Method get_random_in_range

ciphercore-base/src/random.rs:106–121

Generates a random 64-bit integer modulo a given modulus. To avoid modulo bias, rejection sampling is performed. Rejection sampling bound is 2^64 - (2^64 mod modulus). Each sampling attempt can succeed with probability 1 - (2^64 mod modulus)/2^64. Thus, the expected number of sampling rounds is 2^64/(2^64 - (2^64 mod modulus)) < 2 according to the geometric distribution. WARNING**: this function

(&mut self, modulus: Option<u64>)

    //
    // **WARNING**: this function might leak modulus bits. Don't use it if you want to hide the modulus.
    pub fn get_random_in_range(&mut self, modulus: Option<u64>) -> Result<u64> {
        if let Some(m) = modulus {
            let rem = ((u64::MAX % m) + 1) % m;
            let rejection_bound = u64::MAX - rem;
            let mut r;
            loop {
                r = vec_u64_from_bytes(&self.get_random_bytes(8)?, UINT64)?[0];
                if r <= rejection_bound {
                    break;
                }
            }
            Ok(r % m)
        } else {
            Ok(vec_u64_from_bytes(&self.get_random_bytes(8)?, UINT64)?[0])
        }
    }
}

/// Pseudo-random function (Prf/PRF) based on AES-128.