class GPGME::Ctx

def sign(plain, sig = Data.new, mode = GPGME::SIG_MODE_NORMAL)

+sig+ is a data object where the generated signature is stored.
+plain+ is a data object which contains the text.
Create a signature for the text. 
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