aes_dec.c

/* aes.c */
/*
    This file is part of the AVR-Crypto-Lib.
    Copyright (C) 2006-2015 Daniel Otte (bg@nerilex.org)
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
 
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
 
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/
 
#include <stdint.h>
#include <string.h>
#include "../gf256mul/gf256mul.h"
#include "aes.h"
#include "aes_invsbox.h"
#include "aes_dec.h"
 
 
// static
// void aes_invshiftrow(void *data, uint8_t shift){
//  uint8_t tmp[4];
//  tmp[0] = ((uint8_t*)data)[(4+0-shift)&3];
//  tmp[1] = ((uint8_t*)data)[(4+1-shift)&3];
//  tmp[2] = ((uint8_t*)data)[(4+2-shift)&3];
//  tmp[3] = ((uint8_t*)data)[(4+3-shift)&3];
//  memcpy(data, tmp, 4);
// }
 
static
void aes_invshiftcol(void *data, uint8_t shift){
    uint8_t tmp[4];
    tmp[0] = ((uint8_t*)data)[ 0];
    tmp[1] = ((uint8_t*)data)[ 4];
    tmp[2] = ((uint8_t*)data)[ 8];
    tmp[3] = ((uint8_t*)data)[12];
    ((uint8_t*)data)[ 0] = tmp[(4-shift+0)&3];
    ((uint8_t*)data)[ 4] = tmp[(4-shift+1)&3];
    ((uint8_t*)data)[ 8] = tmp[(4-shift+2)&3];
    ((uint8_t*)data)[12] = tmp[(4-shift+3)&3];
}
static
void aes_dec_round(aes_cipher_state_t *state, const aes_roundkey_t *k){
    uint8_t tmp[16];
    uint8_t i;
    uint8_t t,u,v,w;
    /* keyAdd */
    for(i=0; i<16; ++i){
        tmp[i] = state->s[i] ^ k->ks[i];
    }
    /* mixColums */
    for(i=0; i<4; ++i){
        t = tmp[4*i+3] ^ tmp[4*i+2];
        u = tmp[4*i+1] ^ tmp[4*i+0];
        v = t ^ u;
        v = gf256mul(0x09, v, 0x1b);
        w = v ^ gf256mul(0x04, tmp[4*i+2] ^ tmp[4*i+0], 0x1b);
        v = v ^ gf256mul(0x04, tmp[4*i+3] ^ tmp[4*i+1], 0x1b);
        state->s[4*i+3] = tmp[4*i+3] ^ v ^ gf256mul(0x02, tmp[4*i+0] ^ tmp[4*i+3], 0x1b);
        state->s[4*i+2] = tmp[4*i+2] ^ w ^ gf256mul(0x02, t, 0x1b);
        state->s[4*i+1] = tmp[4*i+1] ^ v ^ gf256mul(0x02, tmp[4*i+2] ^ tmp[4*i+1], 0x1b);
        state->s[4*i+0] = tmp[4*i+0] ^ w ^ gf256mul(0x02, u, 0x1b);
        
        /*
        state->s[4*i+0] =
              gf256mul(0xe, tmp[4*i+0], 0x1b)
            ^ gf256mul(0xb, tmp[4*i+1], 0x1b)
            ^ gf256mul(0xd, tmp[4*i+2], 0x1b)
            ^ gf256mul(0x9, tmp[4*i+3], 0x1b);
        state->s[4*i+1] =
              gf256mul(0x9, tmp[4*i+0], 0x1b)
            ^ gf256mul(0xe, tmp[4*i+1], 0x1b)
            ^ gf256mul(0xb, tmp[4*i+2], 0x1b)
            ^ gf256mul(0xd, tmp[4*i+3], 0x1b);
        state->s[4*i+2] =
              gf256mul(0xd, tmp[4*i+0], 0x1b)
            ^ gf256mul(0x9, tmp[4*i+1], 0x1b)
            ^ gf256mul(0xe, tmp[4*i+2], 0x1b)
            ^ gf256mul(0xb, tmp[4*i+3], 0x1b);
        state->s[4*i+3] =
              gf256mul(0xb, tmp[4*i+0], 0x1b)
            ^ gf256mul(0xd, tmp[4*i+1], 0x1b)
            ^ gf256mul(0x9, tmp[4*i+2], 0x1b)
            ^ gf256mul(0xe, tmp[4*i+3], 0x1b);
        */
    }   
    /* shiftRows */
    aes_invshiftcol(state->s+1, 1);
    aes_invshiftcol(state->s+2, 2);
    aes_invshiftcol(state->s+3, 3);     
    /* subBytes */
    for(i=0; i<16; ++i){
        state->s[i] = ROMEM_READ_BYTE(aes_invsbox_T_p + state->s[i]);
    }
}
 
 
static
void aes_dec_firstround(aes_cipher_state_t *state, const aes_roundkey_t *k){
    uint8_t i;
    /* keyAdd */
    for(i=0; i<16; ++i){
        state->s[i] ^= k->ks[i];
    }
    /* shiftRows */
    aes_invshiftcol(state->s+1, 1);
    aes_invshiftcol(state->s+2, 2);
    aes_invshiftcol(state->s+3, 3);     
    /* subBytes */
    for(i=0; i<16; ++i){
        state->s[i] = ROMEM_READ_BYTE(aes_invsbox_T_p + state->s[i]);
    }
}
 
void aes_decrypt_core(aes_cipher_state_t *state, const aes_genctx_t *ks, uint8_t rounds){
    uint8_t i;
    aes_dec_firstround(state, &(ks->key[i=rounds]));
    for(;rounds>1;--rounds){
        --i;
        aes_dec_round(state, &(ks->key[i]));
    }
    for(i=0; i<16; ++i){
        state->s[i] ^= ks->key[0].ks[i];
    }
}
 
void aes128_dec(void *buffer, aes128_ctx_t *ctx){
    aes_decrypt_core(buffer, (aes_genctx_t*)ctx, 10);
}
 
void aes192_dec(void *buffer, aes192_ctx_t *ctx){
    aes_decrypt_core(buffer, (aes_genctx_t*)ctx, 12);
}
 
void aes256_dec(void *buffer, aes256_ctx_t *ctx){
    aes_decrypt_core(buffer, (aes_genctx_t*)ctx, 14);
}