deattack.c

   1 /* $OpenBSD: deattack.c,v 1.29 2006/08/03 03:34:42 deraadt 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 <sys/types.h>
  24
  25 #include <string.h>
  26 #include <stdio.h>
  27 #include <stdarg.h>
  28
  29 #include "xmalloc.h"
  30 #include "deattack.h"
  31 #include "log.h"
  32 #include "crc32.h"
  33 #include "misc.h"
  34
  35 /* SSH Constants */
  36 #define SSH_MAXBLOCKS   (32 * 1024)
  37 #define SSH_BLOCKSIZE   (8)
  38
  39 /* Hashing constants */
  40 #define HASH_MINSIZE    (8 * 1024)
  41 #define HASH_ENTRYSIZE  (2)
  42 #define HASH_FACTOR(x)  ((x)*3/2)
  43 #define HASH_UNUSEDCHAR (0xff)
  44 #define HASH_UNUSED     (0xffff)
  45 #define HASH_IV         (0xfffe)
  46
  47 #define HASH_MINBLOCKS  (7*SSH_BLOCKSIZE)
  48
  49
  50 /* Hash function (Input keys are cipher results) */
  51 #define HASH(x)         get_u32(x)
  52
  53 #define CMP(a, b)       (memcmp(a, b, SSH_BLOCKSIZE))
  54
  55 static void
  56 crc_update(u_int32_t *a, u_int32_t b)
  57 {
  58         b ^= *a;
  59         *a = ssh_crc32((u_char *)&b, sizeof(b));
  60 }
  61
  62 /* detect if a block is used in a particular pattern */
  63 static int
  64 check_crc(u_char *S, u_char *buf, u_int32_t len)
  65 {
  66         u_int32_t crc;
  67         u_char *c;
  68
  69         crc = 0;
  70         for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
  71                 if (!CMP(S, c)) {
  72                         crc_update(&crc, 1);
  73                         crc_update(&crc, 0);
  74                 } else {
  75                         crc_update(&crc, 0);
  76                         crc_update(&crc, 0);
  77                 }
  78         }
  79         return (crc == 0);
  80 }
  81
  82
  83 /* Detect a crc32 compensation attack on a packet */
  84 int
  85 detect_attack(u_char *buf, u_int32_t len)
  86 {
  87         static u_int16_t *h = (u_int16_t *) NULL;
  88         static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
  89         u_int32_t i, j;
  90         u_int32_t l;
  91         u_char *c;
  92         u_char *d;
  93
  94         if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
  95             len % SSH_BLOCKSIZE != 0) {
  96                 fatal("detect_attack: bad length %d", len);
  97         }
  98         for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
  99                 ;
 100
 101         if (h == NULL) {
 102                 debug("Installing crc compensation attack detector.");
 103                 h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
 104                 n = l;
 105         } else {
 106                 if (l > n) {
 107                         h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
 108                         n = l;
 109                 }
 110         }
 111
 112         if (len <= HASH_MINBLOCKS) {
 113                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
 114                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
 115                                 if (!CMP(c, d)) {
 116                                         if ((check_crc(c, buf, len)))
 117                                                 return (DEATTACK_DETECTED);
 118                                         else
 119                                                 break;
 120                                 }
 121                         }
 122                 }
 123                 return (DEATTACK_OK);
 124         }
 125         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 126
 127         for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
 128                 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
 129                     i = (i + 1) & (n - 1)) {
 130                         if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
 131                                 if (check_crc(c, buf, len))
 132                                         return (DEATTACK_DETECTED);
 133                                 else
 134                                         break;
 135                         }
 136                 }
 137                 h[i] = j;
 138         }
 139         return (DEATTACK_OK);
 140 }