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author | Dag-Erling Smørgrav <des@FreeBSD.org> | 2006-09-30 13:29:51 +0000 |
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committer | Dag-Erling Smørgrav <des@FreeBSD.org> | 2006-09-30 13:29:51 +0000 |
commit | 761efaa70c2ed8d35722b7bc234a46bf2457f876 (patch) | |
tree | bba6f2fe7855d7b0095f9dc7720dc27bea4d1fdf /crypto/openssh/deattack.c | |
parent | 30c2033ae748d441213eed2e5c3fae760cc8ea61 (diff) | |
download | src-test2-761efaa70c2ed8d35722b7bc234a46bf2457f876.tar.gz src-test2-761efaa70c2ed8d35722b7bc234a46bf2457f876.zip |
Notes
Diffstat (limited to 'crypto/openssh/deattack.c')
-rw-r--r-- | crypto/openssh/deattack.c | 76 |
1 files changed, 40 insertions, 36 deletions
diff --git a/crypto/openssh/deattack.c b/crypto/openssh/deattack.c index 8b55d668681a..1b37e4dabe59 100644 --- a/crypto/openssh/deattack.c +++ b/crypto/openssh/deattack.c @@ -1,3 +1,4 @@ +/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */ /* * Cryptographic attack detector for ssh - source code * @@ -18,14 +19,36 @@ */ #include "includes.h" -RCSID("$OpenBSD: deattack.c,v 1.19 2003/09/18 08:49:45 markus Exp $"); +#include <sys/types.h> + +#include <string.h> +#include <stdio.h> +#include <stdarg.h> + +#include "xmalloc.h" #include "deattack.h" #include "log.h" #include "crc32.h" -#include "getput.h" -#include "xmalloc.h" -#include "deattack.h" +#include "misc.h" + +/* + * CRC attack detection has a worst-case behaviour that is O(N^3) over + * the number of identical blocks in a packet. This behaviour can be + * exploited to create a limited denial of service attack. + * + * However, because we are dealing with encrypted data, identical + * blocks should only occur every 2^35 maximally-sized packets or so. + * Consequently, we can detect this DoS by looking for identical blocks + * in a packet. + * + * The parameter below determines how many identical blocks we will + * accept in a single packet, trading off between attack detection and + * likelihood of terminating a legitimate connection. A value of 32 + * corresponds to an average of 2^40 messages before an attack is + * misdetected + */ +#define MAX_IDENTICAL 32 /* SSH Constants */ #define SSH_MAXBLOCKS (32 * 1024) @@ -43,7 +66,7 @@ RCSID("$OpenBSD: deattack.c,v 1.19 2003/09/18 08:49:45 markus Exp $"); /* Hash function (Input keys are cipher results) */ -#define HASH(x) GET_32BIT(x) +#define HASH(x) get_u32(x) #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE)) @@ -51,22 +74,17 @@ static void crc_update(u_int32_t *a, u_int32_t b) { b ^= *a; - *a = ssh_crc32((u_char *) &b, sizeof(b)); + *a = ssh_crc32((u_char *)&b, sizeof(b)); } /* detect if a block is used in a particular pattern */ static int -check_crc(u_char *S, u_char *buf, u_int32_t len, - u_char *IV) +check_crc(u_char *S, u_char *buf, u_int32_t len) { u_int32_t crc; u_char *c; crc = 0; - if (IV && !CMP(S, IV)) { - crc_update(&crc, 1); - crc_update(&crc, 0); - } for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) { if (!CMP(S, c)) { crc_update(&crc, 1); @@ -82,12 +100,12 @@ check_crc(u_char *S, u_char *buf, u_int32_t len, /* Detect a crc32 compensation attack on a packet */ int -detect_attack(u_char *buf, u_int32_t len, u_char *IV) +detect_attack(u_char *buf, u_int32_t len) { static u_int16_t *h = (u_int16_t *) NULL; static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE; u_int32_t i, j; - u_int32_t l; + u_int32_t l, same; u_char *c; u_char *d; @@ -100,26 +118,20 @@ detect_attack(u_char *buf, u_int32_t len, u_char *IV) if (h == NULL) { debug("Installing crc compensation attack detector."); - h = (u_int16_t *) xmalloc(l * HASH_ENTRYSIZE); + h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE); n = l; } else { if (l > n) { - h = (u_int16_t *) xrealloc(h, l * HASH_ENTRYSIZE); + h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE); n = l; } } if (len <= HASH_MINBLOCKS) { for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) { - if (IV && (!CMP(c, IV))) { - if ((check_crc(c, buf, len, IV))) - return (DEATTACK_DETECTED); - else - break; - } for (d = buf; d < c; d += SSH_BLOCKSIZE) { if (!CMP(c, d)) { - if ((check_crc(c, buf, len, IV))) + if ((check_crc(c, buf, len))) return (DEATTACK_DETECTED); else break; @@ -130,21 +142,13 @@ detect_attack(u_char *buf, u_int32_t len, u_char *IV) } memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE); - if (IV) - h[HASH(IV) & (n - 1)] = HASH_IV; - - for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) { + for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) { for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED; i = (i + 1) & (n - 1)) { - if (h[i] == HASH_IV) { - if (!CMP(c, IV)) { - if (check_crc(c, buf, len, IV)) - return (DEATTACK_DETECTED); - else - break; - } - } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) { - if (check_crc(c, buf, len, IV)) + if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) { + if (++same > MAX_IDENTICAL) + return (DEATTACK_DOS_DETECTED); + if (check_crc(c, buf, len)) return (DEATTACK_DETECTED); else break; |