1 /* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 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>
23 #include <sys/types.h>
36 * CRC attack detection has a worst-case behaviour that is O(N^3) over
37 * the number of identical blocks in a packet. This behaviour can be
38 * exploited to create a limited denial of service attack.
40 * However, because we are dealing with encrypted data, identical
41 * blocks should only occur every 2^35 maximally-sized packets or so.
42 * Consequently, we can detect this DoS by looking for identical blocks
45 * The parameter below determines how many identical blocks we will
46 * accept in a single packet, trading off between attack detection and
47 * likelihood of terminating a legitimate connection. A value of 32
48 * corresponds to an average of 2^40 messages before an attack is
51 #define MAX_IDENTICAL 32
54 #define SSH_MAXBLOCKS (32 * 1024)
55 #define SSH_BLOCKSIZE (8)
57 /* Hashing constants */
58 #define HASH_MINSIZE (8 * 1024)
59 #define HASH_ENTRYSIZE (2)
60 #define HASH_FACTOR(x) ((x)*3/2)
61 #define HASH_UNUSEDCHAR (0xff)
62 #define HASH_UNUSED (0xffff)
63 #define HASH_IV (0xfffe)
65 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
68 /* Hash function (Input keys are cipher results) */
69 #define HASH(x) get_u32(x)
71 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
74 crc_update(u_int32_t *a, u_int32_t b)
77 *a = ssh_crc32((u_char *)&b, sizeof(b));
80 /* detect if a block is used in a particular pattern */
82 check_crc(u_char *S, u_char *buf, u_int32_t len)
88 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
101 /* Detect a crc32 compensation attack on a packet */
103 detect_attack(u_char *buf, u_int32_t len)
105 static u_int16_t *h = (u_int16_t *) NULL;
106 static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
112 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
113 len % SSH_BLOCKSIZE != 0) {
114 fatal("detect_attack: bad length %d", len);
116 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
120 debug("Installing crc compensation attack detector.");
121 h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
125 h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
130 if (len <= HASH_MINBLOCKS) {
131 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
132 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
134 if ((check_crc(c, buf, len)))
135 return (DEATTACK_DETECTED);
141 return (DEATTACK_OK);
143 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
145 for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
146 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
147 i = (i + 1) & (n - 1)) {
148 if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
149 if (++same > MAX_IDENTICAL)
150 return (DEATTACK_DOS_DETECTED);
151 if (check_crc(c, buf, len))
152 return (DEATTACK_DETECTED);
159 return (DEATTACK_OK);