The HMAC Algorithm 
by William Stallings

Listing One
/* 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 */
caddr_t 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 */
}

Listing Two
key =         0x0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
key_len =     16 bytes
data =        "Hi There"
data_len =    8  bytes
digest =      0x9294727a3638bb1c13f48ef8158bfc9d

key =         "Jefe"
data =        "what do ya want for nothing?"
data_len =    28 bytes
digest =      0x750c783e6ab0b503eaa86e310a5db738
key =         0xAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
key_len       16 bytes
data =        0xDDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD
data_len =    50 bytes
digest =      0x56be34521d144c88dbb8c733f0e8b3f6



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