1 /* 2 * gf2_8.c 3 * 4 * GF(256) finite field implementation, with the representation used 5 * in the AES cipher. 6 * 7 * David A. McGrew 8 * Cisco Systems, Inc. 9 */ 10 11 /* 12 * 13 * Copyright (c) 2001-2006, Cisco Systems, Inc. 14 * All rights reserved. 15 * 16 * Redistribution and use in source and binary forms, with or without 17 * modification, are permitted provided that the following conditions 18 * are met: 19 * 20 * Redistributions of source code must retain the above copyright 21 * notice, this list of conditions and the following disclaimer. 22 * 23 * Redistributions in binary form must reproduce the above 24 * copyright notice, this list of conditions and the following 25 * disclaimer in the documentation and/or other materials provided 26 * with the distribution. 27 * 28 * Neither the name of the Cisco Systems, Inc. nor the names of its 29 * contributors may be used to endorse or promote products derived 30 * from this software without specific prior written permission. 31 * 32 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 33 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 34 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 35 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 36 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 37 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 38 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 39 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 40 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 41 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 42 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 43 * OF THE POSSIBILITY OF SUCH DAMAGE. 44 * 45 */ 46 47 48 #include "datatypes.h" 49 #include "gf2_8.h" 50 51 /* gf2_8_shift() moved to gf2_8.h as an inline function */ 52 53 inline gf2_8 54 gf2_8_multiply(gf2_8 x, gf2_8 y) { 55 gf2_8 z = 0; 56 57 if (y & 1) z ^= x; x = gf2_8_shift(x); 58 if (y & 2) z ^= x; x = gf2_8_shift(x); 59 if (y & 4) z ^= x; x = gf2_8_shift(x); 60 if (y & 8) z ^= x; x = gf2_8_shift(x); 61 if (y & 16) z ^= x; x = gf2_8_shift(x); 62 if (y & 32) z ^= x; x = gf2_8_shift(x); 63 if (y & 64) z ^= x; x = gf2_8_shift(x); 64 if (y & 128) z ^= x; 65 66 return z; 67 } 68 69 70 /* this should use the euclidean algorithm */ 71 72 gf2_8 73 gf2_8_compute_inverse(gf2_8 x) { 74 unsigned int i; 75 76 if (x == 0) return 0; /* zero is a special case */ 77 for (i=0; i < 256; i++) 78 if (gf2_8_multiply((gf2_8) i, x) == 1) 79 return (gf2_8) i; 80 81 return 0; 82 } 83 84
