2 * Cryptographic attack detector for ssh - source code
4 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
6 * All rights reserved. Redistribution and use in source and binary
7 * forms, with or without modification, are permitted provided that
8 * this copyright notice is retained.
10 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
11 * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
12 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
13 * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
16 * Ariel Futoransky <futo@core-sdi.com>
17 * <http://www.core-sdi.com>
21 RCSID("$OpenBSD: deattack.c,v 1.19 2003/09/18 08:49:45 markus Exp $");
31 #define SSH_MAXBLOCKS (32 * 1024)
32 #define SSH_BLOCKSIZE (8)
34 /* Hashing constants */
35 #define HASH_MINSIZE (8 * 1024)
36 #define HASH_ENTRYSIZE (2)
37 #define HASH_FACTOR(x) ((x)*3/2)
38 #define HASH_UNUSEDCHAR (0xff)
39 #define HASH_UNUSED (0xffff)
40 #define HASH_IV (0xfffe)
42 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
45 /* Hash function (Input keys are cipher results) */
46 #define HASH(x) GET_32BIT(x)
48 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
51 crc_update(u_int32_t *a, u_int32_t b)
54 *a = ssh_crc32((u_char *) &b, sizeof(b));
57 /* detect if a block is used in a particular pattern */
59 check_crc(u_char *S, u_char *buf, u_int32_t len,
66 if (IV && !CMP(S, IV)) {
70 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
83 /* Detect a crc32 compensation attack on a packet */
85 detect_attack(u_char *buf, u_int32_t len, u_char *IV)
87 static u_int16_t *h = (u_int16_t *) NULL;
88 static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
94 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
95 len % SSH_BLOCKSIZE != 0) {
96 fatal("detect_attack: bad length %d", len);
98 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
102 debug("Installing crc compensation attack detector.");
103 h = (u_int16_t *) xmalloc(l * HASH_ENTRYSIZE);
107 h = (u_int16_t *) xrealloc(h, l * HASH_ENTRYSIZE);
112 if (len <= HASH_MINBLOCKS) {
113 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
114 if (IV && (!CMP(c, IV))) {
115 if ((check_crc(c, buf, len, IV)))
116 return (DEATTACK_DETECTED);
120 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
122 if ((check_crc(c, buf, len, IV)))
123 return (DEATTACK_DETECTED);
129 return (DEATTACK_OK);
131 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
134 h[HASH(IV) & (n - 1)] = HASH_IV;
136 for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
137 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
138 i = (i + 1) & (n - 1)) {
139 if (h[i] == HASH_IV) {
141 if (check_crc(c, buf, len, IV))
142 return (DEATTACK_DETECTED);
146 } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
147 if (check_crc(c, buf, len, IV))
148 return (DEATTACK_DETECTED);
155 return (DEATTACK_OK);