1 /* gf128mul.h - GF(2^128) multiplication functions
3 * Copyright (c) 2003, Dr Brian Gladman, Worcester, UK.
4 * Copyright (c) 2006 Rik Snel <rsnel@cube.dyndns.org>
6 * Based on Dr Brian Gladman's (GPL'd) work published at
7 * http://fp.gladman.plus.com/cryptography_technology/index.htm
8 * See the original copyright notice below.
10 * This program is free software; you can redistribute it and/or modify it
11 * under the terms of the GNU General Public License as published by the Free
12 * Software Foundation; either version 2 of the License, or (at your option)
16 ---------------------------------------------------------------------------
17 Copyright (c) 2003, Dr Brian Gladman, Worcester, UK. All rights reserved.
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22 form is allowed (with or without changes) provided that:
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43 ---------------------------------------------------------------------------
44 Issue Date: 31/01/2006
46 An implementation of field multiplication in Galois Field GF(2^128)
49 #ifndef _CRYPTO_GF128MUL_H
50 #define _CRYPTO_GF128MUL_H
52 #include <crypto/b128ops.h>
53 #include <linux/slab.h>
57 * For some background on GF(2^128) see for example:
58 * http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-revised-spec.pdf
60 * The elements of GF(2^128) := GF(2)[X]/(X^128-X^7-X^2-X^1-1) can
61 * be mapped to computer memory in a variety of ways. Let's examine
64 * Take a look at the 16 binary octets below in memory order. The msb's
65 * are left and the lsb's are right. char b[16] is an array and b[0] is
68 * 10000000 00000000 00000000 00000000 .... 00000000 00000000 00000000
69 * b[0] b[1] b[2] b[3] b[13] b[14] b[15]
71 * Every bit is a coefficient of some power of X. We can store the bits
72 * in every byte in little-endian order and the bytes themselves also in
73 * little endian order. I will call this lle (little-little-endian).
74 * The above buffer represents the polynomial 1, and X^7+X^2+X^1+1 looks
75 * like 11100001 00000000 .... 00000000 = { 0xE1, 0x00, }.
76 * This format was originally implemented in gf128mul and is used
77 * in GCM (Galois/Counter mode) and in ABL (Arbitrary Block Length).
79 * Another convention says: store the bits in bigendian order and the
80 * bytes also. This is bbe (big-big-endian). Now the buffer above
81 * represents X^127. X^7+X^2+X^1+1 looks like 00000000 .... 10000111,
82 * b[15] = 0x87 and the rest is 0. LRW uses this convention and bbe
83 * is partly implemented.
85 * Both of the above formats are easy to implement on big-endian
88 * XTS and EME (the latter of which is patent encumbered) use the ble
89 * format (bits are stored in big endian order and the bytes in little
90 * endian). The above buffer represents X^7 in this case and the
91 * primitive polynomial is b[0] = 0x87.
93 * The common machine word-size is smaller than 128 bits, so to make
94 * an efficient implementation we must split into machine word sizes.
95 * This implementation uses 64-bit words for the moment. Machine
96 * endianness comes into play. The lle format in relation to machine
97 * endianness is discussed below by the original author of gf128mul Dr
100 * Let's look at the bbe and ble format on a little endian machine.
102 * bbe on a little endian machine u32 x[4]:
104 * MS x[0] LS MS x[1] LS
105 * ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
106 * 103..96 111.104 119.112 127.120 71...64 79...72 87...80 95...88
108 * MS x[2] LS MS x[3] LS
109 * ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
110 * 39...32 47...40 55...48 63...56 07...00 15...08 23...16 31...24
112 * ble on a little endian machine
114 * MS x[0] LS MS x[1] LS
115 * ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
116 * 31...24 23...16 15...08 07...00 63...56 55...48 47...40 39...32
118 * MS x[2] LS MS x[3] LS
119 * ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
120 * 95...88 87...80 79...72 71...64 127.120 199.112 111.104 103..96
122 * Multiplications in GF(2^128) are mostly bit-shifts, so you see why
123 * ble (and lbe also) are easier to implement on a little-endian
124 * machine than on a big-endian machine. The converse holds for bbe
127 * Note: to have good alignment, it seems to me that it is sufficient
128 * to keep elements of GF(2^128) in type u64[2]. On 32-bit wordsize
129 * machines this will automatically aligned to wordsize and on a 64-bit
132 /* Multiply a GF(2^128) field element by x. Field elements are
133 held in arrays of bytes in which field bits 8n..8n + 7 are held in
134 byte[n], with lower indexed bits placed in the more numerically
135 significant bit positions within bytes.
137 On little endian machines the bit indexes translate into the bit
138 positions within four 32-bit words in the following way
140 MS x[0] LS MS x[1] LS
141 ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
142 24...31 16...23 08...15 00...07 56...63 48...55 40...47 32...39
144 MS x[2] LS MS x[3] LS
145 ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
146 88...95 80...87 72...79 64...71 120.127 112.119 104.111 96..103
148 On big endian machines the bit indexes translate into the bit
149 positions within four 32-bit words in the following way
151 MS x[0] LS MS x[1] LS
152 ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
153 00...07 08...15 16...23 24...31 32...39 40...47 48...55 56...63
155 MS x[2] LS MS x[3] LS
156 ms ls ms ls ms ls ms ls ms ls ms ls ms ls ms ls
157 64...71 72...79 80...87 88...95 96..103 104.111 112.119 120.127
160 /* A slow generic version of gf_mul, implemented for lle and bbe
161 * It multiplies a and b and puts the result in a */
162 void gf128mul_lle(be128 *a, const be128 *b);
164 void gf128mul_bbe(be128 *a, const be128 *b);
166 /* multiply by x in ble format, needed by XTS */
167 void gf128mul_x_ble(be128 *a, const be128 *b);
169 /* 4k table optimization */
175 struct gf128mul_4k *gf128mul_init_4k_lle(const be128 *g);
176 struct gf128mul_4k *gf128mul_init_4k_bbe(const be128 *g);
177 void gf128mul_4k_lle(be128 *a, const struct gf128mul_4k *t);
178 void gf128mul_4k_bbe(be128 *a, const struct gf128mul_4k *t);
180 static inline void gf128mul_free_4k(struct gf128mul_4k *t)
186 /* 64k table optimization, implemented for bbe */
188 struct gf128mul_64k {
189 struct gf128mul_4k *t[16];
192 /* First initialize with the constant factor with which you
193 * want to multiply and then call gf128mul_64k_bbe with the other
194 * factor in the first argument, and the table in the second.
195 * Afterwards, the result is stored in *a.
197 struct gf128mul_64k *gf128mul_init_64k_bbe(const be128 *g);
198 void gf128mul_free_64k(struct gf128mul_64k *t);
199 void gf128mul_64k_bbe(be128 *a, const struct gf128mul_64k *t);
201 #endif /* _CRYPTO_GF128MUL_H */