Function for Modular exponentiation. Modular exponentiation is the remainder when an integer b (the base) is raised to the power e (the exponent), and divided by a positive integer m (the modulus); that is, c = be mod m. From the definition of division, it follows that 0 ≤ c < m.

lib.rs

fn modPow(base: u64, exponent: u64, modulus: u64) -> u64 {
    if m == 1 {
        return 0;
    }
    let mut r = 1;
    let mut pow = exponent;
    let mut num = base % modulus;
    loop {
        if pow == 0 {
            break;
        }
        if pow % 2 == 1 {
            r = (r * num) % m;
        }
        num = (num * num) % m;
        pow = pow >> 1;
    }
    r
}

Modular Exponentiation