1 /* $OpenBSD: deattack.c,v 1.26 2006/03/25 13:17:01 djm Exp $ */
3 * Cryptographic attack detector for ssh - source code
5 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
7 * All rights reserved. Redistribution and use in source and binary
8 * forms, with or without modification, are permitted provided that
9 * this copyright notice is retained.
11 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
12 * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
13 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
14 * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
17 * Ariel Futoransky <futo@core-sdi.com>
18 * <http://www.core-sdi.com>
30 #define SSH_MAXBLOCKS (32 * 1024)
31 #define SSH_BLOCKSIZE (8)
33 /* Hashing constants */
34 #define HASH_MINSIZE (8 * 1024)
35 #define HASH_ENTRYSIZE (2)
36 #define HASH_FACTOR(x) ((x)*3/2)
37 #define HASH_UNUSEDCHAR (0xff)
38 #define HASH_UNUSED (0xffff)
39 #define HASH_IV (0xfffe)
41 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
44 /* Hash function (Input keys are cipher results) */
45 #define HASH(x) GET_32BIT(x)
47 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
50 crc_update(u_int32_t *a, u_int32_t b)
53 *a = ssh_crc32((u_char *)&b, sizeof(b));
56 /* detect if a block is used in a particular pattern */
58 check_crc(u_char *S, u_char *buf, u_int32_t len)
64 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
77 /* Detect a crc32 compensation attack on a packet */
79 detect_attack(u_char *buf, u_int32_t len)
81 static u_int16_t *h = (u_int16_t *) NULL;
82 static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
88 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
89 len % SSH_BLOCKSIZE != 0) {
90 fatal("detect_attack: bad length %d", len);
92 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
96 debug("Installing crc compensation attack detector.");
97 h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
101 h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
106 if (len <= HASH_MINBLOCKS) {
107 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
108 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
110 if ((check_crc(c, buf, len)))
111 return (DEATTACK_DETECTED);
117 return (DEATTACK_OK);
119 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
121 for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
122 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
123 i = (i + 1) & (n - 1)) {
124 if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
125 if (check_crc(c, buf, len))
126 return (DEATTACK_DETECTED);
133 return (DEATTACK_OK);