_ Git - BearSSL/blob - src/symcipher/aes_ct_dec.c

   1 /*
   2  * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining
   5  * a copy of this software and associated documentation files (the
   6  * "Software"), to deal in the Software without restriction, including
   7  * without limitation the rights to use, copy, modify, merge, publish,
   8  * distribute, sublicense, and/or sell copies of the Software, and to
   9  * permit persons to whom the Software is furnished to do so, subject to
  10  * the following conditions:
  11  *
  12  * The above copyright notice and this permission notice shall be
  13  * included in all copies or substantial portions of the Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  16  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  17  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  18  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  19  * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  20  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  21  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22  * SOFTWARE.
  23  */
  24
  25 #include "inner.h"
  26
  27 /* see inner.h */
  28 void
  29 br_aes_ct_bitslice_invSbox(uint32_t *q)
  30 {
  31         /*
  32          * AES S-box is:
  33          *   S(x) = A(I(x)) ^ 0x63
  34          * where I() is inversion in GF(256), and A() is a linear
  35          * transform (0 is formally defined to be its own inverse).
  36          * Since inversion is an involution, the inverse S-box can be
  37          * computed from the S-box as:
  38          *   iS(x) = B(S(B(x ^ 0x63)) ^ 0x63)
  39          * where B() is the inverse of A(). Indeed, for any y in GF(256):
  40          *   iS(S(y)) = B(A(I(B(A(I(y)) ^ 0x63 ^ 0x63))) ^ 0x63 ^ 0x63) = y
  41          *
  42          * Note: we reuse the implementation of the forward S-box,
  43          * instead of duplicating it here, so that total code size is
  44          * lower. By merging the B() transforms into the S-box circuit
  45          * we could make faster CBC decryption, but CBC decryption is
  46          * already quite faster than CBC encryption because we can
  47          * process two blocks in parallel.
  48          */
  49         uint32_t q0, q1, q2, q3, q4, q5, q6, q7;
  50
  51         q0 = ~q[0];
  52         q1 = ~q[1];
  53         q2 = q[2];
  54         q3 = q[3];
  55         q4 = q[4];
  56         q5 = ~q[5];
  57         q6 = ~q[6];
  58         q7 = q[7];
  59         q[7] = q1 ^ q4 ^ q6;
  60         q[6] = q0 ^ q3 ^ q5;
  61         q[5] = q7 ^ q2 ^ q4;
  62         q[4] = q6 ^ q1 ^ q3;
  63         q[3] = q5 ^ q0 ^ q2;
  64         q[2] = q4 ^ q7 ^ q1;
  65         q[1] = q3 ^ q6 ^ q0;
  66         q[0] = q2 ^ q5 ^ q7;
  67
  68         br_aes_ct_bitslice_Sbox(q);
  69
  70         q0 = ~q[0];
  71         q1 = ~q[1];
  72         q2 = q[2];
  73         q3 = q[3];
  74         q4 = q[4];
  75         q5 = ~q[5];
  76         q6 = ~q[6];
  77         q7 = q[7];
  78         q[7] = q1 ^ q4 ^ q6;
  79         q[6] = q0 ^ q3 ^ q5;
  80         q[5] = q7 ^ q2 ^ q4;
  81         q[4] = q6 ^ q1 ^ q3;
  82         q[3] = q5 ^ q0 ^ q2;
  83         q[2] = q4 ^ q7 ^ q1;
  84         q[1] = q3 ^ q6 ^ q0;
  85         q[0] = q2 ^ q5 ^ q7;
  86 }
  87
  88 static void
  89 add_round_key(uint32_t *q, const uint32_t *sk)
  90 {
  91         int i;
  92
  93         for (i = 0; i < 8; i ++) {
  94                 q[i] ^= sk[i];
  95         }
  96 }
  97
  98 static void
  99 inv_shift_rows(uint32_t *q)
 100 {
 101         int i;
 102
 103         for (i = 0; i < 8; i ++) {
 104                 uint32_t x;
 105
 106                 x = q[i];
 107                 q[i] = (x & 0x000000FF)
 108                         | ((x & 0x00003F00) << 2) | ((x & 0x0000C000) >> 6)
 109                         | ((x & 0x000F0000) << 4) | ((x & 0x00F00000) >> 4)
 110                         | ((x & 0x03000000) << 6) | ((x & 0xFC000000) >> 2);
 111         }
 112 }
 113
 114 static inline uint32_t
 115 rotr16(uint32_t x)
 116 {
 117         return (x << 16) | (x >> 16);
 118 }
 119
 120 static void
 121 inv_mix_columns(uint32_t *q)
 122 {
 123         uint32_t q0, q1, q2, q3, q4, q5, q6, q7;
 124         uint32_t r0, r1, r2, r3, r4, r5, r6, r7;
 125
 126         q0 = q[0];
 127         q1 = q[1];
 128         q2 = q[2];
 129         q3 = q[3];
 130         q4 = q[4];
 131         q5 = q[5];
 132         q6 = q[6];
 133         q7 = q[7];
 134         r0 = (q0 >> 8) | (q0 << 24);
 135         r1 = (q1 >> 8) | (q1 << 24);
 136         r2 = (q2 >> 8) | (q2 << 24);
 137         r3 = (q3 >> 8) | (q3 << 24);
 138         r4 = (q4 >> 8) | (q4 << 24);
 139         r5 = (q5 >> 8) | (q5 << 24);
 140         r6 = (q6 >> 8) | (q6 << 24);
 141         r7 = (q7 >> 8) | (q7 << 24);
 142
 143         q[0] = q5 ^ q6 ^ q7 ^ r0 ^ r5 ^ r7 ^ rotr16(q0 ^ q5 ^ q6 ^ r0 ^ r5);
 144         q[1] = q0 ^ q5 ^ r0 ^ r1 ^ r5 ^ r6 ^ r7 ^ rotr16(q1 ^ q5 ^ q7 ^ r1 ^ r5 ^ r6);
 145         q[2] = q0 ^ q1 ^ q6 ^ r1 ^ r2 ^ r6 ^ r7 ^ rotr16(q0 ^ q2 ^ q6 ^ r2 ^ r6 ^ r7);
 146         q[3] = q0 ^ q1 ^ q2 ^ q5 ^ q6 ^ r0 ^ r2 ^ r3 ^ r5 ^ rotr16(q0 ^ q1 ^ q3 ^ q5 ^ q6 ^ q7 ^ r0 ^ r3 ^ r5 ^ r7);
 147         q[4] = q1 ^ q2 ^ q3 ^ q5 ^ r1 ^ r3 ^ r4 ^ r5 ^ r6 ^ r7 ^ rotr16(q1 ^ q2 ^ q4 ^ q5 ^ q7 ^ r1 ^ r4 ^ r5 ^ r6);
 148         q[5] = q2 ^ q3 ^ q4 ^ q6 ^ r2 ^ r4 ^ r5 ^ r6 ^ r7 ^ rotr16(q2 ^ q3 ^ q5 ^ q6 ^ r2 ^ r5 ^ r6 ^ r7);
 149         q[6] = q3 ^ q4 ^ q5 ^ q7 ^ r3 ^ r5 ^ r6 ^ r7 ^ rotr16(q3 ^ q4 ^ q6 ^ q7 ^ r3 ^ r6 ^ r7);
 150         q[7] = q4 ^ q5 ^ q6 ^ r4 ^ r6 ^ r7 ^ rotr16(q4 ^ q5 ^ q7 ^ r4 ^ r7);
 151 }
 152
 153 /* see inner.h */
 154 void
 155 br_aes_ct_bitslice_decrypt(unsigned num_rounds,
 156         const uint32_t *skey, uint32_t *q)
 157 {
 158         unsigned u;
 159
 160         add_round_key(q, skey + (num_rounds << 3));
 161         for (u = num_rounds - 1; u > 0; u --) {
 162                 inv_shift_rows(q);
 163                 br_aes_ct_bitslice_invSbox(q);
 164                 add_round_key(q, skey + (u << 3));
 165                 inv_mix_columns(q);
 166         }
 167         inv_shift_rows(q);
 168         br_aes_ct_bitslice_invSbox(q);
 169         add_round_key(q, skey);
 170 }