def sign(plain, sig = Data.new, mode = GPGME::SIG_MODE_NORMAL)
      err = GPGME::gpgme_op_sign(self, plain, sig, mode)
      exc = GPGME::error_to_exception(err)
      raise exc if exc
      sig
    end
def sign(plain, sig = Data.new, mode = GPGME::SIG_MODE_NORMAL)
      err = GPGME::gpgme_op_sign(self, plain, sig, mode)
      exc = GPGME::error_to_exception(err)
      raise exc if exc
      sig
    end
Create a signature for the text.
 +plain+ is a data object which contains the text.
 +sig+ is a data object where the generated signature is stored.