2 * $OpenBSD: deattack.c,v 1.7 2000/06/20 01:39:41 markus 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>
29 #define SSH_MAXBLOCKS (32 * 1024)
30 #define SSH_BLOCKSIZE (8)
32 /* Hashing constants */
33 #define HASH_MINSIZE (8 * 1024)
34 #define HASH_ENTRYSIZE (2)
35 #define HASH_FACTOR(x) ((x)*3/2)
36 #define HASH_UNUSEDCHAR (0xff)
37 #define HASH_UNUSED (0xffff)
38 #define HASH_IV (0xfffe)
40 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
43 /* Hash function (Input keys are cipher results) */
44 #define HASH(x) GET_32BIT(x)
46 #define CMP(a,b) (memcmp(a, b, SSH_BLOCKSIZE))
50 crc_update(u_int32_t *a, u_int32_t b)
53 *a = crc32((unsigned char *) &b, sizeof(b));
56 /* detect if a block is used in a particular pattern */
58 check_crc(unsigned char *S, unsigned char *buf, u_int32_t len,
65 if (IV && !CMP(S, IV)) {
69 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
82 /* Detect a crc32 compensation attack on a packet */
84 detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
86 static u_int16_t *h = (u_int16_t *) NULL;
87 static u_int16_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
88 register u_int32_t i, j;
90 register unsigned char *c;
93 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
94 len % SSH_BLOCKSIZE != 0) {
95 fatal("detect_attack: bad length %d", len);
97 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
101 debug("Installing crc compensation attack detector.");
103 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
107 h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
111 if (len <= HASH_MINBLOCKS) {
112 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
113 if (IV && (!CMP(c, IV))) {
114 if ((check_crc(c, buf, len, IV)))
115 return (DEATTACK_DETECTED);
119 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
121 if ((check_crc(c, buf, len, IV)))
122 return (DEATTACK_DETECTED);
128 return (DEATTACK_OK);
130 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
133 h[HASH(IV) & (n - 1)] = HASH_IV;
135 for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
136 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
137 i = (i + 1) & (n - 1)) {
138 if (h[i] == HASH_IV) {
140 if (check_crc(c, buf, len, IV))
141 return (DEATTACK_DETECTED);
145 } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
146 if (check_crc(c, buf, len, IV))
147 return (DEATTACK_DETECTED);
154 return (DEATTACK_OK);