SafeEquals

package com.udby.test.security;
 
import java.util.concurrent.ThreadLocalRandom;
 
/**
 * Utility class that helps comparing stuff in a "safe" mode where the timing is important, not the performance.
 * Ie the methods should execute in about the same time if inputs are equal or not-equal
 */
public class SafeEquals {
	private static final int MIN_COMP = 16;
 
	static class EQNE {
		int eq;
		int ne;
 
		EQNE(final boolean eq) {
			this.eq = eq ? 0 : -1;
			this.ne = !eq ? 1 : 0;
		}
		void add(final boolean eq) {
			this.eq += (eq ? 1 : 0);
			this.ne += (!eq ? 1 : 0);
		}
	}
 
	private SafeEquals() {
	}
 
	public static boolean equals(byte[] a, byte[] b) {
		if (a == null || b == null) {
			throw new NullPointerException("a or b is null. a="+(a==null?"null":"")+"b="+(b==null?"null":""));
		}
		return equals(a, b, false);
	}
 
	public static boolean neverEquals(byte[] a, byte[] b) {
		if (a == null || b == null) {
			throw new NullPointerException("a or b is null. a="+(a==null?"null":"")+"b="+(b==null?"null":""));
		}
		return equals(a, b, true);
	}
 
	private static boolean equals(byte[] a, byte[] b, final boolean never) {
		int len = Integer.max(a.length, b.length);
		if (len < MIN_COMP) {
			len = MIN_COMP; // always spend some time doing comparisons..
		}
		len *= 2;
		byte[] ba = randomBytes(len);
		// bb is the same
		byte[] bb = (byte[])ba.clone();
		//
		System.arraycopy(a, 0, ba, 0, a.length);
		System.arraycopy(b, 0, bb, 0, b.length);
 
		EQNE eqne = new EQNE(a.length == b.length && !never);
 
		for (int i = 0; i < len; i++) {
			eqne.add(ba[i] == bb[i]);
		}
 
		// pls note "short circuit" (& vs &&) as we DO want to do both checks ALWAYS
		return eqne.eq == len & eqne.ne == 0;
	}
 
	private static byte[] randomBytes(int len) {
		byte[] bytes = new byte[len];
		ThreadLocalRandom.current().nextBytes(bytes);
		return bytes;
	}
}

// assuming user-name is email, generate hash (SHA-256)
String userNameHash = Conversions.toHex(DigestHelper.sha256(username));
// convert user-name hash (email hash) to user uid...
String userUid = userHelper.lookupUserByMailHash(userNameHash);
 
// find appropriate LoginParams (note: the returned might "belong" to a dummy user)
LoginParam lp = loginParamManager.lookup(userUid);
 
// get appropriate strategy...
PasswordCheckStrategy pcs = PasswordCheckStrategy.instance(lp);
 
// check password...
final boolean ok = pcs.check(passwordHash);
 
if (ok && !DummyKeys.isDummyUid(lp.userUid())) {
	// Success!!
}