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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 1.19 2003/09/18 08:49:45 markus Exp $"); | |
22 | ||
23 | #include "deattack.h" | |
24 | #include "log.h" | |
25 | #include "crc32.h" | |
26 | #include "getput.h" | |
27 | #include "xmalloc.h" | |
28 | #include "deattack.h" | |
29 | ||
30 | /* SSH Constants */ | |
31 | #define SSH_MAXBLOCKS (32 * 1024) | |
32 | #define SSH_BLOCKSIZE (8) | |
33 | ||
34 | /* Hashing constants */ | |
35 | #define HASH_MINSIZE (8 * 1024) | |
36 | #define HASH_ENTRYSIZE (2) | |
37 | #define HASH_FACTOR(x) ((x)*3/2) | |
38 | #define HASH_UNUSEDCHAR (0xff) | |
39 | #define HASH_UNUSED (0xffff) | |
40 | #define HASH_IV (0xfffe) | |
41 | ||
42 | #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE) | |
43 | ||
44 | ||
45 | /* Hash function (Input keys are cipher results) */ | |
46 | #define HASH(x) GET_32BIT(x) | |
47 | ||
48 | #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE)) | |
49 | ||
50 | static void | |
51 | crc_update(u_int32_t *a, u_int32_t b) | |
52 | { | |
53 | b ^= *a; | |
54 | *a = ssh_crc32((u_char *) &b, sizeof(b)); | |
55 | } | |
56 | ||
57 | /* detect if a block is used in a particular pattern */ | |
58 | static int | |
59 | check_crc(u_char *S, u_char *buf, u_int32_t len, | |
60 | u_char *IV) | |
61 | { | |
62 | u_int32_t crc; | |
63 | u_char *c; | |
64 | ||
65 | crc = 0; | |
66 | if (IV && !CMP(S, IV)) { | |
67 | crc_update(&crc, 1); | |
68 | crc_update(&crc, 0); | |
69 | } | |
70 | for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) { | |
71 | if (!CMP(S, c)) { | |
72 | crc_update(&crc, 1); | |
73 | crc_update(&crc, 0); | |
74 | } else { | |
75 | crc_update(&crc, 0); | |
76 | crc_update(&crc, 0); | |
77 | } | |
78 | } | |
79 | return (crc == 0); | |
80 | } | |
81 | ||
82 | ||
83 | /* Detect a crc32 compensation attack on a packet */ | |
84 | int | |
85 | detect_attack(u_char *buf, u_int32_t len, u_char *IV) | |
86 | { | |
87 | static u_int16_t *h = (u_int16_t *) NULL; | |
88 | static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE; | |
89 | u_int32_t i, j; | |
90 | u_int32_t l; | |
91 | u_char *c; | |
92 | u_char *d; | |
93 | ||
94 | if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) || | |
95 | len % SSH_BLOCKSIZE != 0) { | |
96 | fatal("detect_attack: bad length %d", len); | |
97 | } | |
98 | for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2) | |
99 | ; | |
100 | ||
101 | if (h == NULL) { | |
102 | debug("Installing crc compensation attack detector."); | |
103 | h = (u_int16_t *) xmalloc(l * HASH_ENTRYSIZE); | |
104 | n = l; | |
105 | } else { | |
106 | if (l > n) { | |
107 | h = (u_int16_t *) xrealloc(h, l * HASH_ENTRYSIZE); | |
108 | n = l; | |
109 | } | |
110 | } | |
111 | ||
112 | if (len <= HASH_MINBLOCKS) { | |
113 | for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) { | |
114 | if (IV && (!CMP(c, IV))) { | |
115 | if ((check_crc(c, buf, len, IV))) | |
116 | return (DEATTACK_DETECTED); | |
117 | else | |
118 | break; | |
119 | } | |
120 | for (d = buf; d < c; d += SSH_BLOCKSIZE) { | |
121 | if (!CMP(c, d)) { | |
122 | if ((check_crc(c, buf, len, IV))) | |
123 | return (DEATTACK_DETECTED); | |
124 | else | |
125 | break; | |
126 | } | |
127 | } | |
128 | } | |
129 | return (DEATTACK_OK); | |
130 | } | |
131 | memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE); | |
132 | ||
133 | if (IV) | |
134 | h[HASH(IV) & (n - 1)] = HASH_IV; | |
135 | ||
136 | for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) { | |
137 | for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED; | |
138 | i = (i + 1) & (n - 1)) { | |
139 | if (h[i] == HASH_IV) { | |
140 | if (!CMP(c, IV)) { | |
141 | if (check_crc(c, buf, len, IV)) | |
142 | return (DEATTACK_DETECTED); | |
143 | else | |
144 | break; | |
145 | } | |
146 | } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) { | |
147 | if (check_crc(c, buf, len, IV)) | |
148 | return (DEATTACK_DETECTED); | |
149 | else | |
150 | break; | |
151 | } | |
152 | } | |
153 | h[i] = j; | |
154 | } | |
155 | return (DEATTACK_OK); | |
156 | } |