The Tcpdump Group git mirrors - tcpdump/blob - signature.c

   1 /*
   2  * Redistribution and use in source and binary forms, with or without
   3  * modification, are permitted provided that: (1) source code
   4  * distributions retain the above copyright notice and this paragraph
   5  * in its entirety, and (2) distributions including binary code include
   6  * the above copyright notice and this paragraph in its entirety in
   7  * the documentation or other materials provided with the distribution.
   8  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND
   9  * WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT
  10  * LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  11  * FOR A PARTICULAR PURPOSE.
  12  *
  13  * Functions for signature and digest verification.
  14  *
  15  * Original code by Hannes Gredler (hannes@gredler.at)
  16  */
  17
  18 #ifdef HAVE_CONFIG_H
  19 #include <config.h>
  20 #endif
  21
  22 #include "netdissect-stdinc.h"
  23
  24 #include <string.h>
  25 #include <stdlib.h>
  26
  27 #include "netdissect.h"
  28 #include "signature.h"
  29 #include "diag-control.h"
  30
  31 #ifdef HAVE_LIBCRYPTO
  32 #include <openssl/md5.h>
  33 #endif
  34
  35 const struct tok signature_check_values[] = {
  36     { SIGNATURE_VALID, "valid"},
  37     { SIGNATURE_INVALID, "invalid"},
  38     { CANT_ALLOCATE_COPY, "can't allocate memory"},
  39     { CANT_CHECK_SIGNATURE, "unchecked"},
  40     { 0, NULL }
  41 };
  42
  43
  44 #ifdef HAVE_LIBCRYPTO
  45 /*
  46  * Compute a HMAC MD5 sum.
  47  * Taken from rfc2104, Appendix.
  48  */
  49 DIAG_OFF_DEPRECATION
  50 static void
  51 signature_compute_hmac_md5(const uint8_t *text, int text_len, unsigned char *key,
  52                            unsigned int key_len, uint8_t *digest)
  53 {
  54     MD5_CTX context;
  55     unsigned char k_ipad[65];    /* inner padding - key XORd with ipad */
  56     unsigned char k_opad[65];    /* outer padding - key XORd with opad */
  57     unsigned char tk[16];
  58     int i;
  59
  60     /* if key is longer than 64 bytes reset it to key=MD5(key) */
  61     if (key_len > 64) {
  62
  63         MD5_CTX tctx;
  64
  65         MD5_Init(&tctx);
  66         MD5_Update(&tctx, key, key_len);
  67         MD5_Final(tk, &tctx);
  68
  69         key = tk;
  70         key_len = 16;
  71     }
  72
  73     /*
  74      * the HMAC_MD5 transform looks like:
  75      *
  76      * MD5(K XOR opad, MD5(K XOR ipad, text))
  77      *
  78      * where K is an n byte key
  79      * ipad is the byte 0x36 repeated 64 times
  80      * opad is the byte 0x5c repeated 64 times
  81      * and text is the data being protected
  82      */
  83
  84     /* start out by storing key in pads */
  85     memset(k_ipad, 0, sizeof(k_ipad));
  86     memset(k_opad, 0, sizeof(k_opad));
  87     memcpy(k_ipad, key, key_len);
  88     memcpy(k_opad, key, key_len);
  89
  90     /* XOR key with ipad and opad values */
  91     for (i=0; i<64; i++) {
  92         k_ipad[i] ^= 0x36;
  93         k_opad[i] ^= 0x5c;
  94     }
  95
  96     /*
  97      * perform inner MD5
  98      */
  99     MD5_Init(&context);                   /* init context for 1st pass */
 100     MD5_Update(&context, k_ipad, 64);     /* start with inner pad */
 101     MD5_Update(&context, text, text_len); /* then text of datagram */
 102     MD5_Final(digest, &context);          /* finish up 1st pass */
 103
 104     /*
 105      * perform outer MD5
 106      */
 107     MD5_Init(&context);                   /* init context for 2nd pass */
 108     MD5_Update(&context, k_opad, 64);     /* start with outer pad */
 109     MD5_Update(&context, digest, 16);     /* then results of 1st hash */
 110     MD5_Final(digest, &context);          /* finish up 2nd pass */
 111 }
 112 DIAG_ON_DEPRECATION
 113
 114 /*
 115  * Verify a cryptographic signature of the packet.
 116  * Currently only MD5 is supported.
 117  */
 118 int
 119 signature_verify(netdissect_options *ndo, const u_char *pptr, u_int plen,
 120                  const u_char *sig_ptr, void (*clear_rtn)(void *),
 121                  const void *clear_arg)
 122 {
 123     uint8_t *packet_copy, *sig_copy;
 124     uint8_t sig[16];
 125     unsigned int i;
 126
 127     if (!ndo->ndo_sigsecret) {
 128         return (CANT_CHECK_SIGNATURE);
 129     }
 130
 131     /*
 132      * Do we have all the packet data to be checked?
 133      */
 134     if (!ND_TTEST_LEN(pptr, plen)) {
 135         /* No. */
 136         return (CANT_CHECK_SIGNATURE);
 137     }
 138
 139     /*
 140      * Do we have the entire signature to check?
 141      */
 142     if (!ND_TTEST_LEN(sig_ptr, sizeof(sig))) {
 143         /* No. */
 144         return (CANT_CHECK_SIGNATURE);
 145     }
 146     if (sig_ptr + sizeof(sig) > pptr + plen) {
 147         /* No. */
 148         return (CANT_CHECK_SIGNATURE);
 149     }
 150
 151     /*
 152      * Make a copy of the packet, so we don't overwrite the original.
 153      */
 154     packet_copy = malloc(plen);
 155     if (packet_copy == NULL) {
 156         return (CANT_ALLOCATE_COPY);
 157     }
 158
 159     memcpy(packet_copy, pptr, plen);
 160
 161     /*
 162      * Clear the signature in the copy.
 163      */
 164     sig_copy = packet_copy + (sig_ptr - pptr);
 165     memset(sig_copy, 0, sizeof(sig));
 166
 167     /*
 168      * Clear anything else that needs to be cleared in the copy.
 169      * Our caller is assumed to have vetted the clear_arg pointer.
 170      */
 171     (*clear_rtn)((void *)(packet_copy + ((const uint8_t *)clear_arg - pptr)));
 172
 173     /*
 174      * Compute the signature.
 175      */
 176     signature_compute_hmac_md5(packet_copy, plen,
 177                                (unsigned char *)ndo->ndo_sigsecret,
 178                                strlen(ndo->ndo_sigsecret), sig);
 179
 180     /*
 181      * Free the copy.
 182      */
 183     free(packet_copy);
 184
 185     /*
 186      * Does the computed signature match the signature in the packet?
 187      */
 188     if (memcmp(sig_ptr, sig, sizeof(sig)) == 0) {
 189         /* Yes. */
 190         return (SIGNATURE_VALID);
 191     } else {
 192         /* No - print the computed signature. */
 193         for (i = 0; i < sizeof(sig); ++i) {
 194             ND_PRINT("%02x", sig[i]);
 195         }
 196
 197         return (SIGNATURE_INVALID);
 198     }
 199 }
 200 #else
 201 int
 202 signature_verify(netdissect_options *ndo _U_, const u_char *pptr _U_,
 203                  u_int plen _U_, const u_char *sig_ptr _U_,
 204                  void (*clear_rtn)(void *) _U_, const void *clear_arg _U_)
 205 {
 206     return (CANT_CHECK_SIGNATURE);
 207 }
 208 #endif