text: /* hmac.c - TinyCrypt implementation of the HMAC algorithm */
  
  /*
   *  Copyright (C) 2017 by Intel Corporation, All Rights Reserved.
   *
   *  Redistribution and use in source and binary forms, with or without
   *  modification, are permitted provided that the following conditions are met:
   *
   *    - Redistributions of source code must retain the above copyright notice,
   *     this list of conditions and the following disclaimer.
   *
   *    - Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in the
   *    documentation and/or other materials provided with the distribution.
   *
   *    - Neither the name of Intel Corporation nor the names of its contributors
   *    may be used to endorse or promote products derived from this software
   *    without specific prior written permission.
   *
   *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
   *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
   *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
   *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
   *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   *  POSSIBILITY OF SUCH DAMAGE.
   */
  
  #include <tinycrypt/hmac.h>
  #include <tinycrypt/constants.h>
  #include <tinycrypt/utils.h>
  
  static void rekey(uint8_t *key, const uint8_t *new_key, unsigned int key_size)
  {
  	const uint8_t inner_pad = (uint8_t) 0x36;
  	const uint8_t outer_pad = (uint8_t) 0x5c;
  	unsigned int i;
  
  	for (i = 0; i < key_size; ++i) {
  		key[i] = inner_pad ^ new_key[i];
  		key[i + TC_SHA256_BLOCK_SIZE] = outer_pad ^ new_key[i];
  	}
  	for (; i < TC_SHA256_BLOCK_SIZE; ++i) {
  		key[i] = inner_pad; key[i + TC_SHA256_BLOCK_SIZE] = outer_pad;
  	}
  }
  
  int tc_hmac_set_key(TCHmacState_t ctx, const uint8_t *key,
  		    unsigned int key_size)
  {
  	/* Input sanity check */
  	if (ctx == (TCHmacState_t) 0 ||
  	    key == (const uint8_t *) 0 ||
  	    key_size == 0) {
  		return TC_CRYPTO_FAIL;
  	}
  
  	const uint8_t dummy_key[TC_SHA256_BLOCK_SIZE];
  	struct tc_hmac_state_struct dummy_state;
  	
  	for(int i=0; i<TC_SHA256_BLOCK_SIZE; i++){
  	    dummy_key[i] = i;
  	}
  
  	if (key_size <= TC_SHA256_BLOCK_SIZE) {
  		/*
  		 * The next three calls are dummy calls just to avoid
  		 * certain timing attacks. Without these dummy calls,
  		 * adversaries would be able to learn whether the key_size is
  		 * greater than TC_SHA256_BLOCK_SIZE by measuring the time
  		 * consumed in this process.
  		 */
  		(void)tc_sha256_init(&dummy_state.hash_state);
  		(void)tc_sha256_update(&dummy_state.hash_state,
  				       dummy_key,
  				       key_size);
  		(void)tc_sha256_final(&dummy_state.key[TC_SHA256_DIGEST_SIZE],
  				      &dummy_state.hash_state);
  
  		/* Actual code for when key_size <= TC_SHA256_BLOCK_SIZE: */
  		rekey(ctx->key, key, key_size);
  	} else {
  		(void)tc_sha256_init(&ctx->hash_state);
  		(void)tc_sha256_update(&ctx->hash_state, key, key_size);
  		(void)tc_sha256_final(&ctx->key[TC_SHA256_DIGEST_SIZE],
  				      &ctx->hash_state);
  		rekey(ctx->key,
  		      &ctx->key[TC_SHA256_DIGEST_SIZE],
  		      TC_SHA256_DIGEST_SIZE);
  	}
  
  	return TC_CRYPTO_SUCCESS;
  }
  
  int tc_hmac_init(TCHmacState_t ctx)
  {
  
  	/* input sanity check: */
  	if (ctx == (TCHmacState_t) 0) {
  		return TC_CRYPTO_FAIL;
  	}
  
    (void) tc_sha256_init(&ctx->hash_state);
    (void) tc_sha256_update(&ctx->hash_state, ctx->key, TC_SHA256_BLOCK_SIZE);
  
  	return TC_CRYPTO_SUCCESS;
  }
  
  int tc_hmac_update(TCHmacState_t ctx,
  		   const void *data,
  		   unsigned int data_length)
  {
  
  	/* input sanity check: */
  	if (ctx == (TCHmacState_t) 0) {
  		return TC_CRYPTO_FAIL;
  	}
  
  	(void)tc_sha256_update(&ctx->hash_state, data, data_length);
  
  	return TC_CRYPTO_SUCCESS;
  }
  
  int tc_hmac_final(uint8_t *tag, unsigned int taglen, TCHmacState_t ctx)
  {
  
  	/* input sanity check: */
  	if (tag == (uint8_t *) 0 ||
  	    taglen != TC_SHA256_DIGEST_SIZE ||
  	    ctx == (TCHmacState_t) 0) {
  		return TC_CRYPTO_FAIL;
  	}
  
  	(void) tc_sha256_final(tag, &ctx->hash_state);
  
  	(void)tc_sha256_init(&ctx->hash_state);
  	(void)tc_sha256_update(&ctx->hash_state,
  			       &ctx->key[TC_SHA256_BLOCK_SIZE],
  				TC_SHA256_BLOCK_SIZE);
  	(void)tc_sha256_update(&ctx->hash_state, tag, TC_SHA256_DIGEST_SIZE);
  	(void)tc_sha256_final(tag, &ctx->hash_state);
  
  	/* destroy the current state */
  	_set(ctx, 0, sizeof(*ctx));
  
  	return TC_CRYPTO_SUCCESS;
  }
  