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Damien Millerd4a8b7e1999-10-27 13:42:43 +10001/*
Damien Millerad833b32000-08-23 10:46:23 +10002 * $OpenBSD: deattack.c,v 1.8 2000/08/19 02:17:12 deraadt Exp $
Damien Millerd4a8b7e1999-10-27 13:42:43 +10003 * Cryptographic attack detector for ssh - source code
4 *
5 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
6 *
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.
10 *
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
15 * SOFTWARE.
16 *
17 * Ariel Futoransky <futo@core-sdi.com>
Damien Miller95def091999-11-25 00:26:21 +110018 * <http://www.core-sdi.com>
19 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100020
21#include "includes.h"
22#include "deattack.h"
23#include "ssh.h"
24#include "crc32.h"
25#include "getput.h"
26#include "xmalloc.h"
27
28/* SSH Constants */
Damien Miller95def091999-11-25 00:26:21 +110029#define SSH_MAXBLOCKS (32 * 1024)
30#define SSH_BLOCKSIZE (8)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100031
32/* Hashing constants */
Damien Miller95def091999-11-25 00:26:21 +110033#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)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100039
Damien Miller95def091999-11-25 00:26:21 +110040#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100041
42
43/* Hash function (Input keys are cipher results) */
Damien Miller95def091999-11-25 00:26:21 +110044#define HASH(x) GET_32BIT(x)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100045
Damien Miller95def091999-11-25 00:26:21 +110046#define CMP(a,b) (memcmp(a, b, SSH_BLOCKSIZE))
Damien Millerd4a8b7e1999-10-27 13:42:43 +100047
48
49void
Damien Miller95def091999-11-25 00:26:21 +110050crc_update(u_int32_t *a, u_int32_t b)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100051{
Damien Miller95def091999-11-25 00:26:21 +110052 b ^= *a;
Damien Millerad833b32000-08-23 10:46:23 +100053 *a = ssh_crc32((unsigned char *) &b, sizeof(b));
Damien Millerd4a8b7e1999-10-27 13:42:43 +100054}
55
Damien Miller95def091999-11-25 00:26:21 +110056/* detect if a block is used in a particular pattern */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100057int
Damien Miller95def091999-11-25 00:26:21 +110058check_crc(unsigned char *S, unsigned char *buf, u_int32_t len,
59 unsigned char *IV)
Damien Millerd4a8b7e1999-10-27 13:42:43 +100060{
Damien Miller95def091999-11-25 00:26:21 +110061 u_int32_t crc;
62 unsigned char *c;
Damien Millerd4a8b7e1999-10-27 13:42:43 +100063
Damien Miller95def091999-11-25 00:26:21 +110064 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);
Damien Millerd4a8b7e1999-10-27 13:42:43 +100079}
80
81
Damien Miller95def091999-11-25 00:26:21 +110082/* Detect a crc32 compensation attack on a packet */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100083int
84detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
85{
Damien Miller95def091999-11-25 00:26:21 +110086 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;
89 u_int32_t l;
90 register unsigned char *c;
91 unsigned char *d;
Damien Millerd4a8b7e1999-10-27 13:42:43 +100092
Damien Miller95def091999-11-25 00:26:21 +110093 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
94 len % SSH_BLOCKSIZE != 0) {
95 fatal("detect_attack: bad length %d", len);
Damien Millerd4a8b7e1999-10-27 13:42:43 +100096 }
Damien Miller95def091999-11-25 00:26:21 +110097 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
98 ;
Damien Millerd4a8b7e1999-10-27 13:42:43 +100099
Damien Miller95def091999-11-25 00:26:21 +1100100 if (h == NULL) {
101 debug("Installing crc compensation attack detector.");
102 n = l;
103 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
104 } else {
105 if (l > n) {
106 n = l;
107 h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
108 }
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000109 }
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000110
Damien Miller95def091999-11-25 00:26:21 +1100111 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);
129 }
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;
137 i = (i + 1) & (n - 1)) {
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;
153 }
154 return (DEATTACK_OK);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000155}