pub fn validate_x25519_scalar(s: &[u8]) -> bool {
    if s.len() != 32 {
        return false;
    }
    let mut all_zero = true;
    let mut i = 0usize;
    while i < s.len() {
        if s[i] != 0 {
            all_zero = false;
        }
        i += 1;
    }
    if all_zero {
        return false;
    }
    (s[0] & 7) == 0 && (s[31] & 128) == 0 && (s[31] & 64) != 0
}