gcmdata.c 1.88 KB
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/* gcmdata.c
 *
 * Galois counter mode, specified by NIST,
 * http://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf
 *
 */

/* Generation of fixed multiplication tables. */

/* nettle, low-level cryptographics library
 *
 * Copyright (C) 2011 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
26 27
 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02111-1301, USA.
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 */

#include <stdio.h>
#include <stdlib.h>

#define GHASH_POLYNOMIAL 0xE1


/* When x is shifted out over the block edge, add multiples of the
   defining polynomial to eliminate each bit. */
static unsigned
reduce(unsigned x)
{
  unsigned p = GHASH_POLYNOMIAL << 1;
  unsigned y = 0;
  for (; x; x >>= 1, p <<= 1)
    if (x & 1)
      y ^= p;
  return y;
}

int
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main(int argc, char **argv)
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{
  unsigned i;
  printf("4-bit table:\n");
  
  for (i = 0; i<16; i++)
    {
      unsigned x;
      if (i && !(i%8))
	printf("\n");

      x = reduce(i << 4);
      printf("W(%02x,%02x),", x >> 8, x & 0xff);
    }
  printf("\n\n");
  printf("8-bit table:\n");
  for (i = 0; i<256; i++)
    {
      unsigned x;
      if (i && !(i%8))
	printf("\n");

      x = reduce(i);
      printf("W(%02x,%02x),", x >> 8, x & 0xff);
    }
  printf("\n");
  return EXIT_SUCCESS;
}