-/* $OpenBSD: deattack.c,v 1.26 2006/03/25 13:17:01 djm Exp $ */
+/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */
/*
* Cryptographic attack detector for ssh - source code
*
#include "includes.h"
+#include <sys/types.h>
+
+#include <string.h>
+#include <stdio.h>
+#include <stdarg.h>
+
+#include "xmalloc.h"
#include "deattack.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)
/* Hash function (Input keys are cipher results) */
-#define HASH(x) GET_32BIT(x)
+#define HASH(x) get_u32(x)
#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
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;
+ u_int32_t l, same;
u_char *c;
u_char *d;
}
memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
- for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
+ 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