public static String generateID() {
    var raw = new byte[32];
    sr.nextBytes(raw);
    return encoder.encodeToString(raw);
  }
@Test
  void generate_unique() {

    var previous = new HashSet<>();

    for (int i = 0; i < 100; i++) {

      var next = IdGenerator.generateID();
      if (previous.contains(next)) {
        fail();
      }
      previous.add(next);
    }
  }