crypto/openssh/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 1.18 2002/03/04 17:27:39 stevesk Exp $");
  22
  23 #include "deattack.h"
  24 #include "log.h"
  25 #include "crc32.h"
  26 #include "getput.h"
  27 #include "xmalloc.h"
  28 #include "deattack.h"
  29
  30 /* SSH Constants */
  31 #define SSH_MAXBLOCKS   (32 * 1024)
  32 #define SSH_BLOCKSIZE   (8)
  33
  34 /* Hashing constants */
  35 #define HASH_MINSIZE    (8 * 1024)
  36 #define HASH_ENTRYSIZE  (2)
  37 #define HASH_FACTOR(x)  ((x)*3/2)
  38 #define HASH_UNUSEDCHAR (0xff)
  39 #define HASH_UNUSED     (0xffff)
  40 #define HASH_IV         (0xfffe)
  41
  42 #define HASH_MINBLOCKS  (7*SSH_BLOCKSIZE)
  43
  44
  45 /* Hash function (Input keys are cipher results) */
  46 #define HASH(x)         GET_32BIT(x)
  47
  48 #define CMP(a, b)       (memcmp(a, b, SSH_BLOCKSIZE))
  49
  50 static void
  51 crc_update(u_int32_t *a, u_int32_t b)
  52 {
  53         b ^= *a;
  54         *a = ssh_crc32((u_char *) &b, sizeof(b));
  55 }
  56
  57 /* detect if a block is used in a particular pattern */
  58 static int
  59 check_crc(u_char *S, u_char *buf, u_int32_t len,
  60           u_char *IV)
  61 {
  62         u_int32_t crc;
  63         u_char *c;
  64
  65         crc = 0;
  66         if (IV && !CMP(S, IV)) {
  67                 crc_update(&crc, 1);
  68                 crc_update(&crc, 0);
  69         }
  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, u_char *IV)
  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                 n = l;
 104                 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
 105         } else {
 106                 if (l > n) {
 107                         n = l;
 108                         h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
 109                 }
 110         }
 111
 112         if (len <= HASH_MINBLOCKS) {
 113                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
 114                         if (IV && (!CMP(c, IV))) {
 115                                 if ((check_crc(c, buf, len, IV)))
 116                                         return (DEATTACK_DETECTED);
 117                                 else
 118                                         break;
 119                         }
 120                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
 121                                 if (!CMP(c, d)) {
 122                                         if ((check_crc(c, buf, len, IV)))
 123                                                 return (DEATTACK_DETECTED);
 124                                         else
 125                                                 break;
 126                                 }
 127                         }
 128                 }
 129                 return (DEATTACK_OK);
 130         }
 131         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 132
 133         if (IV)
 134                 h[HASH(IV) & (n - 1)] = HASH_IV;
 135
 136         for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
 137                 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
 138                     i = (i + 1) & (n - 1)) {
 139                         if (h[i] == HASH_IV) {
 140                                 if (!CMP(c, IV)) {
 141                                         if (check_crc(c, buf, len, IV))
 142                                                 return (DEATTACK_DETECTED);
 143                                         else
 144                                                 break;
 145                                 }
 146                         } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
 147                                 if (check_crc(c, buf, len, IV))
 148                                         return (DEATTACK_DETECTED);
 149                                 else
 150                                         break;
 151                         }
 152                 }
 153                 h[i] = j;
 154         }
 155         return (DEATTACK_OK);
 156 }