Signs a 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_threshold_signature_fulfillment(cls, input_, message, key_pairs):
        """Signs a 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.
        """
        input_ = deepcopy(input_)
        message = sha3_256(message.encode())
        if input_.fulfills:
            message.update('{}{}'.format(
                input_.fulfills.txid, input_.fulfills.output).encode())

        for owner_before in set(input_.owners_before):
            # TODO: CC should throw a KeypairMismatchException, instead of
            #       our manual mapping here

            # TODO FOR CC: Naming wise this is not so smart,
            #              `get_subcondition` in fact doesn't return a
            #              condition but a fulfillment

            # TODO FOR CC: `get_subcondition` is singular. One would not
            #              expect to get a list back.
            ccffill = input_.fulfillment
            subffills = ccffill.get_subcondition_from_vk(
                base58.b58decode(owner_before))
            if not subffills:
                raise KeypairMismatchException('Public key {} cannot be found '
                                               'in the fulfillment'
                                               .format(owner_before))
            try:
                private_key = key_pairs[owner_before]
            except KeyError:
                raise KeypairMismatchException('Public key {} is not a pair '
                                               'to any of the private keys'
                                               .format(owner_before))

            # cryptoconditions makes no assumptions of the encoding of the
            # message to sign or verify. It only accepts bytestrings
            for subffill in subffills:
                subffill.sign(
                    message.digest(), base58.b58decode(private_key.encode()))
        return input_