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signature.c revision 1.5 
      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 (at) juniper.net)
     16  */
     17 
     18 #include <sys/cdefs.h>
     19 #ifndef lint
     20 __RCSID("$NetBSD: signature.c,v 1.5 2014/11/20 03:05:03 christos Exp $");
     21 #endif
     22 
     23 #define NETDISSECT_REWORKED
     24 #ifdef HAVE_CONFIG_H
     25 #include "config.h"
     26 #endif
     27 
     28 #include <tcpdump-stdinc.h>
     29 
     30 #include <string.h>
     31 
     32 #include "interface.h"
     33 #include "signature.h"
     34 
     35 #ifdef HAVE_LIBCRYPTO
     36 #include <openssl/md5.h>
     37 #endif
     38 
     39 const struct tok signature_check_values[] = {
     40     { SIGNATURE_VALID, "valid"},
     41     { SIGNATURE_INVALID, "invalid"},
     42     { CANT_CHECK_SIGNATURE, "unchecked"},
     43     { 0, NULL }
     44 };
     45 
     46 
     47 #ifdef HAVE_LIBCRYPTO
     48 /*
     49  * Compute a HMAC MD5 sum.
     50  * Taken from rfc2104, Appendix.
     51  */
     52 USES_APPLE_DEPRECATED_API
     53 static void
     54 signature_compute_hmac_md5(const uint8_t *text, int text_len, unsigned char *key,
     55                            unsigned int key_len, uint8_t *digest)
     56 {
     57     MD5_CTX context;
     58     unsigned char k_ipad[65];    /* inner padding - key XORd with ipad */
     59     unsigned char k_opad[65];    /* outer padding - key XORd with opad */
     60     unsigned char tk[16];
     61     int i;
     62 
     63     /* if key is longer than 64 bytes reset it to key=MD5(key) */
     64     if (key_len > 64) {
     65 
     66         MD5_CTX tctx;
     67 
     68         MD5_Init(&tctx);
     69         MD5_Update(&tctx, key, key_len);
     70         MD5_Final(tk, &tctx);
     71 
     72         key = tk;
     73         key_len = 16;
     74     }
     75 
     76     /*
     77      * the HMAC_MD5 transform looks like:
     78      *
     79      * MD5(K XOR opad, MD5(K XOR ipad, text))
     80      *
     81      * where K is an n byte key
     82      * ipad is the byte 0x36 repeated 64 times
     83      * opad is the byte 0x5c repeated 64 times
     84      * and text is the data being protected
     85      */
     86 
     87     /* start out by storing key in pads */
     88     memset(k_ipad, 0, sizeof k_ipad);
     89     memset(k_opad, 0, sizeof k_opad);
     90     memcpy(k_ipad, key, key_len);
     91     memcpy(k_opad, key, key_len);
     92 
     93     /* XOR key with ipad and opad values */
     94     for (i=0; i<64; i++) {
     95         k_ipad[i] ^= 0x36;
     96         k_opad[i] ^= 0x5c;
     97     }
     98 
     99     /*
    100      * perform inner MD5
    101      */
    102     MD5_Init(&context);                   /* init context for 1st pass */
    103     MD5_Update(&context, k_ipad, 64);     /* start with inner pad */
    104     MD5_Update(&context, text, text_len); /* then text of datagram */
    105     MD5_Final(digest, &context);          /* finish up 1st pass */
    106 
    107     /*
    108      * perform outer MD5
    109      */
    110     MD5_Init(&context);                   /* init context for 2nd pass */
    111     MD5_Update(&context, k_opad, 64);     /* start with outer pad */
    112     MD5_Update(&context, digest, 16);     /* then results of 1st hash */
    113     MD5_Final(digest, &context);          /* finish up 2nd pass */
    114 }
    115 USES_APPLE_RST
    116 #endif
    117 
    118 #ifdef HAVE_LIBCRYPTO
    119 /*
    120  * Verify a cryptographic signature of the packet.
    121  * Currently only MD5 is supported.
    122  */
    123 int
    124 signature_verify(netdissect_options *ndo,
    125                  const u_char *pptr, u_int plen, u_char *sig_ptr)
    126 {
    127     uint8_t rcvsig[16];
    128     uint8_t sig[16];
    129     unsigned int i;
    130 
    131     /*
    132      * Save the signature before clearing it.
    133      */
    134     memcpy(rcvsig, sig_ptr, sizeof(rcvsig));
    135     memset(sig_ptr, 0, sizeof(rcvsig));
    136 
    137     if (!ndo->ndo_sigsecret) {
    138         return (CANT_CHECK_SIGNATURE);
    139     }
    140 
    141     signature_compute_hmac_md5(pptr, plen, (unsigned char *)ndo->ndo_sigsecret,
    142                                strlen(ndo->ndo_sigsecret), sig);
    143 
    144     if (memcmp(rcvsig, sig, sizeof(sig)) == 0) {
    145         return (SIGNATURE_VALID);
    146 
    147     } else {
    148 
    149         for (i = 0; i < sizeof(sig); ++i) {
    150             ND_PRINT((ndo, "%02x", sig[i]));
    151         }
    152 
    153         return (SIGNATURE_INVALID);
    154     }
    155 }
    156 #endif
    157 
    158 /*
    159  * Local Variables:
    160  * c-style: whitesmith
    161  * c-basic-offset: 4
    162  * End:
    163  */
    164