1 /*
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>
17 * <http://www.core-sdi.com>
18 */
19
20 #include "includes.h"
21 RCSID("$OpenBSD: deattack.c,v 1.18 2002/03/04 17:27:39 stevesk Exp $");
22
23 #pragma ident "%Z%%M% %I% %E% SMI"
24
25 #include "deattack.h"
26 #include "log.h"
27 #include "crc32.h"
28 #include "getput.h"
29 #include "xmalloc.h"
30 #include "deattack.h"
31
32 /*
33 * CRC attack detection has a worst-case behaviour that is O(N^2) over
34 * the number of identical blocks in a packet. This behaviour can be
35 * exploited to create a limited denial of service attack.
36 *
37 * However, because we are dealing with encrypted data, identical
38 * blocks should only occur every 2^35 maximally-sized packets or so.
39 * Consequently, we can detect this DoS by looking for identical blocks
40 * in a packet.
41 *
42 * The parameter below determines how many identical blocks we will
43 * accept in a single packet, trading off between attack detection and
44 * likelihood of terminating a legitimate connection. A value of 32
45 * corresponds to an average of 2^40 messages before an attack is
46 * misdetected
47 */
48 #define MAX_IDENTICAL 32
49
50 /* SSH Constants */
51 #define SSH_MAXBLOCKS (32 * 1024)
52 #define SSH_BLOCKSIZE (8)
53
54 /* Hashing constants */
55 #define HASH_MINSIZE (8 * 1024)
56 #define HASH_ENTRYSIZE (2)
57 #define HASH_FACTOR(x) ((x)*3/2)
58 #define HASH_UNUSEDCHAR (0xff)
59 #define HASH_UNUSED (0xffff)
60 #define HASH_IV (0xfffe)
61
62 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
63
64
65 /* Hash function (Input keys are cipher results) */
66 #define HASH(x) GET_32BIT(x)
67
68 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
69
70 static void
crc_update(u_int32_t * a,u_int32_t b)71 crc_update(u_int32_t *a, u_int32_t b)
72 {
73 b ^= *a;
74 *a = ssh_crc32((u_char *) &b, sizeof(b));
75 }
76
77 /* detect if a block is used in a particular pattern */
78 static int
check_crc(u_char * S,u_char * buf,u_int32_t len,u_char * IV)79 check_crc(u_char *S, u_char *buf, u_int32_t len,
80 u_char *IV)
81 {
82 u_int32_t crc;
83 u_char *c;
84
85 crc = 0;
86 if (IV && !CMP(S, IV)) {
87 crc_update(&crc, 1);
88 crc_update(&crc, 0);
89 }
90 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
91 if (!CMP(S, c)) {
92 crc_update(&crc, 1);
93 crc_update(&crc, 0);
94 } else {
95 crc_update(&crc, 0);
96 crc_update(&crc, 0);
97 }
98 }
99 return (crc == 0);
100 }
101
102
103 /* Detect a crc32 compensation attack on a packet */
104 int
detect_attack(u_char * buf,u_int32_t len,u_char * IV)105 detect_attack(u_char *buf, u_int32_t len, u_char *IV)
106 {
107 static u_int16_t *h = (u_int16_t *) NULL;
108 static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
109 u_int32_t i, j;
110 u_int32_t l, same;
111 u_char *c;
112 u_char *d;
113
114 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
115 len % SSH_BLOCKSIZE != 0) {
116 fatal("detect_attack: bad length %d", len);
117 }
118 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
119 ;
120
121 if (h == NULL) {
122 debug("Installing crc compensation attack detector.");
123 n = l;
124 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
125 } else {
126 if (l > n) {
127 n = l;
128 h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
129 }
130 }
131
132 if (len <= HASH_MINBLOCKS) {
133 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
134 if (IV && (!CMP(c, IV))) {
135 if ((check_crc(c, buf, len, IV)))
136 return (DEATTACK_DETECTED);
137 else
138 break;
139 }
140 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
141 if (!CMP(c, d)) {
142 if ((check_crc(c, buf, len, IV)))
143 return (DEATTACK_DETECTED);
144 else
145 break;
146 }
147 }
148 }
149 return (DEATTACK_OK);
150 }
151 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
152
153 if (IV)
154 h[HASH(IV) & (n - 1)] = HASH_IV;
155
156 for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
157 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
158 i = (i + 1) & (n - 1)) {
159 if (h[i] == HASH_IV) {
160 if (!CMP(c, IV)) {
161 if (check_crc(c, buf, len, IV))
162 return (DEATTACK_DETECTED);
163 else
164 break;
165 }
166 } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
167 if (++same > MAX_IDENTICAL)
168 return (DEATTACK_DOS_DETECTED);
169 if (check_crc(c, buf, len, IV))
170 return (DEATTACK_DETECTED);
171 else
172 break;
173 }
174 }
175 h[i] = j;
176 }
177 return (DEATTACK_OK);
178 }
179