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hmac_md5.c

/*
** Function: hmac_md5
*/

#include <string.h>
#include "global.h"
#include "md5.h"
#include "hmac_md5.h"

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

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