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1 | /* | |
2 | * $Id$ | |
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 | ||
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 */ | |
29 | #define SSH_MAXBLOCKS (32 * 1024) | |
30 | #define SSH_BLOCKSIZE (8) | |
31 | ||
32 | /* Hashing constants */ | |
33 | #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) | |
39 | ||
40 | #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE) | |
41 | ||
42 | ||
43 | /* Hash function (Input keys are cipher results) */ | |
44 | #define HASH(x) GET_32BIT(x) | |
45 | ||
46 | #define CMP(a,b) (memcmp(a, b, SSH_BLOCKSIZE)) | |
47 | ||
48 | ||
49 | void | |
50 | crc_update(u_int32_t *a, u_int32_t b) | |
51 | { | |
52 | b ^= *a; | |
53 | *a = crc32((unsigned char *) &b, sizeof(b)); | |
54 | } | |
55 | ||
56 | /* detect if a block is used in a particular pattern */ | |
57 | int | |
58 | check_crc(unsigned char *S, unsigned char *buf, u_int32_t len, | |
59 | unsigned char *IV) | |
60 | { | |
61 | u_int32_t crc; | |
62 | unsigned char *c; | |
63 | ||
64 | 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); | |
79 | } | |
80 | ||
81 | ||
82 | /* Detect a crc32 compensation attack on a packet */ | |
83 | int | |
84 | detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV) | |
85 | { | |
86 | 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; | |
92 | ||
93 | if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) || | |
94 | len % SSH_BLOCKSIZE != 0) { | |
95 | fatal("detect_attack: bad length %d", len); | |
96 | } | |
97 | for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2) | |
98 | ; | |
99 | ||
100 | 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 | } | |
109 | } | |
110 | ||
111 | 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); | |
155 | } |