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Damien Millerd4a8b7e1999-10-27 13:42:43 +10001/*
2 * $Id: deattack.c,v 1.1 1999/10/27 03:42:44 damien Exp $
3 * 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
103
104 assert(len <= (SSH_MAXBLOCKS * SSH_BLOCKSIZE));
105 assert(len % SSH_BLOCKSIZE == 0);
106
107 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2);
108
109 if (h == NULL)
110 {
111 debug("Installing crc compensation attack detector.");
112 n = l;
113 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
114 } else
115 {
116 if (l > n)
117 {
118 n = l;
119 h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
120 }
121 }
122
123
124 if (len <= HASH_MINBLOCKS)
125 {
126 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE)
127 {
128 if (IV && (!CMP(c, IV)))
129 {
130 if ((check_crc(c, buf, len, IV)))
131 return (DEATTACK_DETECTED);
132 else
133 break;
134 }
135 for (d = buf; d < c; d += SSH_BLOCKSIZE)
136 {
137 if (!CMP(c, d))
138 {
139 if ((check_crc(c, buf, len, IV)))
140 return (DEATTACK_DETECTED);
141 else
142 break;
143 }
144 }
145 }
146 return (DEATTACK_OK);
147 }
148 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
149
150 if (IV)
151 h[HASH(IV) & (n - 1)] = HASH_IV;
152
153
154 for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++)
155 {
156 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
157 i = (i + 1) & (n - 1))
158 {
159 if (h[i] == HASH_IV)
160 {
161 if (!CMP(c, IV))
162 {
163 if (check_crc(c, buf, len, IV))
164 return (DEATTACK_DETECTED);
165 else
166 break;
167 }
168 } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE))
169 {
170 if (check_crc(c, buf, len, IV))
171 return (DEATTACK_DETECTED);
172 else
173 break;
174 }
175 }
176 h[i] = j;
177 }
178
179 return (DEATTACK_OK);
180}