deattack.c

   1 /* $OpenBSD: deattack.c,v 1.28 2006/07/22 20:48:23 stevesk Exp $ */
   2 /*
   3  * Cryptographic attack detector for ssh - source code
   4  *
   5  * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
   6  *
   7  * All rights reserved. Redistribution and use in source and binary
   8  * forms, with or without modification, are permitted provided that
   9  * this copyright notice is retained.
  10  *
  11  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
  12  * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
  13  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
  14  * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
  15  * SOFTWARE.
  16  *
  17  * Ariel Futoransky <futo@core-sdi.com>
  18  * <http://www.core-sdi.com>
  19  */
  20
  21 #include "includes.h"
  22
  23 #include <string.h>
  24
  25 #include "deattack.h"
  26 #include "log.h"
  27 #include "crc32.h"
  28 #include "xmalloc.h"
  29 #include "misc.h"
  30
  31 /* SSH Constants */
  32 #define SSH_MAXBLOCKS   (32 * 1024)
  33 #define SSH_BLOCKSIZE   (8)
  34
  35 /* Hashing constants */
  36 #define HASH_MINSIZE    (8 * 1024)
  37 #define HASH_ENTRYSIZE  (2)
  38 #define HASH_FACTOR(x)  ((x)*3/2)
  39 #define HASH_UNUSEDCHAR (0xff)
  40 #define HASH_UNUSED     (0xffff)
  41 #define HASH_IV         (0xfffe)
  42
  43 #define HASH_MINBLOCKS  (7*SSH_BLOCKSIZE)
  44
  45
  46 /* Hash function (Input keys are cipher results) */
  47 #define HASH(x)         get_u32(x)
  48
  49 #define CMP(a, b)       (memcmp(a, b, SSH_BLOCKSIZE))
  50
  51 static void
  52 crc_update(u_int32_t *a, u_int32_t b)
  53 {
  54         b ^= *a;
  55         *a = ssh_crc32((u_char *)&b, sizeof(b));
  56 }
  57
  58 /* detect if a block is used in a particular pattern */
  59 static int
  60 check_crc(u_char *S, u_char *buf, u_int32_t len)
  61 {
  62         u_int32_t crc;
  63         u_char *c;
  64
  65         crc = 0;
  66         for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
  67                 if (!CMP(S, c)) {
  68                         crc_update(&crc, 1);
  69                         crc_update(&crc, 0);
  70                 } else {
  71                         crc_update(&crc, 0);
  72                         crc_update(&crc, 0);
  73                 }
  74         }
  75         return (crc == 0);
  76 }
  77
  78
  79 /* Detect a crc32 compensation attack on a packet */
  80 int
  81 detect_attack(u_char *buf, u_int32_t len)
  82 {
  83         static u_int16_t *h = (u_int16_t *) NULL;
  84         static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
  85         u_int32_t i, j;
  86         u_int32_t l;
  87         u_char *c;
  88         u_char *d;
  89
  90         if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
  91             len % SSH_BLOCKSIZE != 0) {
  92                 fatal("detect_attack: bad length %d", len);
  93         }
  94         for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
  95                 ;
  96
  97         if (h == NULL) {
  98                 debug("Installing crc compensation attack detector.");
  99                 h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
 100                 n = l;
 101         } else {
 102                 if (l > n) {
 103                         h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
 104                         n = l;
 105                 }
 106         }
 107
 108         if (len <= HASH_MINBLOCKS) {
 109                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
 110                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
 111                                 if (!CMP(c, d)) {
 112                                         if ((check_crc(c, buf, len)))
 113                                                 return (DEATTACK_DETECTED);
 114                                         else
 115                                                 break;
 116                                 }
 117                         }
 118                 }
 119                 return (DEATTACK_OK);
 120         }
 121         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 122
 123         for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
 124                 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
 125                     i = (i + 1) & (n - 1)) {
 126                         if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
 127                                 if (check_crc(c, buf, len))
 128                                         return (DEATTACK_DETECTED);
 129                                 else
 130                                         break;
 131                         }
 132                 }
 133                 h[i] = j;
 134         }
 135         return (DEATTACK_OK);
 136 }