aes_dec.c

   1 /* aes.c */
   2 /*
   3     This file is part of the Crypto-avr-lib/microcrypt-lib.
   4     Copyright (C) 2008  Daniel Otte (daniel.otte@rub.de)
   5
   6     This program is free software: you can redistribute it and/or modify
   7     it under the terms of the GNU General Public License as published by
   8     the Free Software Foundation, either version 3 of the License, or
   9     (at your option) any later version.
  10
  11     This program is distributed in the hope that it will be useful,
  12     but WITHOUT ANY WARRANTY; without even the implied warranty of
  13     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14     GNU General Public License for more details.
  15
  16     You should have received a copy of the GNU General Public License
  17     along with this program.  If not, see <http://www.gnu.org/licenses/>.
  18 */
  19
  20 #include <stdint.h>
  21 #include <string.h>
  22 #include "gf256mul.h"
  23 #include "aes.h"
  24 #include "aes_invsbox.h"
  25 #include "aes_dec.h"
  26 #include <avr/pgmspace.h>
  27
  28 void aes_invshiftrow(void* data, uint8_t shift){
  29         uint8_t tmp[4];
  30         tmp[0] = ((uint8_t*)data)[(4+0-shift)&3];
  31         tmp[1] = ((uint8_t*)data)[(4+1-shift)&3];
  32         tmp[2] = ((uint8_t*)data)[(4+2-shift)&3];
  33         tmp[3] = ((uint8_t*)data)[(4+3-shift)&3];
  34         memcpy(data, tmp, 4);
  35 }
  36
  37 static
  38 void aes_dec_round(aes_cipher_state_t* state, const aes_roundkey_t* k){
  39         uint8_t tmp[16];
  40         uint8_t i;
  41         /* keyAdd */
  42         for(i=0; i<16; ++i){
  43                 tmp[i] = state->s[i] ^ k->ks[i];
  44         }
  45         /* mixColums */
  46         for(i=0; i<4; ++i){
  47                 state->s[4*0+i] =
  48                           gf256mul(0xe, tmp[4*0+i], 0x1b)
  49                         ^ gf256mul(0xb, tmp[4*1+i], 0x1b)
  50                         ^ gf256mul(0xd, tmp[4*2+i], 0x1b)
  51                         ^ gf256mul(0x9, tmp[4*3+i], 0x1b);
  52                 state->s[4*1+i] =
  53                           gf256mul(0x9, tmp[4*0+i], 0x1b)
  54                         ^ gf256mul(0xe, tmp[4*1+i], 0x1b)
  55                         ^ gf256mul(0xb, tmp[4*2+i], 0x1b)
  56                         ^ gf256mul(0xd, tmp[4*3+i], 0x1b);
  57                 state->s[4*2+i] =
  58                           gf256mul(0xd, tmp[4*0+i], 0x1b)
  59                         ^ gf256mul(0x9, tmp[4*1+i], 0x1b)
  60                         ^ gf256mul(0xe, tmp[4*2+i], 0x1b)
  61                         ^ gf256mul(0xb, tmp[4*3+i], 0x1b);
  62                 state->s[4*3+i] =
  63                           gf256mul(0xb, tmp[4*0+i], 0x1b)
  64                         ^ gf256mul(0xd, tmp[4*1+i], 0x1b)
  65                         ^ gf256mul(0x9, tmp[4*2+i], 0x1b)
  66                         ^ gf256mul(0xe, tmp[4*3+i], 0x1b);
  67         }
  68         /* shiftRows */
  69         aes_invshiftrow(state->s+4, 1);
  70         aes_invshiftrow(state->s+8, 2);
  71         aes_invshiftrow(state->s+12, 3);
  72         /* subBytes */
  73         for(i=0; i<16; ++i){
  74                 state->s[i] = pgm_read_byte(aes_invsbox+state->s[i]);
  75         }
  76 }
  77
  78
  79 static
  80 void aes_dec_firstround(aes_cipher_state_t* state, const aes_roundkey_t* k){
  81         uint8_t i;
  82         /* keyAdd */
  83         for(i=0; i<16; ++i){
  84                 state->s[i] ^= k->ks[i];
  85         }
  86         /* shiftRows */
  87         aes_invshiftrow(state->s+4, 1);
  88         aes_invshiftrow(state->s+8, 2);
  89         aes_invshiftrow(state->s+12, 3);
  90         /* subBytes */
  91         for(i=0; i<16; ++i){
  92                 state->s[i] = pgm_read_byte(aes_invsbox+state->s[i]);
  93         }
  94 }
  95
  96 void aes_decrypt_core(aes_cipher_state_t* state, const aes_genctx_t* ks, uint8_t rounds){
  97         uint8_t i;
  98         aes_dec_firstround(state, &(ks->key[i=rounds]));
  99         for(;rounds>1;--rounds){
 100                 --i;
 101                 aes_dec_round(state, &(ks->key[i]));
 102         }
 103         for(i=0; i<16; ++i){
 104                 state->s[i] ^= ks->key[0].ks[i];
 105         }
 106 }