def encrypt(recp, plain, cipher = Data.new, flags = 0)
      err = GPGME::gpgme_op_encrypt(self, recp, flags, plain, cipher)
      exc = GPGME::error_to_exception(err)
      raise exc if exc
      cipher
    end
def encrypt(recp, plain, cipher = Data.new, flags = 0)
      err = GPGME::gpgme_op_encrypt(self, recp, flags, plain, cipher)
      exc = GPGME::error_to_exception(err)
      raise exc if exc
      cipher
    end
Encrypt the plaintext in the data object for the recipients and
 return the ciphertext.