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Damien Millere4340be2000-09-16 13:29:08 +11001/* $OpenBSD: deattack.c,v 1.9 2000/09/07 20:27:51 deraadt Exp $ */
2
Damien Millerd4a8b7e1999-10-27 13:42:43 +10003/*
Damien Millerd4a8b7e1999-10-27 13:42:43 +10004 * Cryptographic attack detector for ssh - source code
5 *
6 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
7 *
8 * All rights reserved. Redistribution and use in source and binary
9 * forms, with or without modification, are permitted provided that
10 * this copyright notice is retained.
11 *
12 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
13 * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
14 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
15 * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
16 * SOFTWARE.
17 *
18 * Ariel Futoransky <futo@core-sdi.com>
Damien Miller95def091999-11-25 00:26:21 +110019 * <http://www.core-sdi.com>
20 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100021
22#include "includes.h"
23#include "deattack.h"
24#include "ssh.h"
25#include "crc32.h"
26#include "getput.h"
27#include "xmalloc.h"
28
29/* SSH Constants */
Damien Miller95def091999-11-25 00:26:21 +110030#define SSH_MAXBLOCKS (32 * 1024)
31#define SSH_BLOCKSIZE (8)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100032
33/* Hashing constants */
Damien Miller95def091999-11-25 00:26:21 +110034#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)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100040
Damien Miller95def091999-11-25 00:26:21 +110041#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100042
43
44/* Hash function (Input keys are cipher results) */
Damien Miller95def091999-11-25 00:26:21 +110045#define HASH(x) GET_32BIT(x)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100046
Damien Miller95def091999-11-25 00:26:21 +110047#define CMP(a,b) (memcmp(a, b, SSH_BLOCKSIZE))
Damien Millerd4a8b7e1999-10-27 13:42:43 +100048
49
50void
Damien Miller95def091999-11-25 00:26:21 +110051crc_update(u_int32_t *a, u_int32_t b)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100052{
Damien Miller95def091999-11-25 00:26:21 +110053 b ^= *a;
Damien Millerad833b32000-08-23 10:46:23 +100054 *a = ssh_crc32((unsigned char *) &b, sizeof(b));
Damien Millerd4a8b7e1999-10-27 13:42:43 +100055}
56
Damien Miller95def091999-11-25 00:26:21 +110057/* detect if a block is used in a particular pattern */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100058int
Damien Miller95def091999-11-25 00:26:21 +110059check_crc(unsigned char *S, unsigned char *buf, u_int32_t len,
60 unsigned char *IV)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100061{
Damien Miller95def091999-11-25 00:26:21 +110062 u_int32_t crc;
63 unsigned char *c;
Damien Millerd4a8b7e1999-10-27 13:42:43 +100064
Damien Miller95def091999-11-25 00:26:21 +110065 crc = 0;
66 if (IV && !CMP(S, IV)) {
67 crc_update(&crc, 1);
68 crc_update(&crc, 0);
69 }
70 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
71 if (!CMP(S, c)) {
72 crc_update(&crc, 1);
73 crc_update(&crc, 0);
74 } else {
75 crc_update(&crc, 0);
76 crc_update(&crc, 0);
77 }
78 }
79 return (crc == 0);
Damien Millerd4a8b7e1999-10-27 13:42:43 +100080}
81
82
Damien Miller95def091999-11-25 00:26:21 +110083/* Detect a crc32 compensation attack on a packet */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100084int
85detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
86{
Damien Miller95def091999-11-25 00:26:21 +110087 static u_int16_t *h = (u_int16_t *) NULL;
88 static u_int16_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
89 register u_int32_t i, j;
90 u_int32_t l;
91 register unsigned char *c;
92 unsigned char *d;
Damien Millerd4a8b7e1999-10-27 13:42:43 +100093
Damien Miller95def091999-11-25 00:26:21 +110094 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
95 len % SSH_BLOCKSIZE != 0) {
96 fatal("detect_attack: bad length %d", len);
Damien Millerd4a8b7e1999-10-27 13:42:43 +100097 }
Damien Miller95def091999-11-25 00:26:21 +110098 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
99 ;
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000100
Damien Miller95def091999-11-25 00:26:21 +1100101 if (h == NULL) {
102 debug("Installing crc compensation attack detector.");
103 n = l;
104 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
105 } else {
106 if (l > n) {
107 n = l;
108 h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
109 }
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000110 }
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000111
Damien Miller95def091999-11-25 00:26:21 +1100112 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);
117 else
118 break;
119 }
120 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
121 if (!CMP(c, d)) {
122 if ((check_crc(c, buf, len, IV)))
123 return (DEATTACK_DETECTED);
124 else
125 break;
126 }
127 }
128 }
129 return (DEATTACK_OK);
130 }
131 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
132
133 if (IV)
134 h[HASH(IV) & (n - 1)] = HASH_IV;
135
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) {
140 if (!CMP(c, IV)) {
141 if (check_crc(c, buf, len, IV))
142 return (DEATTACK_DETECTED);
143 else
144 break;
145 }
146 } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
147 if (check_crc(c, buf, len, IV))
148 return (DEATTACK_DETECTED);
149 else
150 break;
151 }
152 }
153 h[i] = j;
154 }
155 return (DEATTACK_OK);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000156}