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author	Cy Schubert <cy@FreeBSD.org>	2025-06-05 02:47:44 +0000
committer	Cy Schubert <cy@FreeBSD.org>	2025-06-05 02:47:53 +0000
commit	7f2fe78b9dd5f51c821d771b63d2e096f6fd49e9 (patch)
tree	99556833ea7ac74dd4e70eb0019d7dbb103de921 /crypto/krb5/doc/html/admin/spake.html
parent	bf6873c5786e333d679a7838d28812febf479a8a (diff)
parent	8f7d3ef26dec89a92ec0665de84a5936310a5574 (diff)

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+<!DOCTYPE html>

+<html>

+ <head>

+ <meta charset="utf-8" />

+ <meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="generator" content="Docutils 0.17.1: http://docutils.sourceforge.net/" />

+ <title>SPAKE Preauthentication — MIT Kerberos Documentation</title>

+ <link rel="stylesheet" type="text/css" href="../_static/pygments.css" />

+ <link rel="stylesheet" type="text/css" href="../_static/agogo.css" />

+ <link rel="stylesheet" type="text/css" href="../_static/kerb.css" />

+ <script data-url_root="../" id="documentation_options" src="../_static/documentation_options.js"></script>

+ <script src="../_static/jquery.js"></script>

+ <script src="../_static/underscore.js"></script>

+ <script src="../_static/doctools.js"></script>

+ <link rel="author" title="About these documents" href="../about.html" />

+ <link rel="index" title="Index" href="../genindex.html" />

+ <link rel="search" title="Search" href="../search.html" />

+ <link rel="copyright" title="Copyright" href="../copyright.html" />

+ <link rel="next" title="Addressing dictionary attack risks" href="dictionary.html" />

+ <link rel="prev" title="OTP Preauthentication" href="otp.html" />

+ </head><body>

+ <div class="header-wrapper">

+ <div class="header">

+ <h1><a href="../index.html">MIT Kerberos Documentation</a></h1>

+ <div class="rel">

+ <a href="../index.html" title="Full Table of Contents"

+ accesskey="C">Contents</a> |

+ <a href="otp.html" title="OTP Preauthentication"

+ accesskey="P">previous</a> |

+ <a href="dictionary.html" title="Addressing dictionary attack risks"

+ accesskey="N">next</a> |

+ <a href="../genindex.html" title="General Index"

+ accesskey="I">index</a> |

+ <a href="../search.html" title="Enter search criteria"

+ accesskey="S">Search</a> |

+ <a href="mailto:krb5-bugs@mit.edu?subject=Documentation__SPAKE Preauthentication">feedback</a>

+ </div>

+ <div class="content-wrapper">

+ <div class="content">

+ <div class="document">

+ <div class="documentwrapper">

+ <div class="bodywrapper">

+ <div class="body" role="main">

+ <section id="spake-preauthentication">

+<span id="spake"></span><h1>SPAKE Preauthentication<a class="headerlink" href="#spake-preauthentication" title="Permalink to this headline">¶</a></h1>

+<p>SPAKE preauthentication (added in release 1.17) uses public key

+cryptography techniques to protect against <a class="reference internal" href="dictionary.html#dictionary"><span class="std std-ref">password dictionary

+attacks</span></a>. Unlike <a class="reference internal" href="pkinit.html#pkinit"><span class="std std-ref">PKINIT</span></a>, it does not

+require any additional infrastructure such as certificates; it simply

+needs to be turned on. Using SPAKE preauthentication may modestly

+increase the CPU and network load on the KDC.</p>

+<p>SPAKE preauthentication can use one of four elliptic curve groups for

+its password-authenticated key exchange. The recommended group is

+<code class="docutils literal notranslate"><span class="pre">edwards25519</span></code>; three NIST curves (<code class="docutils literal notranslate"><span class="pre">P-256</span></code>, <code class="docutils literal notranslate"><span class="pre">P-384</span></code>, and

+<code class="docutils literal notranslate"><span class="pre">P-521</span></code>) are also supported.</p>

+<p>By default, SPAKE with the <code class="docutils literal notranslate"><span class="pre">edwards25519</span></code> group is enabled on

+clients, but the KDC does not offer SPAKE by default. To turn it on,

+set the <strong>spake_preauth_groups</strong> variable in <a class="reference internal" href="conf_files/krb5_conf.html#libdefaults"><span class="std std-ref">[libdefaults]</span></a> to a

+list of allowed groups. This variable affects both the client and the

+KDC. Simply setting it to <code class="docutils literal notranslate"><span class="pre">edwards25519</span></code> is recommended:</p>

+<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">libdefaults</span><span class="p">]</span>

+ <span class="n">spake_preauth_groups</span> <span class="o">=</span> <span class="n">edwards25519</span>

+</pre></div>

+</div>

+<p>Set the <strong>+requires_preauth</strong> and <strong>-allow_svr</strong> flags on client

+principal entries, as you would for any preauthentication mechanism:</p>

+<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">kadmin</span><span class="p">:</span> <span class="n">modprinc</span> <span class="o">+</span><span class="n">requires_preauth</span> <span class="o">-</span><span class="n">allow_svr</span> <span class="n">PRINCNAME</span>

+</pre></div>

+</div>

+<p>Clients which do not implement SPAKE preauthentication will fall back

+to encrypted timestamp.</p>

+<p>An active attacker can force a fallback to encrypted timestamp by

+modifying the initial KDC response, defeating the protection against

+dictionary attacks. To prevent this fallback on clients which do

+implement SPAKE preauthentication, set the

+<strong>disable_encrypted_timestamp</strong> variable to <code class="docutils literal notranslate"><span class="pre">true</span></code> in the

+<a class="reference internal" href="conf_files/krb5_conf.html#realms"><span class="std std-ref">[realms]</span></a> subsection for realms whose KDCs offer SPAKE

+preauthentication.</p>

+<p>By default, SPAKE preauthentication requires an extra network round

+trip to the KDC during initial authentication. If most of the clients

+in a realm support SPAKE, this extra round trip can be eliminated

+using an optimistic challenge, by setting the

+<strong>spake_preauth_kdc_challenge</strong> variable in <a class="reference internal" href="conf_files/kdc_conf.html#kdcdefaults"><span class="std std-ref">[kdcdefaults]</span></a> to a

+single group name:</p>

+<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">kdcdefaults</span><span class="p">]</span>

+ <span class="n">spake_preauth_kdc_challenge</span> <span class="o">=</span> <span class="n">edwards25519</span>

+</pre></div>

+</div>

+<p>Using optimistic challenge will cause the KDC to do extra work for

+initial authentication requests that do not result in SPAKE

+preauthentication, but will save work when SPAKE preauthentication is

+used.</p>

+</section>

+ <div class="clearer"></div>

+ </div>

+ <div class="sidebar">

+ <h2>On this page</h2>

+ <ul>

+<li><a class="reference internal" href="#">SPAKE Preauthentication</a></li>

+</ul>

+ <br/>

+ <h2>Table of contents</h2>

+ <ul class="current">

+<li class="toctree-l1"><a class="reference internal" href="../user/index.html">For users</a></li>

+<li class="toctree-l1 current"><a class="reference internal" href="index.html">For administrators</a><ul class="current">

+<li class="toctree-l2"><a class="reference internal" href="install.html">Installation guide</a></li>

+<li class="toctree-l2"><a class="reference internal" href="conf_files/index.html">Configuration Files</a></li>

+<li class="toctree-l2"><a class="reference internal" href="realm_config.html">Realm configuration decisions</a></li>

+<li class="toctree-l2"><a class="reference internal" href="database.html">Database administration</a></li>

+<li class="toctree-l2"><a class="reference internal" href="dbtypes.html">Database types</a></li>

+<li class="toctree-l2"><a class="reference internal" href="lockout.html">Account lockout</a></li>

+<li class="toctree-l2"><a class="reference internal" href="conf_ldap.html">Configuring Kerberos with OpenLDAP back-end</a></li>

+<li class="toctree-l2"><a class="reference internal" href="appl_servers.html">Application servers</a></li>

+<li class="toctree-l2"><a class="reference internal" href="host_config.html">Host configuration</a></li>

+<li class="toctree-l2"><a class="reference internal" href="backup_host.html">Backups of secure hosts</a></li>

+<li class="toctree-l2"><a class="reference internal" href="pkinit.html">PKINIT configuration</a></li>

+<li class="toctree-l2"><a class="reference internal" href="otp.html">OTP Preauthentication</a></li>

+<li class="toctree-l2 current"><a class="current reference internal" href="#">SPAKE Preauthentication</a></li>

+<li class="toctree-l2"><a class="reference internal" href="dictionary.html">Addressing dictionary attack risks</a></li>

+<li class="toctree-l2"><a class="reference internal" href="princ_dns.html">Principal names and DNS</a></li>

+<li class="toctree-l2"><a class="reference internal" href="enctypes.html">Encryption types</a></li>

+<li class="toctree-l2"><a class="reference internal" href="https.html">HTTPS proxy configuration</a></li>

+<li class="toctree-l2"><a class="reference internal" href="auth_indicator.html">Authentication indicators</a></li>

+<li class="toctree-l2"><a class="reference internal" href="admin_commands/index.html">Administration programs</a></li>

+<li class="toctree-l2"><a class="reference internal" href="../mitK5defaults.html">MIT Kerberos defaults</a></li>

+<li class="toctree-l2"><a class="reference internal" href="env_variables.html">Environment variables</a></li>

+<li class="toctree-l2"><a class="reference internal" href="troubleshoot.html">Troubleshooting</a></li>

+<li class="toctree-l2"><a class="reference internal" href="advanced/index.html">Advanced topics</a></li>

+<li class="toctree-l2"><a class="reference internal" href="various_envs.html">Various links</a></li>

+</ul>

+</li>

+<li class="toctree-l1"><a class="reference internal" href="../appdev/index.html">For application developers</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../plugindev/index.html">For plugin module developers</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../build/index.html">Building Kerberos V5</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../basic/index.html">Kerberos V5 concepts</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../formats/index.html">Protocols and file formats</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../mitK5features.html">MIT Kerberos features</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../build_this.html">How to build this documentation from the source</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../about.html">Contributing to the MIT Kerberos Documentation</a></li>

+<li class="toctree-l1"><a class="reference internal" href="../resources.html">Resources</a></li>

+</ul>

+ <br/>

+ <h4><a href="../index.html">Full Table of Contents</a></h4>

+ <h4>Search</h4>

+ <form class="search" action="../search.html" method="get">

+ <input type="text" name="q" size="18" />

+ <input type="submit" value="Go" />

+ <input type="hidden" name="check_keywords" value="yes" />

+ <input type="hidden" name="area" value="default" />

+ </form>

+ </div>

+ <div class="clearer"></div>

+ </div>

+ <div class="footer-wrapper">

+ <div class="footer" >

+ <div class="right" ><i>Release: 1.21.3</i><br />

+ </div>

+ <div class="left">

+ <a href="../index.html" title="Full Table of Contents"

+ >Contents</a> |

+ <a href="otp.html" title="OTP Preauthentication"

+ >previous</a> |

+ <a href="dictionary.html" title="Addressing dictionary attack risks"

+ >next</a> |

+ <a href="../genindex.html" title="General Index"

+ >index</a> |

+ <a href="../search.html" title="Enter search criteria"

+ >Search</a> |

+ <a href="mailto:krb5-bugs@mit.edu?subject=Documentation__SPAKE Preauthentication">feedback</a>

+ </div>

+ </body>

+</html> \ No newline at end of file