Replacing a SecureRandom.getInstance() algorithm in Java for mocking

note.txt

When writing tests, it can be a burden to make assertions about code with non-deterministic behavior such as random number generation. The above class shows a way of mocking SecureRandom.getInstance() to produce deterministic behaviour without using bytecode-modifying mocking frameworks such as PowerMock or jMockIt, which often cause headaches by interfering with code coverage tools.

Probably no-one besides me will ever need to do this, but after dealing with the side effects of power-mocking tools all week, I liked the solution too much to share :)

ReplaceAlgorithms.java

import java.security.NoSuchAlgorithmException;
import java.security.Provider;
import java.security.SecureRandom;
import java.security.SecureRandomSpi;
import java.security.Security;

public class ReplaceAlgorithms {

	public static void main(String[] args) throws NoSuchAlgorithmException {
		byte[] bytes = new byte[16];

		SecureRandom.getInstance("SHA1PRNG").nextBytes(bytes);
		System.out.println(new String(bytes));
	}

	static {
		// Give fake provider highest priority
		Security.insertProviderAt(new FakeSha1Provider(), 1);
	}

	public static class FakeSha1Provider extends Provider {

		protected FakeSha1Provider() {
			super("FakeSha1Provider", 1.0, null);
			put("SecureRandom.SHA1PRNG", FakeAlgorithm.class.getName());
		}
	}
	
	public static class FakeAlgorithm extends SecureRandomSpi {

		@Override
		protected void engineSetSeed(byte[] seed) {
			for (int i = 0; i < seed.length; i++) {
				// Deterministic result for a given length
				seed[i] = (byte) ('a' + (i % 26));
			}
		}

		@Override
		protected void engineNextBytes(byte[] bytes) {
			for (int i = 0; i < bytes.length; i++) {
				bytes[i] = (byte) ('a' + (i % 26));
			}
		}

		@Override
		protected byte[] engineGenerateSeed(int numBytes) {
			byte[] bytes = new byte[numBytes];
			for (int i = 0; i < numBytes; i++) {
				bytes[i] = (byte) ('a' + (i % 26));
			}
			return bytes;
		}

	}

}