1 /* $OpenBSD: deattack.c,v 1.14 2001/06/23 15:12:18 itojun Exp $ */
4 * Cryptographic attack detector for ssh - source code
6 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
8 * All rights reserved. Redistribution and use in source and binary
9 * forms, with or without modification, are permitted provided that
10 * this copyright notice is retained.
12 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
13 * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
14 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
15 * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
18 * Ariel Futoransky <futo@core-sdi.com>
19 * <http://www.core-sdi.com>
21 * Modified for use in PuTTY by Simon Tatham
28 typedef unsigned char uchar
;
29 typedef unsigned short uint16
;
32 #define SSH_MAXBLOCKS (32 * 1024)
33 #define SSH_BLOCKSIZE (8)
35 /* Hashing constants */
36 #define HASH_MINSIZE (8 * 1024)
37 #define HASH_ENTRYSIZE (sizeof(uint16))
38 #define HASH_FACTOR(x) ((x)*3/2)
39 #define HASH_UNUSEDCHAR (0xff)
40 #define HASH_UNUSED (0xffff)
41 #define HASH_IV (0xfffe)
43 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
45 #define GET_32BIT_MSB_FIRST(cp) \
46 (((unsigned long)(unsigned char)(cp)[0] << 24) | \
47 ((unsigned long)(unsigned char)(cp)[1] << 16) | \
48 ((unsigned long)(unsigned char)(cp)[2] << 8) | \
49 ((unsigned long)(unsigned char)(cp)[3]))
51 /* Hash function (Input keys are cipher results) */
52 #define HASH(x) GET_32BIT_MSB_FIRST(x)
54 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
56 uchar ONE
[4] = { 1, 0, 0, 0 };
57 uchar ZERO
[4] = { 0, 0, 0, 0 };
59 static void crc_update(uint32
*a
, void *b
)
61 *a
= crc32_update(*a
, b
, 4);
64 /* detect if a block is used in a particular pattern */
65 static int check_crc(uchar
*S
, uchar
*buf
, uint32 len
, uchar
*IV
)
71 if (IV
&& !CMP(S
, IV
)) {
72 crc_update(&crc
, ONE
);
73 crc_update(&crc
, ZERO
);
75 for (c
= buf
; c
< buf
+ len
; c
+= SSH_BLOCKSIZE
) {
77 crc_update(&crc
, ONE
);
78 crc_update(&crc
, ZERO
);
80 crc_update(&crc
, ZERO
);
81 crc_update(&crc
, ZERO
);
87 /* Detect a crc32 compensation attack on a packet */
88 int detect_attack(uchar
*buf
, uint32 len
, uchar
*IV
)
90 static uint16
*h
= (uint16
*) NULL
;
91 static uint32 n
= HASH_MINSIZE
/ HASH_ENTRYSIZE
;
97 assert(!(len
> (SSH_MAXBLOCKS
* SSH_BLOCKSIZE
) ||
98 len
% SSH_BLOCKSIZE
!= 0));
99 for (l
= n
; l
< HASH_FACTOR(len
/ SSH_BLOCKSIZE
); l
= l
<< 2)
103 logevent("Installing CRC compensation attack detector");
105 h
= (uint16
*) smalloc(n
* HASH_ENTRYSIZE
);
109 h
= (uint16
*) srealloc(h
, n
* HASH_ENTRYSIZE
);
113 if (len
<= HASH_MINBLOCKS
) {
114 for (c
= buf
; c
< buf
+ len
; c
+= SSH_BLOCKSIZE
) {
115 if (IV
&& (!CMP(c
, IV
))) {
116 if ((check_crc(c
, buf
, len
, IV
)))
117 return 1; /* attack detected */
121 for (d
= buf
; d
< c
; d
+= SSH_BLOCKSIZE
) {
123 if ((check_crc(c
, buf
, len
, IV
)))
124 return 1; /* attack detected */
132 memset(h
, HASH_UNUSEDCHAR
, n
* HASH_ENTRYSIZE
);
135 h
[HASH(IV
) & (n
- 1)] = HASH_IV
;
137 for (c
= buf
, j
= 0; c
< (buf
+ len
); c
+= SSH_BLOCKSIZE
, j
++) {
138 for (i
= HASH(c
) & (n
- 1); h
[i
] != HASH_UNUSED
;
139 i
= (i
+ 1) & (n
- 1)) {
140 if (h
[i
] == HASH_IV
) {
142 if (check_crc(c
, buf
, len
, IV
))
143 return 1; /* attack detected */
147 } else if (!CMP(c
, buf
+ h
[i
] * SSH_BLOCKSIZE
)) {
148 if (check_crc(c
, buf
, len
, IV
))
149 return 1; /* attack detected */