AES-in-C

Advanced Encryption Standard (AES) in C language

```c
you can calculate sbox through following C code

define ROTL8(x,shift) ((uint8_t) ((x) << (shift)) | ((x) >> (8 - (shift))))

void initialize_aes_sbox(uint8_t sbox[256]) {
uint8_t 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 */
    uint8_t 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