diff options
Diffstat (limited to 'crypto/math/gf2_8.c')
| -rw-r--r-- | crypto/math/gf2_8.c | 83 |
1 files changed, 83 insertions, 0 deletions
diff --git a/crypto/math/gf2_8.c b/crypto/math/gf2_8.c new file mode 100644 index 0000000..8a112ba --- /dev/null +++ b/crypto/math/gf2_8.c @@ -0,0 +1,83 @@ +/* + * 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; +} + |