+/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */
/*
- * $Id$
* Cryptographic attack detector for ssh - source code
*
* Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
* SOFTWARE.
*
* Ariel Futoransky <futo@core-sdi.com>
- * <http://www.core-sdi.com> */
+ * <http://www.core-sdi.com>
+ */
#include "includes.h"
+
+#include <sys/types.h>
+
+#include <string.h>
+#include <stdio.h>
+#include <stdarg.h>
+
+#include "xmalloc.h"
#include "deattack.h"
-#include "ssh.h"
+#include "log.h"
#include "crc32.h"
-#include "getput.h"
-#include "xmalloc.h"
+#include "misc.h"
+
+/*
+ * CRC attack detection has a worst-case behaviour that is O(N^3) over
+ * the number of identical blocks in a packet. This behaviour can be
+ * exploited to create a limited denial of service attack.
+ *
+ * However, because we are dealing with encrypted data, identical
+ * blocks should only occur every 2^35 maximally-sized packets or so.
+ * Consequently, we can detect this DoS by looking for identical blocks
+ * in a packet.
+ *
+ * The parameter below determines how many identical blocks we will
+ * accept in a single packet, trading off between attack detection and
+ * likelihood of terminating a legitimate connection. A value of 32
+ * corresponds to an average of 2^40 messages before an attack is
+ * misdetected
+ */
+#define MAX_IDENTICAL 32
/* SSH Constants */
-#define SSH_MAXBLOCKS (32 * 1024)
-#define SSH_BLOCKSIZE (8)
+#define SSH_MAXBLOCKS (32 * 1024)
+#define SSH_BLOCKSIZE (8)
/* Hashing constants */
-#define HASH_MINSIZE (8 * 1024)
-#define HASH_ENTRYSIZE (2)
-#define HASH_FACTOR(x) ((x)*3/2)
-#define HASH_UNUSEDCHAR (0xff)
-#define HASH_UNUSED (0xffff)
-#define HASH_IV (0xfffe)
+#define HASH_MINSIZE (8 * 1024)
+#define HASH_ENTRYSIZE (2)
+#define HASH_FACTOR(x) ((x)*3/2)
+#define HASH_UNUSEDCHAR (0xff)
+#define HASH_UNUSED (0xffff)
+#define HASH_IV (0xfffe)
-#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
+#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
/* Hash function (Input keys are cipher results) */
-#define HASH(x) GET_32BIT(x)
-
-#define CMP(a,b) (memcmp(a, b, SSH_BLOCKSIZE))
+#define HASH(x) get_u32(x)
+#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
-void
-crc_update(u_int32_t * a, u_int32_t b)
+static void
+crc_update(u_int32_t *a, u_int32_t b)
{
- b ^= *a;
- *a = crc32((unsigned char *) &b, sizeof(b));
+ b ^= *a;
+ *a = ssh_crc32((u_char *)&b, sizeof(b));
}
-/*
- check_crc
- detects if a block is used in a particular pattern
- */
-
-int
-check_crc(unsigned char *S, unsigned char *buf, u_int32_t len, unsigned char *IV)
+/* detect if a block is used in a particular pattern */
+static int
+check_crc(u_char *S, u_char *buf, u_int32_t len)
{
- u_int32_t crc;
- unsigned char *c;
-
- crc = 0;
- if (IV && !CMP(S, IV))
- {
- crc_update(&crc, 1);
- crc_update(&crc, 0);
- }
- for (c = buf; c < buf + len; c += SSH_BLOCKSIZE)
- {
- if (!CMP(S, c))
- {
- crc_update(&crc, 1);
- crc_update(&crc, 0);
- } else
- {
- crc_update(&crc, 0);
- crc_update(&crc, 0);
- }
- }
-
- return (crc == 0);
+ u_int32_t crc;
+ u_char *c;
+
+ crc = 0;
+ for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
+ if (!CMP(S, c)) {
+ crc_update(&crc, 1);
+ crc_update(&crc, 0);
+ } else {
+ crc_update(&crc, 0);
+ crc_update(&crc, 0);
+ }
+ }
+ return (crc == 0);
}
-/*
- detect_attack
- Detects a crc32 compensation attack on a packet
- */
+/* Detect a crc32 compensation attack on a packet */
int
-detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
+detect_attack(u_char *buf, u_int32_t len)
{
- static u_int16_t *h = (u_int16_t *) NULL;
- static u_int16_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
- register u_int32_t i, j;
- u_int32_t l;
- register unsigned char *c;
- unsigned char *d;
-
-
- assert(len <= (SSH_MAXBLOCKS * SSH_BLOCKSIZE));
- assert(len % SSH_BLOCKSIZE == 0);
-
- for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2);
-
- if (h == NULL)
- {
- debug("Installing crc compensation attack detector.");
- n = l;
- h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
- } else
- {
- if (l > n)
- {
- n = l;
- h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
- }
- }
-
-
- if (len <= HASH_MINBLOCKS)
- {
- for (c = buf; c < buf + len; c += SSH_BLOCKSIZE)
- {
- if (IV && (!CMP(c, IV)))
- {
- if ((check_crc(c, buf, len, IV)))
- return (DEATTACK_DETECTED);
- else
- break;
- }
- for (d = buf; d < c; d += SSH_BLOCKSIZE)
- {
- if (!CMP(c, d))
- {
- if ((check_crc(c, buf, len, IV)))
- return (DEATTACK_DETECTED);
- else
- break;
+ static u_int16_t *h = (u_int16_t *) NULL;
+ static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
+ u_int32_t i, j;
+ u_int32_t l, same;
+ u_char *c;
+ u_char *d;
+
+ if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
+ len % SSH_BLOCKSIZE != 0) {
+ fatal("detect_attack: bad length %d", len);
+ }
+ for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
+ ;
+
+ if (h == NULL) {
+ debug("Installing crc compensation attack detector.");
+ h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
+ n = l;
+ } else {
+ if (l > n) {
+ h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
+ n = l;
+ }
+ }
+
+ if (len <= HASH_MINBLOCKS) {
+ for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
+ for (d = buf; d < c; d += SSH_BLOCKSIZE) {
+ if (!CMP(c, d)) {
+ if ((check_crc(c, buf, len)))
+ return (DEATTACK_DETECTED);
+ else
+ break;
+ }
+ }
+ }
+ return (DEATTACK_OK);
}
- }
- }
- return (DEATTACK_OK);
- }
- memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
-
- if (IV)
- h[HASH(IV) & (n - 1)] = HASH_IV;
-
-
- for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++)
- {
- for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
- i = (i + 1) & (n - 1))
- {
- if (h[i] == HASH_IV)
- {
- if (!CMP(c, IV))
- {
- if (check_crc(c, buf, len, IV))
- return (DEATTACK_DETECTED);
- else
- break;
+ memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
+
+ for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
+ for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
+ i = (i + 1) & (n - 1)) {
+ if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
+ if (++same > MAX_IDENTICAL)
+ return (DEATTACK_DOS_DETECTED);
+ if (check_crc(c, buf, len))
+ return (DEATTACK_DETECTED);
+ else
+ break;
+ }
+ }
+ h[i] = j;
}
- } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE))
- {
- if (check_crc(c, buf, len, IV))
- return (DEATTACK_DETECTED);
- else
- break;
- }
- }
- h[i] = j;
- }
-
- return (DEATTACK_OK);
+ return (DEATTACK_OK);
}
andersk / openssh.git / blobdiff