@Override
  @Nullable
  public Integer getIdRequirementOrNull() {
    return getPublicKey().getIdRequirementOrNull();
  }
Returns a {@link Bytes} instance, which is prefixed to every ciphertext.

<p>Returns the same as {@code getPublicKey().getOutputPrefix()}.
java_src/src/main/java/com/google/crypto/tink/hybrid/HybridPrivateKey.java