aes_decrypt.c

void ReverseSubBytes(unsigned char * state) {
    for (int i = 0; i < AES_SIZE; i++) {
        state[i] = inv_s[state[i]];
    }
}

void ReverseShiftRows(unsigned char * plain_text) {
    unsigned char temp_block[AES_SIZE];

    for (int i = 0; i < AES_SIZE; i += 4) {
        //incrementing by 5 causes the diagonal shift effect
        temp_block[i] = plain_text[i];
        temp_block[(i + 5) % AES_SIZE] = plain_text[i+1];
        temp_block[(i + 10) % AES_SIZE] = plain_text[i+2];
        temp_block[(i + 15) % AES_SIZE] = plain_text[i+3];
    }

    for (int i = 0; i < AES_SIZE; i++) {
        plain_text[i] = temp_block[i];
    }
}

void ReverseMixColumns(unsigned char *plain_text) {

    unsigned char temp_block[AES_SIZE];

    for (int i = 0; i < AES_SIZE; i += 4) { 
    //inverse multiplication > 9,11,13,14
        temp_block[i] = gmul(plain_text[i], (unsigned char) 14) ^ gmul(plain_text[i + 1], (unsigned char)11) ^ 
            gmul(plain_text[i + 2], (unsigned char)13) ^ gmul(plain_text[i + 3], (unsigned char)9);

        temp_block[i + 1] = gmul(plain_text[i], (unsigned char) 9) ^ gmul(plain_text[i + 1], (unsigned char)14) ^ 
            gmul(plain_text[i + 2], (unsigned char)11) ^ gmul(plain_text[i + 3], (unsigned char)13);

        temp_block[i + 2] = gmul(plain_text[i], (unsigned char)13) ^ gmul(plain_text[i + 1], (unsigned char)9) ^ 
            gmul(plain_text[i + 2], (unsigned char)14) ^ gmul(plain_text[i + 3], (unsigned char)11);

        temp_block[i + 3] = gmul(plain_text[i], (unsigned char)11) ^ gmul(plain_text[i + 1], (unsigned char)13) ^ 
            gmul(plain_text[i + 2], (unsigned char)9) ^ gmul(plain_text[i + 3], (unsigned char)14);
    }

 for (int i = 0; i < AES_SIZE; i++) {
        plain_text[i] = temp_block[i];
    }
}


void AES_decrypt(unsigned char * plain_text, unsigned char * key) {

    unsigned char expandedKey[AES_SIZE * (NUM_ROUNDS + 1)];

    KeyExpansion(key, expandedKey);

    AddRoundKey(plain_text, expandedKey + (AES_SIZE * NUM_ROUNDS ));

    ReverseShiftRows(plain_text);
    ReverseSubBytes(plain_text);

    for (int i = NUM_ROUNDS -1; i >= 1; i--) {
        AddRoundKey(plain_text, expandedKey + (AES_SIZE * i));
        ReverseMixColumns(plain_text);
        ReverseShiftRows(plain_text);
        ReverseSubBytes(plain_text);
    }
    AddRoundKey(plain_text, expandedKey);
}