using retrieved key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - decrypt packet.authenticator into decr_authenticator - using decr_ticket.key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - if (decr_authenticator.{cname,crealm} != - decr_ticket.{cname,crealm}) then - error_out(KRB_AP_ERR_BADMATCH); - endif - if (decr_ticket.caddr is present) then - if (sender_address(packet) is not in decr_ticket.caddr) then - error_out(KRB_AP_ERR_BADADDR); - endif - elseif (application requires addresses) then - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - error_out(KRB_AP_ERR_BADADDR); - endif - if (not in_clock_skew(decr_authenticator.ctime, - decr_authenticator.cusec)) then - error_out(KRB_AP_ERR_SKEW); - endif - if (repeated(decr_authenticator.{ctime,cusec,cname,crealm})) then - error_out(KRB_AP_ERR_REPEAT); - endif - save_identifier(decr_authenticator.{ctime,cusec,cname,crealm}); - get system_time; - if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or - (decr_ticket.flags.INVALID is set)) then - /* it hasn't yet become valid */ - error_out(KRB_AP_ERR_TKT_NYV); - endif - if (system_time-decr_ticket.endtime > CLOCK_SKEW) then - error_out(KRB_AP_ERR_TKT_EXPIRED); - endif - if (decr_ticket.transited) then - /* caller may ignore the TRANSITED-POLICY-CHECKED and do - * check anyway */ - if (decr_ticket.flags.TRANSITED-POLICY-CHECKED not set) then - if (check_transited_field(decr_ticket.transited) then - error_out(KDC_AP_PATH_NOT_ACCPETED); - endif - endif - endif - /* caller must check decr_ticket.flags for any pertinent details */ - return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED); - -A.11. KRB_AP_REP generation - - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_AP_REP */ - - body.ctime := packet.ctime; - body.cusec := packet.cusec; - if (selecting sub-session key) then - select sub-session key; - body.subkey := sub-session key; - endif - if (using sequence numbers) then - select initial sequence number; - body.seq-number := initial sequence; - endif - - encode body into OCTET STRING; - - select encryption type; - encrypt OCTET STRING into packet.enc-part; - -A.12. KRB_AP_REP verification - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_AP_REP) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - cleartext := decrypt(packet.enc-part) using ticket's session key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - if (cleartext.ctime != authenticator.ctime) then - error_out(KRB_AP_ERR_MUT_FAIL); - endif - if (cleartext.cusec != authenticator.cusec) then - error_out(KRB_AP_ERR_MUT_FAIL); - endif - if (cleartext.subkey is present) then - save cleartext.subkey for future use; - endif - if (cleartext.seq-number is present) then - save cleartext.seq-number for future verifications; - endif - return(AUTHENTICATION_SUCCEEDED); - -A.13. KRB_SAFE generation - - collect user data in buffer; - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_SAFE */ - - body.user-data := buffer; /* DATA */ - if (using timestamp) then - get system_time; - body.timestamp, body.usec := system_time; - endif - if (using sequence numbers) then - body.seq-number := sequence number; - endif - body.s-address := sender host addresses; - if (only one recipient) then - body.r-address := recipient host address; - endif - checksum.cksumtype := checksum type; - compute checksum over body; - checksum.checksum := checksum value; /* checksum.checksum */ - packet.cksum := checksum; - packet.safe-body := body; - -A.14. KRB_SAFE verification - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_SAFE) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - if (packet.checksum.cksumtype is not both collision-proof and keyed) then - error_out(KRB_AP_ERR_INAPP_CKSUM); - endif - if (safe_priv_common_checks_ok(packet)) then - set computed_checksum := checksum(packet.body); - if (computed_checksum != packet.checksum) then - error_out(KRB_AP_ERR_MODIFIED); - endif - return (packet, PACKET_IS_GENUINE); - else - return common_checks_error; - endif - -A.15. KRB_SAFE and KRB_PRIV common checks - - if (packet.s-address != O/S_sender(packet)) then - /* O/S report of sender not who claims to have sent it */ - error_out(KRB_AP_ERR_BADADDR); - endif - if ((packet.r-address is present) and - (packet.r-address != local_host_address)) then - /* was not sent to proper place */ - error_out(KRB_AP_ERR_BADADDR); - endif - if (((packet.timestamp is present) and - (not in_clock_skew(packet.timestamp,packet.usec))) or - (packet.timestamp is not present and timestamp expected)) then - error_out(KRB_AP_ERR_SKEW); - endif - if (repeated(packet.timestamp,packet.usec,packet.s-address)) then - error_out(KRB_AP_ERR_REPEAT); - endif - - if (((packet.seq-number is present) and - ((not in_sequence(packet.seq-number)))) or - (packet.seq-number is not present and sequence expected)) then - error_out(KRB_AP_ERR_BADORDER); - endif - if (packet.timestamp not present and packet.seq-number not present) - then - error_out(KRB_AP_ERR_MODIFIED); - endif - - save_identifier(packet.{timestamp,usec,s-address}, - sender_principal(packet)); - - return PACKET_IS_OK; - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -A.16. KRB_PRIV generation - - collect user data in buffer; - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_PRIV */ - - packet.enc-part.etype := encryption type; - - body.user-data := buffer; - if (using timestamp) then - get system_time; - body.timestamp, body.usec := system_time; - endif - if (using sequence numbers) then - body.seq-number := sequence number; - endif - body.s-address := sender host addresses; - if (only one recipient) then - body.r-address := recipient host address; - endif - - encode body into OCTET STRING; - - select encryption type; - encrypt OCTET STRING into packet.enc-part.cipher; - -A.17. KRB_PRIV verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_PRIV) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - - cleartext := decrypt(packet.enc-part) using negotiated key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - - if (safe_priv_common_checks_ok(cleartext)) then - return(cleartext.DATA, PACKET_IS_GENUINE_AND_UNMODIFIED); - else - return common_checks_error; - endif - -A.18. KRB_CRED generation - - invoke KRB_TGS; /* obtain tickets to be provided to peer */ - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_CRED */ - - for (tickets[n] in tickets to be forwarded) do - packet.tickets[n] = tickets[n].ticket; - done - - packet.enc-part.etype := encryption type; - - for (ticket[n] in tickets to be forwarded) do - body.ticket-info[n].key = tickets[n].session; - body.ticket-info[n].prealm = tickets[n].crealm; - body.ticket-info[n].pname = tickets[n].cname; - body.ticket-info[n].flags = tickets[n].flags; - body.ticket-info[n].authtime = tickets[n].authtime; - body.ticket-info[n].starttime = tickets[n].starttime; - body.ticket-info[n].endtime = tickets[n].endtime; - body.ticket-info[n].renew-till = tickets[n].renew-till; - body.ticket-info[n].srealm = tickets[n].srealm; - body.ticket-info[n].sname = tickets[n].sname; - body.ticket-info[n].caddr = tickets[n].caddr; - done - - get system_time; - body.timestamp, body.usec := system_time; - - if (using nonce) then - body.nonce := nonce; - endif - - if (using s-address) then - body.s-address := sender host addresses; - endif - if (limited recipients) then - body.r-address := recipient host address; - endif - - encode body into OCTET STRING; - - select encryption type; - encrypt OCTET STRING into packet.enc-part.cipher - using negotiated encryption key; - -A.19. KRB_CRED verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_CRED) then - error_out(KRB_AP_ERR_MSG_TYPE); - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - endif - - cleartext := decrypt(packet.enc-part) using negotiated key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - if ((packet.r-address is present or required) and - (packet.s-address != O/S_sender(packet)) then - /* O/S report of sender not who claims to have sent it */ - error_out(KRB_AP_ERR_BADADDR); - endif - if ((packet.r-address is present) and - (packet.r-address != local_host_address)) then - /* was not sent to proper place */ - error_out(KRB_AP_ERR_BADADDR); - endif - if (not in_clock_skew(packet.timestamp,packet.usec)) then - error_out(KRB_AP_ERR_SKEW); - endif - if (repeated(packet.timestamp,packet.usec,packet.s-address)) then - error_out(KRB_AP_ERR_REPEAT); - endif - if (packet.nonce is required or present) and - (packet.nonce != expected-nonce) then - error_out(KRB_AP_ERR_MODIFIED); - endif - - for (ticket[n] in tickets that were forwarded) do - save_for_later(ticket[n],key[n],principal[n], - server[n],times[n],flags[n]); - return - -A.20. KRB_ERROR generation - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_ERROR */ - - get system_time; - packet.stime, packet.susec := system_time; - packet.realm, packet.sname := server name; - - if (client time available) then - packet.ctime, packet.cusec := client_time; - endif - packet.error-code := error code; - if (client name available) then - packet.cname, packet.crealm := client name; - endif - if (error text available) then - packet.e-text := error text; - endif - if (error data available) then - packet.e-data := error data; - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - endif - -B. Definition of common authorization data elements - -This appendix contains the definitions of common authorization data -elements. These common authorization data elements are recursivly defined, -meaning the ad-data for these types will itself contain a sequence of -authorization data whose interpretation is affected by the encapsulating -element. Depending on the meaning of the encapsulating element, the -encapsulated elements may be ignored, might be interpreted as issued -directly by the KDC, or they might be stored in a separate plaintext part of -the ticket. The types of the encapsulating elements are specified as part of -the Kerberos specification ebcause the behavior based on these values should -be understood across implementations whereas other elements need only be -understood by the applications which they affect. - -In the definitions that follow, the value of the ad-type for the element -will be specified in the subsection number, and the value of the ad-data -will be as shown in the ASN.1 structure that follows the subsection heading. - -B.1. KDC Issued - -AD-KDCIssued SEQUENCE { - ad-checksum[0] Checksum, - i-realm[1] Realm OPTIONAL, - i-sname[2] PrincipalName OPTIONAL, - elements[3] AuthorizationData. -} - -ad-checksum - A checksum over the elements field using a cryptographic checksum - method that is identical to the checksum used to protect the ticket - itself (i.e. using the same hash function and the same encryption - algorithm used to encrypt the ticket) and using a key derived from the - same key used to protect the ticket. -i-realm, i-sname - The name of the issuing principal if different from the KDC itself. - This field would be used when the KDC can verify the authenticity of - elements signed by the issuing principal and it allows this KDC to - notify the application server of the validity of those elements. -elements - A sequence of authorization data elements issued by the KDC. - -The KDC-issued ad-data field is intended to provide a means for Kerberos -principal credentials to embed within themselves privilege attributes and -other mechanisms for positive authorization, amplifying the priveleges of -the principal beyond what can be done using a credentials without such an -a-data element. - -This can not be provided without this element because the definition of the -authorization-data field allows elements to be added at will by the bearer -of a TGT at the time that they request service tickets and elements may also -be added to a delegated ticket by inclusion in the authenticator. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -For KDC-issued elements this is prevented because the elements are signed by -the KDC by including a checksum encrypted using the server's key (the same -key used to encrypt the ticket - or a key derived from that key). Elements -encapsulated with in the KDC-issued element will be ignored by the -application server if this "signature" is not present. Further, elements -encapsulated within this element from a ticket granting ticket may be -interpreted by the KDC, and used as a basis according to policy for -including new signed elements within derivative tickets, but they will not -be copied to a derivative ticket directly. If they are copied directly to a -derivative ticket by a KDC that is not aware of this element, the signature -will not be correct for the application ticket elements, and the field will -be ignored by the application server. - -This element and the elements it encapulates may be safely ignored by -applications, application servers, and KDCs that do not implement this -element. - -B.2. Intended for server - -AD-INTENDED-FOR-SERVER SEQUENCE { - intended-server[0] SEQUENCE OF PrincipalName - elements[1] AuthorizationData -} - -AD elements encapsulated within the intended-for-server element may be -ignored if the application server is not in the list of principal names of -intended servers. Further, a KDC issuing a ticket for an application server -can remove this element if the application server is not in the list of -intended servers. - -Application servers should check for their principal name in the -intended-server field of this element. If their principal name is not found, -this element should be ignored. If found, then the encapsulated elements -should be evaluated in the same manner as if they were present in the top -level authorization data field. Applications and application servers that do -not implement this element should reject tickets that contain authorization -data elements of this type. - -B.3. Intended for application class - -AD-INTENDED-FOR-APPLICATION-CLASS SEQUENCE { intended-application-class[0] -SEQUENCE OF GeneralString elements[1] AuthorizationData } AD elements -encapsulated within the intended-for-application-class element may be -ignored if the application server is not in one of the named classes of -application servers. Examples of application server classes include -"FILESYSTEM", and other kinds of servers. - -This element and the elements it encapulates may be safely ignored by -applications, application servers, and KDCs that do not implement this -element. - -B.4. If relevant - -AD-IF-RELEVANT AuthorizationData - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -AD elements encapsulated within the if-relevant element are intended for -interpretation only by application servers that understand the particular -ad-type of the embedded element. Application servers that do not understand -the type of an element embedded within the if-relevant element may ignore -the uninterpretable element. This element promotes interoperability across -implementations which may have local extensions for authorization. - -B.5. And-Or - -AD-AND-OR SEQUENCE { - condition-count[0] INTEGER, - elements[1] AuthorizationData -} - -When restrictive AD elements encapsulated within the and-or element are -encountered, only the number specified in condition-count of the -encapsulated conditions must be met in order to satisfy this element. This -element may be used to implement an "or" operation by setting the -condition-count field to 1, and it may specify an "and" operation by setting -the condition count to the number of embedded elements. Application servers -that do not implement this element must reject tickets that contain -authorization data elements of this type. - -B.6. Mandatory ticket extensions - -AD-Mandatory-Ticket-Extensions Checksum - -An authorization data element of type mandatory-ticket-extensions specifies -a collision-proof checksum using the same has angorithm used to protect the -integrity of the ticket itself. This checksum will be calculated over the -entire extensions field. If there are more than one extension, all will be -covered by the checksum. This restriction indicates that the ticket should -not be accepted if the checksum does not match that calculated over the -ticket extensions. Application servers that do not implement this element -must reject tickets that contain authorization data elements of this type. - -B.7. Authorization Data in ticket extensions - -AD-IN-Ticket-Extensions Checksum - -An authorization data element of type in-ticket-extensions specifies a -collision-proof checksum using the same has angorithm used to protect the -integrity of the ticket itself. This checksum is calculated over a separate -external AuthorizationData field carried in the ticket extensions. -Application servers that do not implement this element must reject tickets -that contain authorization data elements of this type. Application servers -that do implement this element will search the ticket extensions for -authorization data fields, calculate the specified checksum over each -authorization data field and look for one matching the checksum in this -in-ticket-extensions element. If not found, then the ticket must be -rejected. If found, the corresponding authorization data elements will be -interpreted in the same manner as if they were contained in the top level -authorization data field. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -Note that if multiple external authorization data fields are present in a -ticket, each will have a corresponding element of type in-ticket-extensions -in the top level authorization data field, and the external entries will be -linked to the corresponding element by their checksums. - -C. Definition of common ticket extensions - -This appendix contains the definitions of common ticket extensions. Support -for these extensions is optional. However, certain extensions have -associated authorization data elements that may require rejection of a -ticket containing an extension by application servers that do not implement -the particular extension. Other extensions have been defined beyond those -described in this specification. Such extensions are described elswhere and -for some of those extensions the reserved number may be found in the list of -constants. - -It is known that older versions of Kerberos did not support this field, and -that some clients will strip this field from a ticket when they parse and -then reassemble a ticket as it is passed to the application servers. The -presence of the extension will not break such clients, but any functionaly -dependent on the extensions will not work when such tickets are handled by -old clients. In such situations, some implementation may use alternate -methods to transmit the information in the extensions field. - -C.1. Null ticket extension - -TE-NullExtension OctetString -- The empty Octet String - -The te-data field in the null ticket extension is an octet string of lenght -zero. This extension may be included in a ticket granting ticket so that the -KDC can determine on presentation of the ticket granting ticket whether the -client software will strip the extensions field. - -C.2. External Authorization Data - -TE-ExternalAuthorizationData AuthorizationData - -The te-data field in the external authorization data ticket extension is -field of type AuthorizationData containing one or more authorization data -elements. If present, a corresponding authorization data element will be -present in the primary authorization data for the ticket and that element -will contain a checksum of the external authorization data ticket extension. ----------------------------------------------------------------------------- -[TM] Project Athena, Athena, and Kerberos are trademarks of the -Massachusetts Institute of Technology (MIT). No commercial use of these -trademarks may be made without prior written permission of MIT. - -[1] Note, however, that many applications use Kerberos' functions only upon -the initiation of a stream-based network connection. Unless an application -subsequently provides integrity protection for the data stream, the identity -verification applies only to the initiation of the connection, and does not -guarantee that subsequent messages on the connection originate from the same -principal. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -[2] Secret and private are often used interchangeably in the literature. In -our usage, it takes two (or more) to share a secret, thus a shared DES key -is a secret key. Something is only private when no one but its owner knows -it. Thus, in public key cryptosystems, one has a public and a private key. - -[3] Of course, with appropriate permission the client could arrange -registration of a separately-named prin- cipal in a remote realm, and engage -in normal exchanges with that realm's services. However, for even small -numbers of clients this becomes cumbersome, and more automatic methods as -described here are necessary. - -[4] Though it is permissible to request or issue tick- ets with no network -addresses specified. - -[5] The password-changing request must not be honored unless the requester -can provide the old password (the user's current secret key). Otherwise, it -would be possible for someone to walk up to an unattended ses- sion and -change another user's password. - -[6] To authenticate a user logging on to a local system, the credentials -obtained in the AS exchange may first be used in a TGS exchange to obtain -credentials for a local server. Those credentials must then be verified by a -local server through successful completion of the Client/Server exchange. - -[7] "Random" means that, among other things, it should be impossible to -guess the next session key based on knowledge of past session keys. This can -only be achieved in a pseudo-random number generator if it is based on -cryptographic principles. It is more desirable to use a truly random number -generator, such as one based on measurements of random physical phenomena. - -[8] Tickets contain both an encrypted and unencrypted portion, so cleartext -here refers to the entire unit, which can be copied from one message and -replayed in another without any cryptographic skill. - -[9] Note that this can make applications based on unreliable transports -difficult to code correctly. If the transport might deliver duplicated -messages, either a new authenticator must be generated for each retry, or -the application server must match requests and replies and replay the first -reply in response to a detected duplicate. - -[10] This is used for user-to-user authentication as described in [8]. - -[11] Note that the rejection here is restricted to authenticators from the -same principal to the same server. Other client principals communicating -with the same server principal should not be have their authenticators -rejected if the time and microsecond fields happen to match some other -client's authenticator. - -[12] In the Kerberos version 4 protocol, the timestamp in the reply was the -client's timestamp plus one. This is not necessary in version 5 because -version 5 messages are formatted in such a way that it is not possible to -create the reply by judicious message surgery (even in encrypted form) -without knowledge of the appropriate encryption keys. - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - - -[13] Note that for encrypting the KRB_AP_REP message, the sub-session key is -not used, even if present in the Authenticator. - -[14] Implementations of the protocol may wish to provide routines to choose -subkeys based on session keys and random numbers and to generate a -negotiated key to be returned in the KRB_AP_REP message. - -[15]This can be accomplished in several ways. It might be known beforehand -(since the realm is part of the principal identifier), it might be stored in -a nameserver, or it might be obtained from a configura- tion file. If the -realm to be used is obtained from a nameserver, there is a danger of being -spoofed if the nameservice providing the realm name is not authenti- cated. -This might result in the use of a realm which has been compromised, and -would result in an attacker's ability to compromise the authentication of -the application server to the client. - -[16] If the client selects a sub-session key, care must be taken to ensure -the randomness of the selected sub- session key. One approach would be to -generate a random number and XOR it with the session key from the -ticket-granting ticket. - -[17] This allows easy implementation of user-to-user authentication [8], -which uses ticket-granting ticket session keys in lieu of secret server keys -in situa- tions where such secret keys could be easily comprom- ised. - -[18] For the purpose of appending, the realm preceding the first listed -realm is considered to be the null realm (""). - -[19] For the purpose of interpreting null subfields, the client's realm is -considered to precede those in the transited field, and the server's realm -is considered to follow them. - -[20] This means that a client and server running on the same host and -communicating with one another using the KRB_SAFE messages should not share -a common replay cache to detect KRB_SAFE replays. - -[21] The implementation of the Kerberos server need not combine the database -and the server on the same machine; it is feasible to store the principal -database in, say, a network name service, as long as the entries stored -therein are protected from disclosure to and modification by unauthorized -parties. However, we recommend against such strategies, as they can make -system management and threat analysis quite complex. - -[22] See the discussion of the padata field in section 5.4.2 for details on -why this can be useful. - -[23] Warning for implementations that unpack and repack data structures -during the generation and verification of embedded checksums: Because any -checksums applied to data structures must be checked against the original -data the length of bit strings must be preserved within a data structure -between the time that a checksum is generated through transmission to the -time that the checksum is verified. - - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -[24] It is NOT recommended that this time value be used to adjust the -workstation's clock since the workstation cannot reliably determine that -such a KRB_AS_REP actually came from the proper KDC in a timely manner. - -[25] Note, however, that if the time is used as the nonce, one must make -sure that the workstation time is monotonically increasing. If the time is -ever reset backwards, there is a small, but finite, probability that a nonce -will be reused. - -[27] An application code in the encrypted part of a message provides an -additional check that the message was decrypted properly. - -[29] An application code in the encrypted part of a message provides an -additional check that the message was decrypted properly. - -[31] An application code in the encrypted part of a message provides an -additional check that the message was decrypted properly. - -[32] If supported by the encryption method in use, an initialization vector -may be passed to the encryption procedure, in order to achieve proper cipher -chaining. The initialization vector might come from the last block of the -ciphertext from the previous KRB_PRIV message, but it is the application's -choice whether or not to use such an initialization vector. If left out, the -default initialization vector for the encryption algorithm will be used. - -[33] This prevents an attacker who generates an incorrect AS request from -obtaining verifiable plaintext for use in an off-line password guessing -attack. - -[35] In the above specification, UNTAGGED OCTET STRING(length) is the -notation for an octet string with its tag and length removed. It is not a -valid ASN.1 type. The tag bits and length must be removed from the -confounder since the purpose of the confounder is so that the message starts -with random data, but the tag and its length are fixed. For other fields, -the length and tag would be redundant if they were included because they are -specified by the encryption type. [36] The ordering of the fields in the -CipherText is important. Additionally, messages encoded in this format must -include a length as part of the msg-seq field. This allows the recipient to -verify that the message has not been truncated. Without a length, an -attacker could use a chosen plaintext attack to generate a message which -could be truncated, while leaving the checksum intact. Note that if the -msg-seq is an encoding of an ASN.1 SEQUENCE or OCTET STRING, then the length -is part of that encoding. - -[37] In some cases, it may be necessary to use a different "mix-in" string -for compatibility reasons; see the discussion of padata in section 5.4.2. - -[38] In some cases, it may be necessary to use a different "mix-in" string -for compatibility reasons; see the discussion of padata in section 5.4.2. - -[39] A variant of the key is used to limit the use of a key to a particular -function, separating the functions of generating a checksum from other -encryption performed using the session key. The constant F0F0F0F0F0F0F0F0 -was chosen because it maintains key parity. The properties of DES precluded - - -draft-ietf-cat-kerberos-r-01 Expires 21 May 1998 - -the use of the complement. The same constant is used for similar purpose in -the Message Integrity Check in the Privacy Enhanced Mail standard. - -[40] This error carries additional information in the e- data field. The -contents of the e-data field for this message is described in section 5.9.1. |