aes-set-decrypt-key.c

   1 /* aes-set-decrypt-key.c
   2  *
   3  * Inverse key setup for the aes/rijndael block cipher.
   4  */
   5
   6 /* nettle, low-level cryptographics library
   7  *
   8  * Copyright (C) 2000, 2001, 2002 Rafael R. Sevilla, Niels Möller
   9  *
  10  * The nettle library is free software; you can redistribute it and/or modify
  11  * it under the terms of the GNU Lesser General Public License as published by
  12  * the Free Software Foundation; either version 2.1 of the License, or (at your
  13  * option) any later version.
  14  *
  15  * The nettle library is distributed in the hope that it will be useful, but
  16  * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  17  * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  18  * License for more details.
  19  *
  20  * You should have received a copy of the GNU Lesser General Public License
  21  * along with the nettle library; see the file COPYING.LIB.  If not, write to
  22  * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  23  * MA 02111-1307, USA.
  24  */
  25
  26 /* Originally written by Rafael R. Sevilla <dido@pacific.net.ph> */
  27
  28 #if HAVE_CONFIG_H
  29 # include "config.h"
  30 #endif
  31
  32 #include "aes-internal.h"
  33
  34 /* Tables for computations in the AES GF2 field. */
  35 static const uint8_t gf2_log[0x100] =
  36 {
  37   0x00,0x00,0x19,0x01,0x32,0x02,0x1a,0xc6,
  38   0x4b,0xc7,0x1b,0x68,0x33,0xee,0xdf,0x03,
  39   0x64,0x04,0xe0,0x0e,0x34,0x8d,0x81,0xef,
  40   0x4c,0x71,0x08,0xc8,0xf8,0x69,0x1c,0xc1,
  41   0x7d,0xc2,0x1d,0xb5,0xf9,0xb9,0x27,0x6a,
  42   0x4d,0xe4,0xa6,0x72,0x9a,0xc9,0x09,0x78,
  43   0x65,0x2f,0x8a,0x05,0x21,0x0f,0xe1,0x24,
  44   0x12,0xf0,0x82,0x45,0x35,0x93,0xda,0x8e,
  45   0x96,0x8f,0xdb,0xbd,0x36,0xd0,0xce,0x94,
  46   0x13,0x5c,0xd2,0xf1,0x40,0x46,0x83,0x38,
  47   0x66,0xdd,0xfd,0x30,0xbf,0x06,0x8b,0x62,
  48   0xb3,0x25,0xe2,0x98,0x22,0x88,0x91,0x10,
  49   0x7e,0x6e,0x48,0xc3,0xa3,0xb6,0x1e,0x42,
  50   0x3a,0x6b,0x28,0x54,0xfa,0x85,0x3d,0xba,
  51   0x2b,0x79,0x0a,0x15,0x9b,0x9f,0x5e,0xca,
  52   0x4e,0xd4,0xac,0xe5,0xf3,0x73,0xa7,0x57,
  53   0xaf,0x58,0xa8,0x50,0xf4,0xea,0xd6,0x74,
  54   0x4f,0xae,0xe9,0xd5,0xe7,0xe6,0xad,0xe8,
  55   0x2c,0xd7,0x75,0x7a,0xeb,0x16,0x0b,0xf5,
  56   0x59,0xcb,0x5f,0xb0,0x9c,0xa9,0x51,0xa0,
  57   0x7f,0x0c,0xf6,0x6f,0x17,0xc4,0x49,0xec,
  58   0xd8,0x43,0x1f,0x2d,0xa4,0x76,0x7b,0xb7,
  59   0xcc,0xbb,0x3e,0x5a,0xfb,0x60,0xb1,0x86,
  60   0x3b,0x52,0xa1,0x6c,0xaa,0x55,0x29,0x9d,
  61   0x97,0xb2,0x87,0x90,0x61,0xbe,0xdc,0xfc,
  62   0xbc,0x95,0xcf,0xcd,0x37,0x3f,0x5b,0xd1,
  63   0x53,0x39,0x84,0x3c,0x41,0xa2,0x6d,0x47,
  64   0x14,0x2a,0x9e,0x5d,0x56,0xf2,0xd3,0xab,
  65   0x44,0x11,0x92,0xd9,0x23,0x20,0x2e,0x89,
  66   0xb4,0x7c,0xb8,0x26,0x77,0x99,0xe3,0xa5,
  67   0x67,0x4a,0xed,0xde,0xc5,0x31,0xfe,0x18,
  68   0x0d,0x63,0x8c,0x80,0xc0,0xf7,0x70,0x07,
  69 };
  70
  71 static const uint8_t gf2_exp[0x100] =
  72 {
  73   0x01,0x03,0x05,0x0f,0x11,0x33,0x55,0xff,
  74   0x1a,0x2e,0x72,0x96,0xa1,0xf8,0x13,0x35,
  75   0x5f,0xe1,0x38,0x48,0xd8,0x73,0x95,0xa4,
  76   0xf7,0x02,0x06,0x0a,0x1e,0x22,0x66,0xaa,
  77   0xe5,0x34,0x5c,0xe4,0x37,0x59,0xeb,0x26,
  78   0x6a,0xbe,0xd9,0x70,0x90,0xab,0xe6,0x31,
  79   0x53,0xf5,0x04,0x0c,0x14,0x3c,0x44,0xcc,
  80   0x4f,0xd1,0x68,0xb8,0xd3,0x6e,0xb2,0xcd,
  81   0x4c,0xd4,0x67,0xa9,0xe0,0x3b,0x4d,0xd7,
  82   0x62,0xa6,0xf1,0x08,0x18,0x28,0x78,0x88,
  83   0x83,0x9e,0xb9,0xd0,0x6b,0xbd,0xdc,0x7f,
  84   0x81,0x98,0xb3,0xce,0x49,0xdb,0x76,0x9a,
  85   0xb5,0xc4,0x57,0xf9,0x10,0x30,0x50,0xf0,
  86   0x0b,0x1d,0x27,0x69,0xbb,0xd6,0x61,0xa3,
  87   0xfe,0x19,0x2b,0x7d,0x87,0x92,0xad,0xec,
  88   0x2f,0x71,0x93,0xae,0xe9,0x20,0x60,0xa0,
  89   0xfb,0x16,0x3a,0x4e,0xd2,0x6d,0xb7,0xc2,
  90   0x5d,0xe7,0x32,0x56,0xfa,0x15,0x3f,0x41,
  91   0xc3,0x5e,0xe2,0x3d,0x47,0xc9,0x40,0xc0,
  92   0x5b,0xed,0x2c,0x74,0x9c,0xbf,0xda,0x75,
  93   0x9f,0xba,0xd5,0x64,0xac,0xef,0x2a,0x7e,
  94   0x82,0x9d,0xbc,0xdf,0x7a,0x8e,0x89,0x80,
  95   0x9b,0xb6,0xc1,0x58,0xe8,0x23,0x65,0xaf,
  96   0xea,0x25,0x6f,0xb1,0xc8,0x43,0xc5,0x54,
  97   0xfc,0x1f,0x21,0x63,0xa5,0xf4,0x07,0x09,
  98   0x1b,0x2d,0x77,0x99,0xb0,0xcb,0x46,0xca,
  99   0x45,0xcf,0x4a,0xde,0x79,0x8b,0x86,0x91,
 100   0xa8,0xe3,0x3e,0x42,0xc6,0x51,0xf3,0x0e,
 101   0x12,0x36,0x5a,0xee,0x29,0x7b,0x8d,0x8c,
 102   0x8f,0x8a,0x85,0x94,0xa7,0xf2,0x0d,0x17,
 103   0x39,0x4b,0xdd,0x7c,0x84,0x97,0xa2,0xfd,
 104   0x1c,0x24,0x6c,0xb4,0xc7,0x52,0xf6,0x01,
 105 };
 106
 107 static unsigned
 108 mult(unsigned a, unsigned b)
 109 {
 110   return (a && b) ? gf2_exp[ (gf2_log[a] + gf2_log[b]) % 255] : 0;
 111 }
 112
 113 static void
 114 inv_mix_column(uint32_t *a)
 115 {
 116   uint8_t c[4][4];
 117   unsigned i, j;
 118
 119   for (j = 0; j < 4; j++)
 120     {
 121       for(i = 0; i < 4; i++)
 122         {
 123           c[j][i] = mult(0xe, (a[j] >> i*8) & 0xff)
 124             ^ mult(0xb, (a[j] >> ((i+1)%4)*8) & 0xff)
 125             ^ mult(0xd, (a[j] >> ((i+2)%4)*8) & 0xff)
 126             ^ mult(0x9, (a[j] >> ((i+3)%4)*8) & 0xff);
 127         }
 128     }
 129   for (i = 0; i < 4; i++)
 130     {
 131       a[i] = 0;
 132       for(j = 0; j < 4; j++)
 133         a[i] |= c[i][j] << (j*8);
 134     }
 135 }
 136
 137 #define SWAP(a, b) \
 138 do { uint32_t t_swap = (a); (a) = (b); (b) = t_swap; } while(0)
 139
 140 void
 141 aes_invert_key(struct aes_ctx *dst,
 142                const struct aes_ctx *src)
 143 {
 144   unsigned nrounds;
 145   unsigned i;
 146
 147   nrounds = src->nrounds;
 148
 149   /* Reverse the order of subkeys, in groups of 4. */
 150   /* FIXME: Instead of reordering the subkeys, change the access order
 151      of aes_decrypt, since it's a separate function anyway? */
 152   if (src == dst)
 153     {
 154       unsigned j, k;
 155
 156       for (i = 0, j = nrounds * 4;
 157            i < j;
 158            i += 4, j -= 4)
 159         for (k = 0; k<4; k++)
 160           SWAP(dst->keys[i+k], dst->keys[j+k]);
 161     }
 162   else
 163     {
 164       unsigned k;
 165
 166       dst->nrounds = nrounds;
 167       for (i = 0; i <= nrounds * 4; i += 4)
 168         for (k = 0; k < 4; k++)
 169           dst->keys[i+k] = src->keys[nrounds * 4 - i + k];
 170     }
 171
 172   /* Transform all subkeys but the first and last. */
 173   for (i = 4; i < 4 * nrounds; i += 4)
 174     inv_mix_column(dst->keys + i);
 175 }
 176
 177 void
 178 aes_set_decrypt_key(struct aes_ctx *ctx,
 179                     unsigned keysize, const uint8_t *key)
 180 {
 181   /* We first create subkeys for encryption,
 182    * then modify the subkeys for decryption. */
 183   aes_set_encrypt_key(ctx, keysize, key);
 184   aes_invert_key(ctx, ctx);
 185 }
 186