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@juniper.net)
  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
  30 #ifdef HAVE_LIBCRYPTO
  31 #include <openssl/md5.h>
  32 #endif
  33
  34 const struct tok signature_check_values[] = {
  35     { SIGNATURE_VALID, "valid"},
  36     { SIGNATURE_INVALID, "invalid"},
  37     { CANT_ALLOCATE_COPY, "can't allocate memory"},
  38     { CANT_CHECK_SIGNATURE, "unchecked"},
  39     { 0, NULL }
  40 };
  41
  42
  43 #ifdef HAVE_LIBCRYPTO
  44 /*
  45  * Compute a HMAC MD5 sum.
  46  * Taken from rfc2104, Appendix.
  47  */
  48 USES_APPLE_DEPRECATED_API
  49 static void
  50 signature_compute_hmac_md5(const uint8_t *text, int text_len, unsigned char *key,
  51                            unsigned int key_len, uint8_t *digest)
  52 {
  53     MD5_CTX context;
  54     unsigned char k_ipad[65];    /* inner padding - key XORd with ipad */
  55     unsigned char k_opad[65];    /* outer padding - key XORd with opad */
  56     unsigned char tk[16];
  57     int i;
  58
  59     /* if key is longer than 64 bytes reset it to key=MD5(key) */
  60     if (key_len > 64) {
  61
  62         MD5_CTX tctx;
  63
  64         MD5_Init(&tctx);
  65         MD5_Update(&tctx, key, key_len);
  66         MD5_Final(tk, &tctx);
  67
  68         key = tk;
  69         key_len = 16;
  70     }
  71
  72     /*
  73      * the HMAC_MD5 transform looks like:
  74      *
  75      * MD5(K XOR opad, MD5(K XOR ipad, text))
  76      *
  77      * where K is an n byte key
  78      * ipad is the byte 0x36 repeated 64 times
  79      * opad is the byte 0x5c repeated 64 times
  80      * and text is the data being protected
  81      */
  82
  83     /* start out by storing key in pads */
  84     memset(k_ipad, 0, sizeof k_ipad);
  85     memset(k_opad, 0, sizeof k_opad);
  86     memcpy(k_ipad, key, key_len);
  87     memcpy(k_opad, key, key_len);
  88
  89     /* XOR key with ipad and opad values */
  90     for (i=0; i<64; i++) {
  91         k_ipad[i] ^= 0x36;
  92         k_opad[i] ^= 0x5c;
  93     }
  94
  95     /*
  96      * perform inner MD5
  97      */
  98     MD5_Init(&context);                   /* init context for 1st pass */
  99     MD5_Update(&context, k_ipad, 64);     /* start with inner pad */
 100     MD5_Update(&context, text, text_len); /* then text of datagram */
 101     MD5_Final(digest, &context);          /* finish up 1st pass */
 102
 103     /*
 104      * perform outer MD5
 105      */
 106     MD5_Init(&context);                   /* init context for 2nd pass */
 107     MD5_Update(&context, k_opad, 64);     /* start with outer pad */
 108     MD5_Update(&context, digest, 16);     /* then results of 1st hash */
 109     MD5_Final(digest, &context);          /* finish up 2nd pass */
 110 }
 111 USES_APPLE_RST
 112 #endif
 113
 114 #ifdef HAVE_LIBCRYPTO
 115 /*
 116  * Verify a cryptographic signature of the packet.
 117  * Currently only MD5 is supported.
 118  */
 119 int
 120 signature_verify(netdissect_options *ndo, const u_char *pptr, u_int plen,
 121                  const u_char *sig_ptr, void (*clear_rtn)(void *),
 122                  const void *clear_arg)
 123 {
 124     uint8_t *packet_copy, *sig_copy;
 125     uint8_t sig[16];
 126     unsigned int i;
 127
 128     if (!ndo->ndo_sigsecret) {
 129         return (CANT_CHECK_SIGNATURE);
 130     }
 131
 132     /*
 133      * Do we have all the packet data to be checked?
 134      */
 135     if (!ND_TTEST2(pptr, plen)) {
 136         /* No. */
 137         return (CANT_CHECK_SIGNATURE);
 138     }
 139
 140     /*
 141      * Do we have the entire signature to check?
 142      */
 143     if (!ND_TTEST2(sig_ptr, sizeof(sig))) {
 144         /* No. */
 145         return (CANT_CHECK_SIGNATURE);
 146     }
 147     if ((sig_ptr + sizeof(sig) - pptr) > plen) {
 148         /* No. */
 149         return (CANT_CHECK_SIGNATURE);
 150     }
 151
 152     /*
 153      * Make a copy of the packet, so we don't overwrite the original.
 154      */
 155     packet_copy = malloc(plen);
 156     if (packet_copy == NULL) {
 157         return (CANT_ALLOCATE_COPY);
 158     }
 159
 160     memcpy(packet_copy, pptr, plen);
 161
 162     /*
 163      * Clear the signature in the copy.
 164      */
 165     sig_copy = packet_copy + (sig_ptr - pptr);
 166     memset(sig_copy, 0, sizeof(sig));
 167
 168     /*
 169      * Clear anything else that needs to be cleared in the copy.
 170      * Our caller is assumed to have vetted the clear_arg pointer.
 171      */
 172     (*clear_rtn)((void *)(packet_copy + ((const u_int8_t *)clear_arg - pptr)));
 173
 174     /*
 175      * Compute the signature.
 176      */
 177     signature_compute_hmac_md5(packet_copy, plen,
 178                                (unsigned char *)ndo->ndo_sigsecret,
 179                                strlen(ndo->ndo_sigsecret), sig);
 180
 181     /*
 182      * Free the copy.
 183      */
 184     free(packet_copy);
 185
 186     /*
 187      * Does the computed signature match the signature in the packet?
 188      */
 189     if (memcmp(sig_ptr, sig, sizeof(sig)) == 0) {
 190         /* Yes. */
 191         return (SIGNATURE_VALID);
 192     } else {
 193         /* No - print the computed signature. */
 194         for (i = 0; i < sizeof(sig); ++i) {
 195             ND_PRINT((ndo, "%02x", sig[i]));
 196         }
 197
 198         return (SIGNATURE_INVALID);
 199     }
 200 }
 201 #endif
 202
 203 /*
 204  * Local Variables:
 205  * c-style: whitesmith
 206  * c-basic-offset: 4
 207  * End:
 208  */