code: public static byte[] signMessageWithPrivateKey(byte[] keyData, byte[] message)
              throws InvalidKeyException, NoSuchAlgorithmException, InvalidKeySpecException,
              SignatureException {
          return signMessageWithPrivateKey(keyData, message, DEFAULT_SIGNATURE_ALGORITHM);
      }
docs: Sign a message with RSA private key, using {@link #DEFAULT_SIGNATURE_ALGORITHM}.
  
  @param keyData
  RSA private key data (value of {@link RSAPrivateKey#getEncoded()})
  @param message
  @return
  @throws InvalidKeyException
  @throws NoSuchAlgorithmException
  @throws InvalidKeySpecException
  @throws SignatureException