aes.c

#include "aes.h"

void initialize_aes_sbox(u8 sbox[256]) {
  u8 p = 1, q = 1;
  
  /* loop invariant: p * q == 1 in the Galois field */
  do {
    /* multiply p by 3 */
    p = p ^ (p << 1) ^ (p & 0x80 ? 0x1B : 0);

    /* divide q by 3 (equals multiplication by 0xf6) */
    q ^= q << 1;
    q ^= q << 2;
    q ^= q << 4;
    q ^= q & 0x80 ? 0x09 : 0;

    /* compute the affine transformation */
    u8 xformed = q ^ ROTL8(q, 1) ^ ROTL8(q, 2) ^ ROTL8(q, 3) ^ ROTL8(q, 4);

    sbox[p] = xformed ^ 0x63;
  } while (p != 1);

  /* 0 is a special case since it has no inverse */
  sbox[0] = 0x63;
}

void printState (u8 in[16]) {
  for(int i=0; i < 4; i++) {
    printf("%.2X %.2X %.2X %.2X\n", in[i], in[i+4], in[i+8], in[i+12]);
  }
  printf("\n");
}

void subBytes (u8 state[16]) {
  for (int i = 0; i < 16; i++)
    state[i] = SBox[state[i]];
}


void shiftRows (u8 state[16]) {
  u8 out[16];
  int shiftTab[16] = {0, 5, 10, 15, 4, 9, 14, 3, 8, 13, 2, 7, 12, 1, 6, 11};
  for (int i = 0; i < 16; i++) {
    out[i] = state[shiftTab[i]];
  }
  memcpy(state, out, sizeof(out));
}

void addRoundKey (u8 state[16], u8 roundKey[16]) {
  for (int i = 0; i < 16; i++)
    state[i] ^= roundKey[i];
}

u8 gMul (u8 a, u8 b) {
  u8 p = 0;
  u8 hi_bit_set;

  for (int i = 0; i < 8; i++) {
    if ((b & 1) == 1)
      p ^= a;
    hi_bit_set = (a & 0x80);
    a <<= 1;
    if (hi_bit_set == 0x80)
      a ^= 0x1b;
    b >>= 1;
  }
return p;
}
void mixColumns (u8 state[16]) {
  u8 out[16];
  for (int i = 0; i < 4; i++) {
    out[4*i] = gMul(2, state[4*i]) ^ gMul(3, state[4*i + 1]) ^ state[4*i + 2] ^ state[4*i + 3];
    out[4*i + 1] = state[4*i] ^ gMul(2, state[4*i + 1]) ^ gMul(3, state[4*i + 2]) ^ state[4*i + 3];
    out[4*i + 2] = state[4*i] ^ state[4*i + 1] ^ gMul(2, state[4*i + 2]) ^ gMul(3, state[4*i + 3]);
    out[4*i + 3] = gMul(3, state[4*i]) ^ state[4*i+1] ^ state[4*i + 2] ^ gMul(2, state[4*i + 3]);
  }
  
  memcpy(state, out, sizeof(out));
}

void expandKey (u8 key[16], u8 expandedKey[176]) {
  u8 tmp[4];
  int i = 0;

  for (i = 0; i < 4; i++) {
    expandedKey[4*i] = key[4*i];
    expandedKey[4*i + 1] = key[4*i + 1];
    expandedKey[4*i + 2] = key[4*i + 2];
    expandedKey[4*i + 3] = key[4*i + 3];
  }

  for (i = 4; i < 44; i++) {
    tmp[0] = expandedKey[4*(i-1)];
    tmp[1] = expandedKey[4*(i-1) + 1];
    tmp[2] = expandedKey[4*(i-1) + 2];
    tmp[3] = expandedKey[4*(i-1) + 3];

    if (i % 4 == 0) {
        int k = tmp[0];
        tmp[0] = SBox[tmp[1]] ^ rCon[i/4];
        tmp[1] = SBox[tmp[2]];
        tmp[2] = SBox[tmp[3]];
        tmp[3] = SBox[k];

    }
    expandedKey[4*i] = expandedKey[4*(i-4)] ^ tmp[0];
    expandedKey[4*i + 1] = expandedKey[4*(i-4) + 1] ^ tmp[1];
    expandedKey[4*i + 2] = expandedKey[4*(i-4) + 2] ^ tmp[2];
    expandedKey[4*i + 3] = expandedKey[4*(i-4) + 3] ^ tmp[3];
  }
}


void aes_128_encrypt (u8 input[16], u8 output[16]) {
  u8 expandedKey[176];
  u8 key[16] = {0};

  expandKey (key, expandedKey);

  for (int i = 0; i < 9; i++) {

    shiftRows (input); 
    shiftRows (expandedKey+16*i);
    addRoundKey (input, expandedKey + 16*i);
    subBytes (input);
    mixColumns (input);

  }

  shiftRows (input);
  shiftRows (expandedKey + 144);
  addRoundKey (input, expandedKey + 144);
  subBytes (input);
  addRoundKey (input, expandedKey + 160);

  for (int i = 0; i < 16; i++)
    output[i] = input[i];

}