code: public static byte[] signMessage(RSAPrivateKey key, byte[] message)
              throws InvalidKeyException, NoSuchAlgorithmException, SignatureException {
          return signMessage(key, message, DEFAULT_SIGNATURE_ALGORITHM);
      }
docs: Sign a message with RSA private key, using {@link #DEFAULT_SIGNATURE_ALGORITHM}.
  
  @param key
  @param message
  @return the signature
  @throws InvalidKeyException
  @throws NoSuchAlgorithmException
  @throws SignatureException