/*
**  hmac_md5 -- implements RFC 2104
**  
**  Copyright 2010, markus schnalke <meillo@marmaro.de>
**  
**  Permission to use, copy, modify, and/or distribute this software for any
**  purpose with or without fee is hereby granted, provided that the above
**  copyright notice and this permission notice appear in all copies.
**  
**  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
**  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
**  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
**  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
**  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
**  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
**  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
**  
**  
**  My motivation to write this code was the lack of a nicely licensed
**  hmac_md5 function in C. I programmed it following the RFC's text.
**  Obviously this code is highly similar to the sample code of the RFC.
**  The code is tested against the test vectors of the RFC. Wikipedia's
**  HMAC page helped me to understand the algorithm better.
**  
**  This hmac_md5 function requires an OpenSSL-compatible MD5
**  implementation. There are Public Domain MD5 implementations by Colin
**  Plumb and by Solar Designer. You probably want to use one of these.
*/

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


const int blocksize = 64;
const int hashsize = 16;


/*
**  The computed HMAC will be written to `digest'.
**  Ensure digest points to hashsize bytes of allocated memory.
*/
void
hmac_md5(unsigned char *text, int textlen, unsigned char *key, int keylen,
		unsigned char *digest)
{
	int i;
	MD5_CTX context;
	unsigned char ipad[blocksize];
	unsigned char opad[blocksize];

	/* too long keys are replaced by their hash value */
	if (keylen > blocksize) {
		MD5_Init(&context);
		MD5_Update(&context, key, keylen);
		MD5_Final(digest, &context);
		key = digest;
		keylen = hashsize;
	}

        /* copy the key into the pads */
	memset(ipad, 0, sizeof(ipad));
	memcpy(ipad, key, keylen);

	memset(opad, 0, sizeof(opad));
	memcpy(opad, key, keylen);

        /* xor the pads with their ``basic'' value */
	for (i=0; i<blocksize; i++) {
		ipad[i] ^= 0x36;
		opad[i] ^= 0x5c;
	}

	/* inner pass (ipad ++ message) */
	MD5_Init(&context);
	MD5_Update(&context, ipad, sizeof(ipad));
	MD5_Update(&context, text, textlen);
	MD5_Final(digest, &context);

	/* outer pass (opad ++ result of inner pass) */
	MD5_Init(&context);
	MD5_Update(&context, opad, sizeof(opad));
	MD5_Update(&context, digest, hashsize);
	MD5_Final(digest, &context);
}