deattack.c

   1 /*
   2  * Cryptographic attack detector for ssh - source code
   3  *
   4  * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
   5  *
   6  * All rights reserved. Redistribution and use in source and binary
   7  * forms, with or without modification, are permitted provided that
   8  * this copyright notice is retained.
   9  *
  10  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
  11  * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
  12  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
  13  * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
  14  * SOFTWARE.
  15  *
  16  * Ariel Futoransky <futo@core-sdi.com>
  17  * <http://www.core-sdi.com>
  18  */
  19
  20 #include "includes.h"
  21 RCSID("$OpenBSD: deattack.c,v 1.20 2006/02/07 03:59:20 stevesk Exp $");
  22
  23 #include "deattack.h"
  24 #include "log.h"
  25 #include "crc32.h"
  26 #include "getput.h"
  27 #include "xmalloc.h"
  28
  29 /* SSH Constants */
  30 #define SSH_MAXBLOCKS   (32 * 1024)
  31 #define SSH_BLOCKSIZE   (8)
  32
  33 /* Hashing constants */
  34 #define HASH_MINSIZE    (8 * 1024)
  35 #define HASH_ENTRYSIZE  (2)
  36 #define HASH_FACTOR(x)  ((x)*3/2)
  37 #define HASH_UNUSEDCHAR (0xff)
  38 #define HASH_UNUSED     (0xffff)
  39 #define HASH_IV         (0xfffe)
  40
  41 #define HASH_MINBLOCKS  (7*SSH_BLOCKSIZE)
  42
  43
  44 /* Hash function (Input keys are cipher results) */
  45 #define HASH(x)         GET_32BIT(x)
  46
  47 #define CMP(a, b)       (memcmp(a, b, SSH_BLOCKSIZE))
  48
  49 static void
  50 crc_update(u_int32_t *a, u_int32_t b)
  51 {
  52         b ^= *a;
  53         *a = ssh_crc32((u_char *) &b, sizeof(b));
  54 }
  55
  56 /* detect if a block is used in a particular pattern */
  57 static int
  58 check_crc(u_char *S, u_char *buf, u_int32_t len,
  59           u_char *IV)
  60 {
  61         u_int32_t crc;
  62         u_char *c;
  63
  64         crc = 0;
  65         if (IV && !CMP(S, IV)) {
  66                 crc_update(&crc, 1);
  67                 crc_update(&crc, 0);
  68         }
  69         for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
  70                 if (!CMP(S, c)) {
  71                         crc_update(&crc, 1);
  72                         crc_update(&crc, 0);
  73                 } else {
  74                         crc_update(&crc, 0);
  75                         crc_update(&crc, 0);
  76                 }
  77         }
  78         return (crc == 0);
  79 }
  80
  81
  82 /* Detect a crc32 compensation attack on a packet */
  83 int
  84 detect_attack(u_char *buf, u_int32_t len, u_char *IV)
  85 {
  86         static u_int16_t *h = (u_int16_t *) NULL;
  87         static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
  88         u_int32_t i, j;
  89         u_int32_t l;
  90         u_char *c;
  91         u_char *d;
  92
  93         if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
  94             len % SSH_BLOCKSIZE != 0) {
  95                 fatal("detect_attack: bad length %d", len);
  96         }
  97         for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
  98                 ;
  99
 100         if (h == NULL) {
 101                 debug("Installing crc compensation attack detector.");
 102                 h = (u_int16_t *) xmalloc(l * HASH_ENTRYSIZE);
 103                 n = l;
 104         } else {
 105                 if (l > n) {
 106                         h = (u_int16_t *) xrealloc(h, l * HASH_ENTRYSIZE);
 107                         n = l;
 108                 }
 109         }
 110
 111         if (len <= HASH_MINBLOCKS) {
 112                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
 113                         if (IV && (!CMP(c, IV))) {
 114                                 if ((check_crc(c, buf, len, IV)))
 115                                         return (DEATTACK_DETECTED);
 116                                 else
 117                                         break;
 118                         }
 119                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
 120                                 if (!CMP(c, d)) {
 121                                         if ((check_crc(c, buf, len, IV)))
 122                                                 return (DEATTACK_DETECTED);
 123                                         else
 124                                                 break;
 125                                 }
 126                         }
 127                 }
 128                 return (DEATTACK_OK);
 129         }
 130         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 131
 132         if (IV)
 133                 h[HASH(IV) & (n - 1)] = HASH_IV;
 134
 135         for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
 136                 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
 137                     i = (i + 1) & (n - 1)) {
 138                         if (h[i] == HASH_IV) {
 139                                 if (!CMP(c, IV)) {
 140                                         if (check_crc(c, buf, len, IV))
 141                                                 return (DEATTACK_DETECTED);
 142                                         else
 143                                                 break;
 144                                 }
 145                         } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
 146                                 if (check_crc(c, buf, len, IV))
 147                                         return (DEATTACK_DETECTED);
 148                                 else
 149                                         break;
 150                         }
 151                 }
 152                 h[i] = j;
 153         }
 154         return (DEATTACK_OK);
 155 }