void
aes_compute_inv_tables(void) {
int i;
uint8_t x, xe, x9, xd, xb;
v32_t tmp;
/* combine sbox with linear operations to form 8-bit to 32-bit tables */
for (i=0; i < 256; i++) {
x = aes_inv_sbox[i];
xe = gf2_8_multiply(0x0e, x);
x9 = gf2_8_multiply(0x09, x);
xd = gf2_8_multiply(0x0d, x);
xb = gf2_8_multiply(0x0b, x);
tmp.v8[0] = xe;
tmp.v8[1] = x9;
tmp.v8[2] = xd;
tmp.v8[3] = xb;
U0[i] = tmp.value;
tmp.v8[0] = xb;
tmp.v8[1] = xe;
tmp.v8[2] = x9;
tmp.v8[3] = xd;
U1[i] = tmp.value;
tmp.v8[0] = xd;
tmp.v8[1] = xb;
tmp.v8[2] = xe;
tmp.v8[3] = x9;
U2[i] = tmp.value;
tmp.v8[0] = x9;
tmp.v8[1] = xd;
tmp.v8[2] = xb;
tmp.v8[3] = xe;
U3[i] = tmp.value;
tmp.v8[0] = tmp.v8[1] = tmp.v8[2] = tmp.v8[3] = x;
U4[i] = tmp.value;
}
}
gf2_8
gf2_8_compute_inverse(gf2_8 x) {
unsigned int i;
if (x == 0) return 0; /* zero is a special case */
for (i=0; i < 256; i++)
if (gf2_8_multiply((gf2_8) i, x) == 1)
return (gf2_8) i;
return 0;
}
