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/* $OpenBSD: deattack.c,v 1.32 2015/01/20 23:14:00 deraadt Exp $ */
/*
* Cryptographic attack detector for ssh - source code
*
* Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
*
* All rights reserved. Redistribution and use in source and binary
* forms, with or without modification, are permitted provided that
* this copyright notice is retained.
*
* THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
* WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
* CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
* SOFTWARE.
*
* Ariel Futoransky <futo@core-sdi.com>
* <http://www.core-sdi.com>
*/
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "deattack.h"
#include "crc32.h"
#include "sshbuf.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)
/* 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_MINBLOCKS (7*SSH_BLOCKSIZE)
/* Hash function (Input keys are cipher results) */
#define HASH(x) PEEK_U32(x)
#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
static void
crc_update(u_int32_t *a, u_int32_t b)
{
b ^= *a;
*a = ssh_crc32((u_char *)&b, sizeof(b));
}
/* detect if a block is used in a particular pattern */
static int
check_crc(const u_char *S, const u_char *buf, u_int32_t len)
{
u_int32_t crc;
const 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;
}
void
deattack_init(struct deattack_ctx *dctx)
{
bzero(dctx, sizeof(*dctx));
dctx->n = HASH_MINSIZE / HASH_ENTRYSIZE;
}
/* Detect a crc32 compensation attack on a packet */
int
detect_attack(struct deattack_ctx *dctx, const u_char *buf, u_int32_t len)
{
u_int32_t i, j, l, same;
u_int16_t *tmp;
const u_char *c, *d;
if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
len % SSH_BLOCKSIZE != 0)
return DEATTACK_ERROR;
for (l = dctx->n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
;
if (dctx->h == NULL) {
if ((dctx->h = calloc(l, HASH_ENTRYSIZE)) == NULL)
return DEATTACK_ERROR;
dctx->n = l;
} else {
if (l > dctx->n) {
if ((tmp = reallocarray(dctx->h, l, HASH_ENTRYSIZE))
== NULL) {
free(dctx->h);
dctx->h = NULL;
return DEATTACK_ERROR;
}
dctx->h = tmp;
dctx->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;
}
memset(dctx->h, HASH_UNUSEDCHAR, dctx->n * HASH_ENTRYSIZE);
for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
for (i = HASH(c) & (dctx->n - 1); dctx->h[i] != HASH_UNUSED;
i = (i + 1) & (dctx->n - 1)) {
if (!CMP(c, buf + dctx->h[i] * SSH_BLOCKSIZE)) {
if (++same > MAX_IDENTICAL)
return DEATTACK_DOS_DETECTED;
if (check_crc(c, buf, len))
return DEATTACK_DETECTED;
else
break;
}
}
dctx->h[i] = j;
}
return DEATTACK_OK;
}
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