Get deterministic values from SecureRandom?

Question

For benchmarking purposes, I'd like to have a SecureRandom produce deterministic output. Is this possible through the use of a seed (or maybe specification of an algorithm)?

import java.security.SecureRandom;
class TestSecureRandom {
    public static void main(String [] args) {
        SecureRandom rnd = new SecureRandom();
        rnd.setSeed(1);
        System.out.println(rnd.nextInt());
    }
}

For me, the above program produces different values even though the seed is specified.

Answer 1

In order to override the default seed ,pass PRNG algorithm name like this

import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
class TestSecureRandom {
    public static void main(String [] args) throws NoSuchAlgorithmException, UnsupportedEncodingException {
        SecureRandom rnd = SecureRandom.getInstance("SHA1PRNG");
        rnd.setSeed("foo".getBytes("us-ascii"));
        System.out.println(rnd.nextInt());
        System.out.println(rnd.nextInt());
    }
}

Ouput

-207444710
-1693504542

Answer 2

Everything is clearly explained in documentation:

Many SecureRandom implementations are in the form of a pseudo-random number generator (PRNG), which means they use a deterministic algorithm to produce a pseudo-random sequence from a true random seed. Other implementations may produce true random numbers, and yet others may use a combination of both techniques.

If you ask about implementation, in Linux it uses /dev/urandom so the result is unpredictable. However, you can force SecureRandom to use another algorithm:

SecureRandom rnd = SecureRandom.getInstance("SHA1PRNG");

But for many cases, most reasonable idea is to use some OOP patterns (e.g., jon-skeet's answer).

Answer 3

The simplest way of doing this is probably to make most of your code only depend on Random, with an instance being injected (e.g. by passing it into a constructor). That way, for testing purposes you can pass in a simple Random with a fixed seed - but for real runs where security is required, you can pass in an instance of SecureRandom.