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/*
* gf2_8.c
*
* GF(256) finite field implementation, with the representation used
* in the AES cipher.
*
* David A. McGrew
* Cisco Systems, Inc.
*/
/*
*
* Copyright (c) 2001-2006, Cisco Systems, Inc.
* 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 the Cisco Systems, Inc. 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 HOLDERS 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 "datatypes.h"
#include "gf2_8.h"
/* gf2_8_shift() moved to gf2_8.h as an inline function */
inline gf2_8
gf2_8_multiply(gf2_8 x, gf2_8 y) {
gf2_8 z = 0;
if (y & 1) z ^= x; x = gf2_8_shift(x);
if (y & 2) z ^= x; x = gf2_8_shift(x);
if (y & 4) z ^= x; x = gf2_8_shift(x);
if (y & 8) z ^= x; x = gf2_8_shift(x);
if (y & 16) z ^= x; x = gf2_8_shift(x);
if (y & 32) z ^= x; x = gf2_8_shift(x);
if (y & 64) z ^= x; x = gf2_8_shift(x);
if (y & 128) z ^= x;
return z;
}
/* this should use the euclidean algorithm */
gf2_8
gf2_8_compute_inverse(gf2_8 x) {
unsigned int i;
if (x == 0) return 0; /* zero is a special case */
for (i=0; i < 256; i++)
if (gf2_8_multiply((gf2_8) i, x) == 1)
return (gf2_8) i;
return 0;
}
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