def get_op_key(self, operation=None, arg=None):
        """Get the key object associated to the requested opration.
        For example the public RSA key for the 'verify' operation or
        the private EC key for the 'decrypt' operation.

        :param operation: The requested operation.
         The valid set of operations is availble in the
         :data:`JWKOperationsRegistry` registry.
        :param arg: an optional, context specific, argument
         For example a curve name.

        :raises InvalidJWKOperation: if the operation is unknown or
         not permitted with this key.
        :raises InvalidJWKUsage: if the use constraints do not permit
         the operation.
        """
        validops = self._params.get('key_ops',
                                    list(JWKOperationsRegistry.keys()))
        if validops is not list:
            validops = [validops]
        if operation is None:
            if self._params['kty'] == 'oct':
                return self._key['k']
            raise InvalidJWKOperation(operation, validops)
        elif operation == 'sign':
            self._check_constraints('sig', operation)
            return self._get_private_key(arg)
        elif operation == 'verify':
            self._check_constraints('sig', operation)
            return self._get_public_key(arg)
        elif operation == 'encrypt' or operation == 'wrapKey':
            self._check_constraints('enc', operation)
            return self._get_public_key(arg)
        elif operation == 'decrypt' or operation == 'unwrapKey':
            self._check_constraints('enc', operation)
            return self._get_private_key(arg)
        else:
            raise NotImplementedError
Get the key object associated to the requested opration.
        For example the public RSA key for the 'verify' operation or
        the private EC key for the 'decrypt' operation.

        :param operation: The requested operation.
         The valid set of operations is availble in the
         :data:`JWKOperationsRegistry` registry.
        :param arg: an optional, context specific, argument
         For example a curve name.

        :raises InvalidJWKOperation: if the operation is unknown or
         not permitted with this key.
        :raises InvalidJWKUsage: if the use constraints do not permit
         the operation.