deattack.c

   1 /* $OpenBSD: deattack.c,v 1.27 2006/03/30 09:58:15 djm 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 "deattack.h"
  24 #include "log.h"
  25 #include "crc32.h"
  26 #include "xmalloc.h"
  27 #include "misc.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_u32(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 {
  60         u_int32_t crc;
  61         u_char *c;
  62
  63         crc = 0;
  64         for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
  65                 if (!CMP(S, c)) {
  66                         crc_update(&crc, 1);
  67                         crc_update(&crc, 0);
  68                 } else {
  69                         crc_update(&crc, 0);
  70                         crc_update(&crc, 0);
  71                 }
  72         }
  73         return (crc == 0);
  74 }
  75
  76
  77 /* Detect a crc32 compensation attack on a packet */
  78 int
  79 detect_attack(u_char *buf, u_int32_t len)
  80 {
  81         static u_int16_t *h = (u_int16_t *) NULL;
  82         static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
  83         u_int32_t i, j;
  84         u_int32_t l;
  85         u_char *c;
  86         u_char *d;
  87
  88         if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
  89             len % SSH_BLOCKSIZE != 0) {
  90                 fatal("detect_attack: bad length %d", len);
  91         }
  92         for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
  93                 ;
  94
  95         if (h == NULL) {
  96                 debug("Installing crc compensation attack detector.");
  97                 h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
  98                 n = l;
  99         } else {
 100                 if (l > n) {
 101                         h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
 102                         n = l;
 103                 }
 104         }
 105
 106         if (len <= HASH_MINBLOCKS) {
 107                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
 108                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
 109                                 if (!CMP(c, d)) {
 110                                         if ((check_crc(c, buf, len)))
 111                                                 return (DEATTACK_DETECTED);
 112                                         else
 113                                                 break;
 114                                 }
 115                         }
 116                 }
 117                 return (DEATTACK_OK);
 118         }
 119         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 120
 121         for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
 122                 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
 123                     i = (i + 1) & (n - 1)) {
 124                         if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
 125                                 if (check_crc(c, buf, len))
 126                                         return (DEATTACK_DETECTED);
 127                                 else
 128                                         break;
 129                         }
 130                 }
 131                 h[i] = j;
 132         }
 133         return (DEATTACK_OK);
 134 }