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Damien Millerd4a8b7e1999-10-27 13:42:43 +10001/*
Damien Millerfd7c9111999-11-08 16:15:55 +11002 * $Id: deattack.c,v 1.2 1999/11/08 05:15:55 damien 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>
18 * <http://www.core-sdi.com> */
19
20#include "includes.h"
21#include "deattack.h"
22#include "ssh.h"
23#include "crc32.h"
24#include "getput.h"
25#include "xmalloc.h"
26
27/* SSH Constants */
28#define SSH_MAXBLOCKS (32 * 1024)
29#define SSH_BLOCKSIZE (8)
30
31/* Hashing constants */
32#define HASH_MINSIZE (8 * 1024)
33#define HASH_ENTRYSIZE (2)
34#define HASH_FACTOR(x) ((x)*3/2)
35#define HASH_UNUSEDCHAR (0xff)
36#define HASH_UNUSED (0xffff)
37#define HASH_IV (0xfffe)
38
39#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
40
41
42/* Hash function (Input keys are cipher results) */
43#define HASH(x) GET_32BIT(x)
44
45#define CMP(a,b) (memcmp(a, b, SSH_BLOCKSIZE))
46
47
48void
49crc_update(u_int32_t * a, u_int32_t b)
50{
51 b ^= *a;
52 *a = crc32((unsigned char *) &b, sizeof(b));
53}
54
55/*
56 check_crc
57 detects if a block is used in a particular pattern
58 */
59
60int
61check_crc(unsigned char *S, unsigned char *buf, u_int32_t len, unsigned char *IV)
62{
63 u_int32_t crc;
64 unsigned char *c;
65
66 crc = 0;
67 if (IV && !CMP(S, IV))
68 {
69 crc_update(&crc, 1);
70 crc_update(&crc, 0);
71 }
72 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE)
73 {
74 if (!CMP(S, c))
75 {
76 crc_update(&crc, 1);
77 crc_update(&crc, 0);
78 } else
79 {
80 crc_update(&crc, 0);
81 crc_update(&crc, 0);
82 }
83 }
84
85 return (crc == 0);
86}
87
88
89/*
90 detect_attack
91 Detects a crc32 compensation attack on a packet
92 */
93int
94detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
95{
96 static u_int16_t *h = (u_int16_t *) NULL;
97 static u_int16_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
98 register u_int32_t i, j;
99 u_int32_t l;
100 register unsigned char *c;
101 unsigned char *d;
102
Damien Millerfd7c9111999-11-08 16:15:55 +1100103 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
104 len % SSH_BLOCKSIZE != 0) {
105 fatal("detect_attack: bad length %d", len);
106 }
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000107
108 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2);
109
110 if (h == NULL)
111 {
112 debug("Installing crc compensation attack detector.");
113 n = l;
114 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
115 } else
116 {
117 if (l > n)
118 {
119 n = l;
120 h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
121 }
122 }
123
124
125 if (len <= HASH_MINBLOCKS)
126 {
127 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE)
128 {
129 if (IV && (!CMP(c, IV)))
130 {
131 if ((check_crc(c, buf, len, IV)))
132 return (DEATTACK_DETECTED);
133 else
134 break;
135 }
136 for (d = buf; d < c; d += SSH_BLOCKSIZE)
137 {
138 if (!CMP(c, d))
139 {
140 if ((check_crc(c, buf, len, IV)))
141 return (DEATTACK_DETECTED);
142 else
143 break;
144 }
145 }
146 }
147 return (DEATTACK_OK);
148 }
149 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
150
151 if (IV)
152 h[HASH(IV) & (n - 1)] = HASH_IV;
153
154
155 for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++)
156 {
157 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
158 i = (i + 1) & (n - 1))
159 {
160 if (h[i] == HASH_IV)
161 {
162 if (!CMP(c, IV))
163 {
164 if (check_crc(c, buf, len, IV))
165 return (DEATTACK_DETECTED);
166 else
167 break;
168 }
169 } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE))
170 {
171 if (check_crc(c, buf, len, IV))
172 return (DEATTACK_DETECTED);
173 else
174 break;
175 }
176 }
177 h[i] = j;
178 }
179
180 return (DEATTACK_OK);
181}