aes-decrypt-internal.c

/* aes-decrypt-internal.c
 *
 * Decryption function for the aes/rijndael block cipher.
 */

/* nettle, low-level cryptographics library
 *
 * Copyright (C) 2002, 2013 Niels Möller
 *  
 * The nettle library is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or (at your
 * option) any later version.
 * 
 * The nettle library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 * License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with the nettle library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02111-1301, USA.
 */

#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <assert.h>

#include "aes-internal.h"
#include "macros.h"

void
_nettle_aes_decrypt(unsigned rounds, const uint32_t *keys,
		    const struct aes_table *T,
		    size_t length, uint8_t *dst,
		    const uint8_t *src)
{
  FOR_BLOCKS(length, dst, src, AES_BLOCK_SIZE)
    {
      uint32_t w0, w1, w2, w3;		/* working ciphertext */
      uint32_t t0, t1, t2, t3;
      unsigned i;
      
      /* Get clear text, using little-endian byte order.
       * Also XOR with the first subkey. */

      w0 = LE_READ_UINT32(src)      ^ keys[0];
      w1 = LE_READ_UINT32(src + 4)  ^ keys[1];
      w2 = LE_READ_UINT32(src + 8)  ^ keys[2];
      w3 = LE_READ_UINT32(src + 12) ^ keys[3];

      for (i = 1; i < rounds; i++)
	{
	  t0 = AES_ROUND(T, w0, w3, w2, w1, keys[4*i]);
	  t1 = AES_ROUND(T, w1, w0, w3, w2, keys[4*i + 1]);
	  t2 = AES_ROUND(T, w2, w1, w0, w3, keys[4*i + 2]);
	  t3 = AES_ROUND(T, w3, w2, w1, w0, keys[4*i + 3]);

	  /* We could unroll the loop twice, to avoid these
	     assignments. If all eight variables fit in registers,
	     that should give a slight speedup. */
	  w0 = t0;
	  w1 = t1;
	  w2 = t2;
	  w3 = t3;
	}

      /* Final round */

      t0 = AES_FINAL_ROUND(T, w0, w3, w2, w1, keys[4*i]);
      t1 = AES_FINAL_ROUND(T, w1, w0, w3, w2, keys[4*i + 1]);
      t2 = AES_FINAL_ROUND(T, w2, w1, w0, w3, keys[4*i + 2]);
      t3 = AES_FINAL_ROUND(T, w3, w2, w1, w0, keys[4*i + 3]);

      LE_WRITE_UINT32(dst, t0);
      LE_WRITE_UINT32(dst + 4, t1);
      LE_WRITE_UINT32(dst + 8, t2);
      LE_WRITE_UINT32(dst + 12, t3);
    }
}