Credentials done right

1. static byte[] salt() — 16 random bytes (SecureRandom). 2. static byte[] hash(byte[] salt, String password) — SHA-256 over salt bytes followed by UTF-8 password bytes (concatenate the two arrays; MessageDigest from java.security). 3. static boolean verify(byte[] salt, String password, byte[] expectedHash) — recompute hash(salt, password) and compare with MessageDigest.isEqual (constant time). 4. static boolean samePasswordDifferentSalt(byte[] s1, byte[] s2, String pw) — true iff hash(s1,pw) differs from hash(s2,pw) — demonstrating that two users with the same password produce different stored bytes. 5. static boolean timingSafe(byte[] a, byte[] b) — MessageDigest.isEqual(a, b).