Alexandre Lision | 8af73cb | 2013-12-10 14:11:20 -0500 | [diff] [blame] | 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 | |