pub fn private_key(p: u64) -> u64 {
    p/2
}
pub fn public_key(p: u64, g: u64, a: u64) -> u64 {
    let me = |(r, b), e| (if (a >> e) % 2 != 0 { r*b % p } else { r }, b*b % p);
    (0..(64 - a.leading_zeros())).fold((1, g), me).0
}
pub fn secret(p: u64, b_pub: u64, a: u64) -> u64 {
    public_key(p, b_pub, a)
}