hmac_md5.c

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/* $Id: hmac_md5.c,v 1.2 2003/10/20 18:59:57 tomcollins Exp $ */

#include "global.h"
#include "md5.h"

/*
** Function: hmac_md5
*/

void hmac_md5(text, text_len, key, key_len, digest)
unsigned char*  text;                /* pointer to data stream */
int             text_len;            /* length of data stream */
unsigned char*  key;                 /* pointer to authentication key */
int             key_len;             /* length of authentication key */
unsigned char   *digest;              /* caller digest to be filled in */

{
        MD5_CTX context;
        unsigned char k_ipad[65];    /* inner padding -
                                      * key XORd with ipad
                                      */
        unsigned char k_opad[65];    /* outer padding -
                                      * key XORd with opad
                                      */
        unsigned char tk[16];
        int i;
        /* if key is longer than 64 bytes reset it to key=MD5(key) */
        if (key_len > 64) {

                MD5_CTX      tctx;

                MD5Init(&tctx);
                MD5Update(&tctx, key, key_len);
                MD5Final(tk, &tctx);

                key = tk;
                key_len = 16;
        }

        /*
         * the HMAC_MD5 transform looks like:
         *
         * MD5(K XOR opad, MD5(K XOR ipad, text))
         *
         * where K is an n byte key
         * ipad is the byte 0x36 repeated 64 times
         * opad is the byte 0x5c repeated 64 times
         * and text is the data being protected
         */

        /* start out by storing key in pads */
        bzero( k_ipad, sizeof k_ipad);
        bzero( k_opad, sizeof k_opad);
        bcopy( key, k_ipad, key_len);
        bcopy( key, k_opad, key_len);

        /* XOR key with ipad and opad values */
        for (i=0; i<64; i++) {
                k_ipad[i] ^= 0x36;
                k_opad[i] ^= 0x5c;
        }
        /*
         * perform inner MD5
         */
        MD5Init(&context);                   /* init context for 1st pass */
        MD5Update(&context, k_ipad, 64);      /* start with inner pad */
        MD5Update(&context, text, text_len); /* then text of datagram */
        MD5Final(digest, &context);          /* finish up 1st pass */
        /*
         * perform outer MD5
         */
        MD5Init(&context);                   /* init context for 2nd
                                              * pass */
        MD5Update(&context, k_opad, 64);     /* start with outer pad */
        MD5Update(&context, digest, 16);     /* then results of 1st
                                              * hash */
        MD5Final(digest, &context);          /* finish up 2nd pass */
}