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
gf2_8_multiply(gf2_8 x,gf2_8 y)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
gf2_8_compute_inverse(gf2_8 x)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