@Test
public void testSignature() throws Exception {

  Signer signer = new Signer("secret".getBytes());
  String s1 = signer.sign("ok");
  String s2 = signer.sign("ok");
  String s3 = signer.sign("wrong");
  Assert.assertEquals(s1, s2);
  Assert.assertNotSame(s1, s3);
}