a simple rust snippet to compute an AES/Rijndael Sbox.

sbox.rs

// Substitution BOX, a lookup table to optimyze the substitution step
pub type Sbox = [u8; 256];

// shift 8 bits left 
fn rot_l8(x: u8,shift: isize) -> u8 {
    // check that input is in bounds
    assert!(shift < 8);
    assert!(shift > -8);
    // actualy do it
    ((x) << (shift)) | ((x) >> (8 - (shift)))
}

// code addapeded from https://en.wikipedia.org/wiki/Rijndael_S-box
pub fn make_sub_box() -> Sbox {

    let mut sbox: Sbox = [0_u8; 256];

    let mut p: u8 = 1;
    let mut q: u8 = 1;
    loop {
        
        let x = match (p & 0x80) {
            0 => 0,
            _ => 0x1B
        };
    
        p = p ^ (p << 1) ^ x;
        
        q ^= q << 1;
		q ^= q << 2;
		q ^= q << 4;
		q ^= match (q & 0x80) {
            0 => 0,
            _ => 0x09
        };
        
        let transformed = q ^ rot_l8(q, 1) ^ rot_l8(q, 2) ^ rot_l8(q, 3) ^ rot_l8(q, 4);
        
        sbox[p as usize] = transformed ^ 0x63;
        
        // stop if p is one
        if p == 1 {
            break;
        }
    
    }
    
    //zero is specal
    sbox[0] = 0x63;
    
    return sbox;
}

// we need a way to undo the sbox.
pub fn invert_sub_box(sbox :&Sbox) -> Sbox {
    // buffer for inverted sbox
    let mut isbox = [0_u8; 256];

    // i is the location in the sbox, v is the value at that location
    for (i, v) in sbox.iter().enumerate() {
        isbox[*v as usize] = i as u8;
    }

    return isbox;
}