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 /* Hash function (Input keys are cipher results) */
46 #define HASH(x) GET_32BIT_MSB_FIRST(x)
48 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
50 uchar ONE
[4] = { 1, 0, 0, 0 };
51 uchar ZERO
[4] = { 0, 0, 0, 0 };
58 void *crcda_make_context(void)
60 struct crcda_ctx
*ret
= snew(struct crcda_ctx
);
62 ret
->n
= HASH_MINSIZE
/ HASH_ENTRYSIZE
;
66 void crcda_free_context(void *handle
)
68 struct crcda_ctx
*ctx
= (struct crcda_ctx
*)handle
;
76 static void crc_update(uint32
*a
, void *b
)
78 *a
= crc32_update(*a
, b
, 4);
81 /* detect if a block is used in a particular pattern */
82 static int check_crc(uchar
*S
, uchar
*buf
, uint32 len
, uchar
*IV
)
88 if (IV
&& !CMP(S
, IV
)) {
89 crc_update(&crc
, ONE
);
90 crc_update(&crc
, ZERO
);
92 for (c
= buf
; c
< buf
+ len
; c
+= SSH_BLOCKSIZE
) {
94 crc_update(&crc
, ONE
);
95 crc_update(&crc
, ZERO
);
97 crc_update(&crc
, ZERO
);
98 crc_update(&crc
, ZERO
);
104 /* Detect a crc32 compensation attack on a packet */
105 int detect_attack(void *handle
, uchar
*buf
, uint32 len
, uchar
*IV
)
107 struct crcda_ctx
*ctx
= (struct crcda_ctx
*)handle
;
108 register uint32 i
, j
;
113 assert(!(len
> (SSH_MAXBLOCKS
* SSH_BLOCKSIZE
) ||
114 len
% SSH_BLOCKSIZE
!= 0));
115 for (l
= ctx
->n
; l
< HASH_FACTOR(len
/ SSH_BLOCKSIZE
); l
= l
<< 2)
118 if (ctx
->h
== NULL
) {
120 ctx
->h
= snewn(ctx
->n
, uint16
);
124 ctx
->h
= sresize(ctx
->h
, ctx
->n
, uint16
);
128 if (len
<= HASH_MINBLOCKS
) {
129 for (c
= buf
; c
< buf
+ len
; c
+= SSH_BLOCKSIZE
) {
130 if (IV
&& (!CMP(c
, IV
))) {
131 if ((check_crc(c
, buf
, len
, IV
)))
132 return 1; /* attack detected */
136 for (d
= buf
; d
< c
; d
+= SSH_BLOCKSIZE
) {
138 if ((check_crc(c
, buf
, len
, IV
)))
139 return 1; /* attack detected */
147 memset(ctx
->h
, HASH_UNUSEDCHAR
, ctx
->n
* HASH_ENTRYSIZE
);
150 ctx
->h
[HASH(IV
) & (ctx
->n
- 1)] = HASH_IV
;
152 for (c
= buf
, j
= 0; c
< (buf
+ len
); c
+= SSH_BLOCKSIZE
, j
++) {
153 for (i
= HASH(c
) & (ctx
->n
- 1); ctx
->h
[i
] != HASH_UNUSED
;
154 i
= (i
+ 1) & (ctx
->n
- 1)) {
155 if (ctx
->h
[i
] == HASH_IV
) {
157 if (check_crc(c
, buf
, len
, IV
))
158 return 1; /* attack detected */
162 } else if (!CMP(c
, buf
+ ctx
->h
[i
] * SSH_BLOCKSIZE
)) {
163 if (check_crc(c
, buf
, len
, IV
))
164 return 1; /* attack detected */