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8efc0c15 1/*
8efc0c15 2 * Cryptographic attack detector for ssh - source code
3 *
4 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
5 *
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.
9 *
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
14 * SOFTWARE.
15 *
16 * Ariel Futoransky <futo@core-sdi.com>
5260325f 17 * <http://www.core-sdi.com>
18 */
8efc0c15 19
20#include "includes.h"
db5a9db9 21RCSID("$OpenBSD: deattack.c,v 1.20 2006/02/07 03:59:20 stevesk Exp $");
93c3b6de 22
8efc0c15 23#include "deattack.h"
42f11eb2 24#include "log.h"
8efc0c15 25#include "crc32.h"
26#include "getput.h"
27#include "xmalloc.h"
28
29/* SSH Constants */
5260325f 30#define SSH_MAXBLOCKS (32 * 1024)
31#define SSH_BLOCKSIZE (8)
8efc0c15 32
33/* Hashing constants */
5260325f 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)
184eed6a 39#define HASH_IV (0xfffe)
8efc0c15 40
5260325f 41#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
8efc0c15 42
43
44/* Hash function (Input keys are cipher results) */
5260325f 45#define HASH(x) GET_32BIT(x)
8efc0c15 46
12bf85ed 47#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
8efc0c15 48
396c147e 49static void
5260325f 50crc_update(u_int32_t *a, u_int32_t b)
8efc0c15 51{
5260325f 52 b ^= *a;
1e3b8b07 53 *a = ssh_crc32((u_char *) &b, sizeof(b));
8efc0c15 54}
55
5260325f 56/* detect if a block is used in a particular pattern */
396c147e 57static int
1e3b8b07 58check_crc(u_char *S, u_char *buf, u_int32_t len,
59 u_char *IV)
8efc0c15 60{
5260325f 61 u_int32_t crc;
1e3b8b07 62 u_char *c;
5260325f 63
64 crc = 0;
65 if (IV && !CMP(S, IV)) {
66 crc_update(&crc, 1);
67 crc_update(&crc, 0);
68 }
69 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
70 if (!CMP(S, c)) {
71 crc_update(&crc, 1);
72 crc_update(&crc, 0);
73 } else {
74 crc_update(&crc, 0);
75 crc_update(&crc, 0);
76 }
77 }
78 return (crc == 0);
8efc0c15 79}
80
81
5260325f 82/* Detect a crc32 compensation attack on a packet */
8efc0c15 83int
1e3b8b07 84detect_attack(u_char *buf, u_int32_t len, u_char *IV)
8efc0c15 85{
5260325f 86 static u_int16_t *h = (u_int16_t *) NULL;
b850ecd9 87 static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
44c2ab73 88 u_int32_t i, j;
5260325f 89 u_int32_t l;
44c2ab73 90 u_char *c;
1e3b8b07 91 u_char *d;
5260325f 92
93 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
94 len % SSH_BLOCKSIZE != 0) {
95 fatal("detect_attack: bad length %d", len);
96 }
97 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
98 ;
99
100 if (h == NULL) {
101 debug("Installing crc compensation attack detector.");
c46e584f 102 h = (u_int16_t *) xmalloc(l * HASH_ENTRYSIZE);
5260325f 103 n = l;
5260325f 104 } else {
105 if (l > n) {
c46e584f 106 h = (u_int16_t *) xrealloc(h, l * HASH_ENTRYSIZE);
5260325f 107 n = l;
5260325f 108 }
109 }
110
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);
116 else
117 break;
118 }
119 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
120 if (!CMP(c, d)) {
121 if ((check_crc(c, buf, len, IV)))
122 return (DEATTACK_DETECTED);
123 else
124 break;
125 }
126 }
127 }
128 return (DEATTACK_OK);
8efc0c15 129 }
5260325f 130 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
131
132 if (IV)
133 h[HASH(IV) & (n - 1)] = HASH_IV;
134
135 for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
136 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
184eed6a 137 i = (i + 1) & (n - 1)) {
5260325f 138 if (h[i] == HASH_IV) {
139 if (!CMP(c, IV)) {
140 if (check_crc(c, buf, len, IV))
141 return (DEATTACK_DETECTED);
142 else
143 break;
144 }
145 } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
146 if (check_crc(c, buf, len, IV))
147 return (DEATTACK_DETECTED);
148 else
149 break;
150 }
151 }
152 h[i] = j;
8efc0c15 153 }
5260325f 154 return (DEATTACK_OK);
8efc0c15 155}
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