// The sub_bytes Function Substitutes the values in the
// state matrix with values in an S-box.
void sub_bytes()
{
int i, j;
for (i = 0; i < 4; i++) {
for (j = 0; j < 4; j++) {
state[i][j] = get_sbox_value(state[i][j]);
}
}
}
// This function produces nb(nr+1) round keys. The round keys are used in each round to encrypt the states.
void key_expansion()
{
int i, j;
unsigned char temp[4], k;
// The first round key is the key itself.
for (i = 0; i < nk; i++) {
round_key[i * 4] = key[i * 4];
round_key[i * 4 + 1] = key[i * 4 + 1];
round_key[i * 4 + 2] = key[i * 4 + 2];
round_key[i * 4 + 3] = key[i * 4 + 3];
}
// All other round keys are found from the previous round keys.
while (i < (nb * (nr + 1))) {
for (j = 0; j < 4; j++) {
temp[j] = round_key[(i - 1) * 4 + j];
}
if (i % nk == 0) {
// This function rotates the 4 bytes in a word to the left once.
// [a0,a1,a2,a3] becomes [a1,a2,a3,a0]
// Function RotWord()
{
k = temp[0];
temp[0] = temp[1];
temp[1] = temp[2];
temp[2] = temp[3];
temp[3] = k;
}
// SubWord() is a function that takes a four-byte input word and
// applies the S-box to each of the four bytes to produce an output word.
// Function Subword()
{
temp[0] = get_sbox_value(temp[0]);
temp[1] = get_sbox_value(temp[1]);
temp[2] = get_sbox_value(temp[2]);
temp[3] = get_sbox_value(temp[3]);
}
temp[0] = temp[0] ^ rcon[i / nk];
} else if (nk > 6 && i % nk == 4) {
// Function Subword()
{
temp[0] = get_sbox_value(temp[0]);
temp[1] = get_sbox_value(temp[1]);
temp[2] = get_sbox_value(temp[2]);
temp[3] = get_sbox_value(temp[3]);
}
}
round_key[i * 4 + 0] = round_key[(i - nk) * 4 + 0] ^ temp[0];
round_key[i * 4 + 1] = round_key[(i - nk) * 4 + 1] ^ temp[1];
round_key[i * 4 + 2] = round_key[(i - nk) * 4 + 2] ^ temp[2];
round_key[i * 4 + 3] = round_key[(i - nk) * 4 + 3] ^ temp[3];
i++;
}
}
