public String generate(int secretLength) {
    byte[] randomBytes = new byte[secretLength];
    payloadSecretRng.nextBytes(randomBytes);
    return BaseEncoding.base16().lowerCase().encode(randomBytes);
  }
@Test
  public void generate_always_generatesExpectedSecretString() {
    PayloadSecretGenerator secretGenerator = new PayloadSecretGenerator(TEST_RNG);

    assertThat(secretGenerator.generate(4)).isEqualTo("ffffffff");
  }