deattack.c

/*
 * $Id: deattack.c,v 1.2 1999/11/08 05:15:55 damien 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 "includes.h"
#include "deattack.h"
#include "ssh.h"
#include "crc32.h"
#include "getput.h"
#include "xmalloc.h"

/* 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) GET_32BIT(x)

#define CMP(a,b) (memcmp(a, b, SSH_BLOCKSIZE))


void
crc_update(u_int32_t * a, u_int32_t b)
{
  b ^= *a;
  *a = crc32((unsigned 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)
{
  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);
}


/*
   detect_attack
   Detects a crc32 compensation attack on a packet
 */
int
detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
{
  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;

  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.");
    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;
	}
      }
    }
    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;
	}
      } 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);
}