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/* $OpenBSD: deattack.c,v 1.14 2001/06/23 15:12:18 itojun 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>
*
* Modified for use in PuTTY by Simon Tatham
*/
#include <assert.h>
#include "misc.h"
#include "ssh.h"
/* SSH Constants */
#define SSH_MAXBLOCKS (32 * 1024)
#define SSH_BLOCKSIZE (8)
/* Hashing constants */
#define HASH_MINSIZE (8 * 1024)
#define HASH_ENTRYSIZE (sizeof(uint16_t))
#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) GET_32BIT_MSB_FIRST(x)
#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
uint8_t ONE[4] = { 1, 0, 0, 0 };
uint8_t ZERO[4] = { 0, 0, 0, 0 };
struct crcda_ctx {
uint16_t *h;
uint32_t n;
};
struct crcda_ctx *crcda_make_context(void)
{
struct crcda_ctx *ret = snew(struct crcda_ctx);
ret->h = NULL;
ret->n = HASH_MINSIZE / HASH_ENTRYSIZE;
return ret;
}
void crcda_free_context(struct crcda_ctx *ctx)
{
if (ctx) {
sfree(ctx->h);
ctx->h = NULL;
sfree(ctx);
}
}
static void crc_update(uint32_t *a, const void *b)
{
*a = crc32_update(*a, make_ptrlen(b, 4));
}
/* detect if a block is used in a particular pattern */
static bool check_crc(const uint8_t *S, const uint8_t *buf,
uint32_t len, const uint8_t *IV)
{
uint32_t crc;
const uint8_t *c;
crc = 0;
if (IV && !CMP(S, IV)) {
crc_update(&crc, ONE);
crc_update(&crc, ZERO);
}
for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
if (!CMP(S, c)) {
crc_update(&crc, ONE);
crc_update(&crc, ZERO);
} else {
crc_update(&crc, ZERO);
crc_update(&crc, ZERO);
}
}
return (crc == 0);
}
/* Detect a crc32 compensation attack on a packet */
bool detect_attack(struct crcda_ctx *ctx,
const unsigned char *buf, uint32_t len,
const unsigned char *IV)
{
register uint32_t i, j;
uint32_t l;
register const uint8_t *c;
const uint8_t *d;
assert(!(len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
len % SSH_BLOCKSIZE != 0));
for (l = ctx->n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
;
if (ctx->h == NULL) {
ctx->n = l;
ctx->h = snewn(ctx->n, uint16_t);
} else {
if (l > ctx->n) {
ctx->n = l;
ctx->h = sresize(ctx->h, ctx->n, uint16_t);
}
}
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 true; /* attack detected */
else
break;
}
for (d = buf; d < c; d += SSH_BLOCKSIZE) {
if (!CMP(c, d)) {
if ((check_crc(c, buf, len, IV)))
return true; /* attack detected */
else
break;
}
}
}
return false; /* ok */
}
memset(ctx->h, HASH_UNUSEDCHAR, ctx->n * HASH_ENTRYSIZE);
if (IV)
ctx->h[HASH(IV) & (ctx->n - 1)] = HASH_IV;
for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
for (i = HASH(c) & (ctx->n - 1); ctx->h[i] != HASH_UNUSED;
i = (i + 1) & (ctx->n - 1)) {
if (ctx->h[i] == HASH_IV) {
assert(IV); /* or we wouldn't have stored HASH_IV above */
if (!CMP(c, IV)) {
if (check_crc(c, buf, len, IV))
return true; /* attack detected */
else
break;
}
} else if (!CMP(c, buf + ctx->h[i] * SSH_BLOCKSIZE)) {
if (check_crc(c, buf, len, IV))
return true; /* attack detected */
else
break;
}
}
ctx->h[i] = j;
}
return false; /* ok */
}
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