deattack.c revision 69587 
1/*	$OpenBSD: deattack.c,v 1.10 2000/10/31 13:18:53 markus Exp $	*/
2
3/*
4 * Cryptographic attack detector for ssh - source code
5 *
6 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
7 *
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.
11 *
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
16 * SOFTWARE.
17 *
18 * Ariel Futoransky <futo@core-sdi.com>
19 * <http://www.core-sdi.com>
20 */
21
22#include "includes.h"
23#include "deattack.h"
24#include "ssh.h"
25#include "crc32.h"
26#include "getput.h"
27#include "xmalloc.h"
28
29/* SSH Constants */
30#define SSH_MAXBLOCKS	(32 * 1024)
31#define SSH_BLOCKSIZE	(8)
32
33/* Hashing constants */
34#define HASH_MINSIZE	(8 * 1024)
35#define HASH_ENTRYSIZE	(2)
36#define HASH_FACTOR(x)	((x)*3/2)
37#define HASH_UNUSEDCHAR	(0xff)
38#define HASH_UNUSED	(0xffff)
39#define HASH_IV     	(0xfffe)
40
41#define HASH_MINBLOCKS	(7*SSH_BLOCKSIZE)
42
43
44/* Hash function (Input keys are cipher results) */
45#define HASH(x)		GET_32BIT(x)
46
47#define CMP(a,b)	(memcmp(a, b, SSH_BLOCKSIZE))
48
49
50void
51crc_update(u_int32_t *a, u_int32_t b)
52{
53	b ^= *a;
54	*a = ssh_crc32((unsigned char *) &b, sizeof(b));
55}
56
57/* detect if a block is used in a particular pattern */
58int
59check_crc(unsigned char *S, unsigned char *buf, u_int32_t len,
60	  unsigned char *IV)
61{
62	u_int32_t crc;
63	unsigned 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 */
84int
85detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
86{
87	static u_int16_t *h = (u_int16_t *) NULL;
88	static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
89	register u_int32_t i, j;
90	u_int32_t l;
91	register unsigned char *c;
92	unsigned 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		n = l;
104		h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
105	} else {
106		if (l > n) {
107			n = l;
108			h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
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}
157