Appendix - Sample Code

For the sake of illustration we provide the following sample code for the implementation of HMAC-MD5 as well as some corresponding test vectors (the code is based on MD5 code as described in [MD5]).

/*
** 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 */
}

Test Vectors

Trailing \0 of a character string not included in test:

Test Case 1:

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

Test Case 2:

key =         "Jefe"
data =        "what do ya want for nothing?"
data_len =    28 bytes
digest =      0x750c783e6ab0b503eaa86e310a5db738

Test Case 3:

key =         0xAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
key_len       16 bytes
data =        0xDDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD...
              ..DDDDDDDDDDDDDDDDDDDD
data_len =    50 bytes
digest =      0x56be34521d144c88dbb8c733f0e8b3f6