def curve(self):
        """
        Returns information about the curve used for an EC key

        :raises:
            ValueError - when the key is not an EC key

        :return:
            A two-element tuple, with the first element being a unicode string
            of "implicit_ca", "specified" or "named". If the first element is
            "implicit_ca", the second is None. If "specified", the second is
            an OrderedDict that is the native version of SpecifiedECDomain. If
            "named", the second is a unicode string of the curve name.
        """

        if self.algorithm != 'ec':
            raise ValueError(unwrap(
                '''
                Only EC keys have a curve, this key is %s
                ''',
                self.algorithm.upper()
            ))

        params = self['algorithm']['parameters']
        chosen = params.chosen

        if params.name == 'implicit_ca':
            value = None
        else:
            value = chosen.native

        return (params.name, value)