michael@0: /*
michael@0:  * gf2_8.c
michael@0:  *
michael@0:  * GF(256) finite field implementation, with the representation used
michael@0:  * in the AES cipher.
michael@0:  * 
michael@0:  * David A. McGrew
michael@0:  * Cisco Systems, Inc.
michael@0:  */
michael@0: 
michael@0: /*
michael@0:  *	
michael@0:  * Copyright (c) 2001-2006, Cisco Systems, Inc.
michael@0:  * All rights reserved.
michael@0:  * 
michael@0:  * Redistribution and use in source and binary forms, with or without
michael@0:  * modification, are permitted provided that the following conditions
michael@0:  * are met:
michael@0:  * 
michael@0:  *   Redistributions of source code must retain the above copyright
michael@0:  *   notice, this list of conditions and the following disclaimer.
michael@0:  * 
michael@0:  *   Redistributions in binary form must reproduce the above
michael@0:  *   copyright notice, this list of conditions and the following
michael@0:  *   disclaimer in the documentation and/or other materials provided
michael@0:  *   with the distribution.
michael@0:  * 
michael@0:  *   Neither the name of the Cisco Systems, Inc. nor the names of its
michael@0:  *   contributors may be used to endorse or promote products derived
michael@0:  *   from this software without specific prior written permission.
michael@0:  * 
michael@0:  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
michael@0:  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
michael@0:  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
michael@0:  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
michael@0:  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
michael@0:  * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
michael@0:  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
michael@0:  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
michael@0:  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
michael@0:  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
michael@0:  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
michael@0:  * OF THE POSSIBILITY OF SUCH DAMAGE.
michael@0:  *
michael@0:  */
michael@0: 
michael@0: 
michael@0: #include "datatypes.h"
michael@0: #include "gf2_8.h"
michael@0: 
michael@0: /* gf2_8_shift() moved to gf2_8.h as an inline function */
michael@0: 
michael@0: gf2_8
michael@0: gf2_8_multiply(gf2_8 x, gf2_8 y) {
michael@0:   gf2_8 z = 0;
michael@0: 
michael@0:   if (y &   1) z ^= x; x = gf2_8_shift(x);
michael@0:   if (y &   2) z ^= x; x = gf2_8_shift(x);
michael@0:   if (y &   4) z ^= x; x = gf2_8_shift(x);
michael@0:   if (y &   8) z ^= x; x = gf2_8_shift(x);
michael@0:   if (y &  16) z ^= x; x = gf2_8_shift(x);
michael@0:   if (y &  32) z ^= x; x = gf2_8_shift(x);
michael@0:   if (y &  64) z ^= x; x = gf2_8_shift(x);
michael@0:   if (y & 128) z ^= x; 
michael@0:   
michael@0:   return z;
michael@0: }
michael@0: 
michael@0: 
michael@0: /* this should use the euclidean algorithm */
michael@0: 
michael@0: gf2_8
michael@0: gf2_8_compute_inverse(gf2_8 x) {
michael@0:   unsigned int i;
michael@0: 
michael@0:   if (x == 0) return 0;    /* zero is a special case */
michael@0:   for (i=0; i < 256; i++)
michael@0:     if (gf2_8_multiply((gf2_8) i, x) == 1)
michael@0:       return (gf2_8) i;
michael@0: 
michael@0:     return 0;
michael@0: }
michael@0: