aes_dec.c

   1 /* aes.c */
   2 /*
   3     This file is part of the Crypto-avr-lib/microcrypt-lib.
   4     Copyright (C) 2008, 2009  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 void aes_invshiftcol(void* data, uint8_t shift){
  38         uint8_t tmp[4];
  39         tmp[0] = ((uint8_t*)data)[ 0];
  40         tmp[1] = ((uint8_t*)data)[ 4];
  41         tmp[2] = ((uint8_t*)data)[ 8];
  42         tmp[3] = ((uint8_t*)data)[12];
  43         ((uint8_t*)data)[ 0] = tmp[(4-shift+0)&3];
  44         ((uint8_t*)data)[ 4] = tmp[(4-shift+1)&3];
  45         ((uint8_t*)data)[ 8] = tmp[(4-shift+2)&3];
  46         ((uint8_t*)data)[12] = tmp[(4-shift+3)&3];
  47 }
  48 static
  49 void aes_dec_round(aes_cipher_state_t* state, const aes_roundkey_t* k){
  50         uint8_t tmp[16];
  51         uint8_t i;
  52         /* keyAdd */
  53         for(i=0; i<16; ++i){
  54                 tmp[i] = state->s[i] ^ k->ks[i];
  55         }
  56         /* mixColums */
  57         for(i=0; i<4; ++i){
  58                 state->s[4*i+0] =
  59                           gf256mul(0xe, tmp[4*i+0], 0x1b)
  60                         ^ gf256mul(0xb, tmp[4*i+1], 0x1b)
  61                         ^ gf256mul(0xd, tmp[4*i+2], 0x1b)
  62                         ^ gf256mul(0x9, tmp[4*i+3], 0x1b);
  63                 state->s[4*i+1] =
  64                           gf256mul(0x9, tmp[4*i+0], 0x1b)
  65                         ^ gf256mul(0xe, tmp[4*i+1], 0x1b)
  66                         ^ gf256mul(0xb, tmp[4*i+2], 0x1b)
  67                         ^ gf256mul(0xd, tmp[4*i+3], 0x1b);
  68                 state->s[4*i+2] =
  69                           gf256mul(0xd, tmp[4*i+0], 0x1b)
  70                         ^ gf256mul(0x9, tmp[4*i+1], 0x1b)
  71                         ^ gf256mul(0xe, tmp[4*i+2], 0x1b)
  72                         ^ gf256mul(0xb, tmp[4*i+3], 0x1b);
  73                 state->s[4*i+3] =
  74                           gf256mul(0xb, tmp[4*i+0], 0x1b)
  75                         ^ gf256mul(0xd, tmp[4*i+1], 0x1b)
  76                         ^ gf256mul(0x9, tmp[4*i+2], 0x1b)
  77                         ^ gf256mul(0xe, tmp[4*i+3], 0x1b);
  78         }
  79         /* shiftRows */
  80         aes_invshiftcol(state->s+1, 1);
  81         aes_invshiftcol(state->s+2, 2);
  82         aes_invshiftcol(state->s+3, 3);
  83         /* subBytes */
  84         for(i=0; i<16; ++i){
  85                 state->s[i] = pgm_read_byte(aes_invsbox+state->s[i]);
  86         }
  87 }
  88
  89
  90 static
  91 void aes_dec_firstround(aes_cipher_state_t* state, const aes_roundkey_t* k){
  92         uint8_t i;
  93         /* keyAdd */
  94         for(i=0; i<16; ++i){
  95                 state->s[i] ^= k->ks[i];
  96         }
  97         /* shiftRows */
  98         aes_invshiftcol(state->s+1, 1);
  99         aes_invshiftcol(state->s+2, 2);
 100         aes_invshiftcol(state->s+3, 3);
 101         /* subBytes */
 102         for(i=0; i<16; ++i){
 103                 state->s[i] = pgm_read_byte(aes_invsbox+state->s[i]);
 104         }
 105 }
 106
 107 void aes_decrypt_core(aes_cipher_state_t* state, const aes_genctx_t* ks, uint8_t rounds){
 108         uint8_t i;
 109         aes_dec_firstround(state, &(ks->key[i=rounds]));
 110         for(;rounds>1;--rounds){
 111                 --i;
 112                 aes_dec_round(state, &(ks->key[i]));
 113         }
 114         for(i=0; i<16; ++i){
 115                 state->s[i] ^= ks->key[0].ks[i];
 116         }
 117 }