| /* |

| * 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; |

| } |

| |