1 /* $OpenBSD: deattack.c,v 1.32 2015/01/20 23:14:00 deraadt Exp $ */
3 * Cryptographic attack detector for ssh - source code
5 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
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.
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
17 * Ariel Futoransky <futo@core-sdi.com>
18 * <http://www.core-sdi.com>
33 * CRC attack detection has a worst-case behaviour that is O(N^3) over
34 * the number of identical blocks in a packet. This behaviour can be
35 * exploited to create a limited denial of service attack.
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
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
48 #define MAX_IDENTICAL 32
51 #define SSH_MAXBLOCKS (32 * 1024)
52 #define SSH_BLOCKSIZE (8)
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)
62 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
65 /* Hash function (Input keys are cipher results) */
66 #define HASH(x) PEEK_U32(x)
68 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
71 crc_update(u_int32_t
*a
, u_int32_t b
)
74 *a
= ssh_crc32((u_char
*)&b
, sizeof(b
));
77 /* detect if a block is used in a particular pattern */
79 check_crc(const u_char
*S
, const u_char
*buf
, u_int32_t len
)
85 for (c
= buf
; c
< buf
+ len
; c
+= SSH_BLOCKSIZE
) {
98 deattack_init(struct deattack_ctx
*dctx
)
100 bzero(dctx
, sizeof(*dctx
));
101 dctx
->n
= HASH_MINSIZE
/ HASH_ENTRYSIZE
;
104 /* Detect a crc32 compensation attack on a packet */
106 detect_attack(struct deattack_ctx
*dctx
, const u_char
*buf
, u_int32_t len
)
108 u_int32_t i
, j
, l
, same
;
112 if (len
> (SSH_MAXBLOCKS
* SSH_BLOCKSIZE
) ||
113 len
% SSH_BLOCKSIZE
!= 0)
114 return DEATTACK_ERROR
;
115 for (l
= dctx
->n
; l
< HASH_FACTOR(len
/ SSH_BLOCKSIZE
); l
= l
<< 2)
118 if (dctx
->h
== NULL
) {
119 if ((dctx
->h
= calloc(l
, HASH_ENTRYSIZE
)) == NULL
)
120 return DEATTACK_ERROR
;
124 if ((tmp
= reallocarray(dctx
->h
, l
, HASH_ENTRYSIZE
))
128 return DEATTACK_ERROR
;
135 if (len
<= HASH_MINBLOCKS
) {
136 for (c
= buf
; c
< buf
+ len
; c
+= SSH_BLOCKSIZE
) {
137 for (d
= buf
; d
< c
; d
+= SSH_BLOCKSIZE
) {
139 if ((check_crc(c
, buf
, len
)))
140 return DEATTACK_DETECTED
;
148 memset(dctx
->h
, HASH_UNUSEDCHAR
, dctx
->n
* HASH_ENTRYSIZE
);
150 for (c
= buf
, same
= j
= 0; c
< (buf
+ len
); c
+= SSH_BLOCKSIZE
, j
++) {
151 for (i
= HASH(c
) & (dctx
->n
- 1); dctx
->h
[i
] != HASH_UNUSED
;
152 i
= (i
+ 1) & (dctx
->n
- 1)) {
153 if (!CMP(c
, buf
+ dctx
->h
[i
] * SSH_BLOCKSIZE
)) {
154 if (++same
> MAX_IDENTICAL
)
155 return DEATTACK_DOS_DETECTED
;
156 if (check_crc(c
, buf
, len
))
157 return DEATTACK_DETECTED
;