Signs a single Input.

            Note:
                This method works only for the following Cryptoconditions
                currently:
                    - Ed25519Fulfillment
                    - ThresholdSha256.

            Args:
                input_ (:class:`~bigchaindb.common.transaction.
                    Input`) The Input to be signed.
                message (str): The message to be signed
                key_pairs (dict): The keys to sign the Transaction with.

def _sign_input(cls, input_, message, key_pairs):
        """Signs a single Input.

            Note:
                This method works only for the following Cryptoconditions
                currently:
                    - Ed25519Fulfillment
                    - ThresholdSha256.

            Args:
                input_ (:class:`~bigchaindb.common.transaction.
                    Input`) The Input to be signed.
                message (str): The message to be signed
                key_pairs (dict): The keys to sign the Transaction with.
        """
        if isinstance(input_.fulfillment, Ed25519Sha256):
            return cls._sign_simple_signature_fulfillment(input_, message,
                                                          key_pairs)
        elif isinstance(input_.fulfillment, ThresholdSha256):
            return cls._sign_threshold_signature_fulfillment(input_, message,
                                                             key_pairs)
        else:
            raise ValueError("Fulfillment couldn't be matched to "
                             'Cryptocondition fulfillment type.')