aes-set-decrypt-key.c 5.2 KB
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/* aes-set-decrypt-key.c
 *
 * Inverse key setup for the aes/rijndael block cipher.
 */

/* nettle, low-level cryptographics library
 *
 * Copyright (C) 2000, 2001, 2002 Rafael R. Sevilla, Niels Mller
 *  
 * 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., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/* Originally written by Rafael R. Sevilla <dido@pacific.net.ph> */

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

#include "aes-internal.h"

/* Tables for computations in the AES GF2 field. */
static const uint8_t gf2_log[0x100] =
{
  0x00,0x00,0x19,0x01,0x32,0x02,0x1a,0xc6,
  0x4b,0xc7,0x1b,0x68,0x33,0xee,0xdf,0x03,
  0x64,0x04,0xe0,0x0e,0x34,0x8d,0x81,0xef,
  0x4c,0x71,0x08,0xc8,0xf8,0x69,0x1c,0xc1,
  0x7d,0xc2,0x1d,0xb5,0xf9,0xb9,0x27,0x6a,
  0x4d,0xe4,0xa6,0x72,0x9a,0xc9,0x09,0x78,
  0x65,0x2f,0x8a,0x05,0x21,0x0f,0xe1,0x24,
  0x12,0xf0,0x82,0x45,0x35,0x93,0xda,0x8e,
  0x96,0x8f,0xdb,0xbd,0x36,0xd0,0xce,0x94,
  0x13,0x5c,0xd2,0xf1,0x40,0x46,0x83,0x38,
  0x66,0xdd,0xfd,0x30,0xbf,0x06,0x8b,0x62,
  0xb3,0x25,0xe2,0x98,0x22,0x88,0x91,0x10,
  0x7e,0x6e,0x48,0xc3,0xa3,0xb6,0x1e,0x42,
  0x3a,0x6b,0x28,0x54,0xfa,0x85,0x3d,0xba,
  0x2b,0x79,0x0a,0x15,0x9b,0x9f,0x5e,0xca,
  0x4e,0xd4,0xac,0xe5,0xf3,0x73,0xa7,0x57,
  0xaf,0x58,0xa8,0x50,0xf4,0xea,0xd6,0x74,
  0x4f,0xae,0xe9,0xd5,0xe7,0xe6,0xad,0xe8,
  0x2c,0xd7,0x75,0x7a,0xeb,0x16,0x0b,0xf5,
  0x59,0xcb,0x5f,0xb0,0x9c,0xa9,0x51,0xa0,
  0x7f,0x0c,0xf6,0x6f,0x17,0xc4,0x49,0xec,
  0xd8,0x43,0x1f,0x2d,0xa4,0x76,0x7b,0xb7,
  0xcc,0xbb,0x3e,0x5a,0xfb,0x60,0xb1,0x86,
  0x3b,0x52,0xa1,0x6c,0xaa,0x55,0x29,0x9d,
  0x97,0xb2,0x87,0x90,0x61,0xbe,0xdc,0xfc,
  0xbc,0x95,0xcf,0xcd,0x37,0x3f,0x5b,0xd1,
  0x53,0x39,0x84,0x3c,0x41,0xa2,0x6d,0x47,
  0x14,0x2a,0x9e,0x5d,0x56,0xf2,0xd3,0xab,
  0x44,0x11,0x92,0xd9,0x23,0x20,0x2e,0x89,
  0xb4,0x7c,0xb8,0x26,0x77,0x99,0xe3,0xa5,
  0x67,0x4a,0xed,0xde,0xc5,0x31,0xfe,0x18,
  0x0d,0x63,0x8c,0x80,0xc0,0xf7,0x70,0x07,
};

static const uint8_t gf2_exp[0x100] =
{
  0x01,0x03,0x05,0x0f,0x11,0x33,0x55,0xff,
  0x1a,0x2e,0x72,0x96,0xa1,0xf8,0x13,0x35,
  0x5f,0xe1,0x38,0x48,0xd8,0x73,0x95,0xa4,
  0xf7,0x02,0x06,0x0a,0x1e,0x22,0x66,0xaa,
  0xe5,0x34,0x5c,0xe4,0x37,0x59,0xeb,0x26,
  0x6a,0xbe,0xd9,0x70,0x90,0xab,0xe6,0x31,
  0x53,0xf5,0x04,0x0c,0x14,0x3c,0x44,0xcc,
  0x4f,0xd1,0x68,0xb8,0xd3,0x6e,0xb2,0xcd,
  0x4c,0xd4,0x67,0xa9,0xe0,0x3b,0x4d,0xd7,
  0x62,0xa6,0xf1,0x08,0x18,0x28,0x78,0x88,
  0x83,0x9e,0xb9,0xd0,0x6b,0xbd,0xdc,0x7f,
  0x81,0x98,0xb3,0xce,0x49,0xdb,0x76,0x9a,
  0xb5,0xc4,0x57,0xf9,0x10,0x30,0x50,0xf0,
  0x0b,0x1d,0x27,0x69,0xbb,0xd6,0x61,0xa3,
  0xfe,0x19,0x2b,0x7d,0x87,0x92,0xad,0xec,
  0x2f,0x71,0x93,0xae,0xe9,0x20,0x60,0xa0,
  0xfb,0x16,0x3a,0x4e,0xd2,0x6d,0xb7,0xc2,
  0x5d,0xe7,0x32,0x56,0xfa,0x15,0x3f,0x41,
  0xc3,0x5e,0xe2,0x3d,0x47,0xc9,0x40,0xc0,
  0x5b,0xed,0x2c,0x74,0x9c,0xbf,0xda,0x75,
  0x9f,0xba,0xd5,0x64,0xac,0xef,0x2a,0x7e,
  0x82,0x9d,0xbc,0xdf,0x7a,0x8e,0x89,0x80,
  0x9b,0xb6,0xc1,0x58,0xe8,0x23,0x65,0xaf,
  0xea,0x25,0x6f,0xb1,0xc8,0x43,0xc5,0x54,
  0xfc,0x1f,0x21,0x63,0xa5,0xf4,0x07,0x09,
  0x1b,0x2d,0x77,0x99,0xb0,0xcb,0x46,0xca,
  0x45,0xcf,0x4a,0xde,0x79,0x8b,0x86,0x91,
  0xa8,0xe3,0x3e,0x42,0xc6,0x51,0xf3,0x0e,
  0x12,0x36,0x5a,0xee,0x29,0x7b,0x8d,0x8c,
  0x8f,0x8a,0x85,0x94,0xa7,0xf2,0x0d,0x17,
  0x39,0x4b,0xdd,0x7c,0x84,0x97,0xa2,0xfd,
  0x1c,0x24,0x6c,0xb4,0xc7,0x52,0xf6,0x01,
};

static unsigned
mult(unsigned a, unsigned b)
{
  return (a && b) ? gf2_exp[ (gf2_log[a] + gf2_log[b]) % 255] : 0;
}

static void
inv_mix_column(uint32_t *a)
{
  uint8_t c[4][4];
  unsigned i, j;
	
  for (j = 0; j < 4; j++)
    {
      for(i = 0; i < 4; i++)
	{
	  c[j][i] = mult(0xe, (a[j] >> i*8) & 0xff)
	    ^ mult(0xb, (a[j] >> ((i+1)%4)*8) & 0xff)
	    ^ mult(0xd, (a[j] >> ((i+2)%4)*8) & 0xff)
	    ^ mult(0x9, (a[j] >> ((i+3)%4)*8) & 0xff);
	}
    }
  for (i = 0; i < 4; i++)
    {
      a[i] = 0;
      for(j = 0; j < 4; j++)
	a[i] |= c[i][j] << (j*8);
    }
}

#define SWAP(a, b) \
do { uint32_t t_swap = (a); (a) = (b); (b) = t_swap; } while(0)

void
aes_set_decrypt_key(struct aes_ctx *ctx,
		    unsigned keysize, const uint8_t *key)
{
  unsigned nkeys;
  unsigned i, j, k;
  
  /* We first create subkeys for encryption,
   * then modify the subkeys for decryption. */
  aes_set_encrypt_key(ctx, keysize, key);

  nkeys = (AES_BLOCK_SIZE/4) * (ctx->nrounds + 1);

  /* Reverse the order of subkeys */
  for (i = 0, j = ctx->nrounds * 4;
       i < j;
       i += 4, j -= 4)
    for (k = 0; k<4; k++)
      SWAP(ctx->keys[i+k], ctx->keys[j+k]);

  /* Transform all subkeys but the first and last. */
  for (i = 4; i < 4 * ctx->nrounds; i += 4)
    inv_mix_column(ctx->keys + i);
}