    def _input_valid(self, input_, operation, message, output_condition_uri=None):
        """Validates a single Input against a single Output.

            Note:
                In case of a `CREATE` Transaction, this method
                does not validate against `output_condition_uri`.

            Args:
                input_ (:class:`~bigchaindb.common.transaction.
                    Input`) The Input to be signed.
                operation (str): The type of Transaction.
                message (str): The fulfillment message.
                output_condition_uri (str, optional): An Output to check the
                    Input against.

            Returns:
                bool: If the Input is valid.
        """
        ccffill = input_.fulfillment
        try:
            parsed_ffill = Fulfillment.from_uri(ccffill.serialize_uri())
        except (TypeError, ValueError,
                ParsingError, ASN1DecodeError, ASN1EncodeError):
            return False

        if operation == self.CREATE:
            # NOTE: In the case of a `CREATE` transaction, the
            #       output is always valid.
            output_valid = True
        else:
            output_valid = output_condition_uri == ccffill.condition_uri

        message = sha3_256(message.encode())
        if input_.fulfills:
            message.update('{}{}'.format(
                input_.fulfills.txid, input_.fulfills.output).encode())

        # NOTE: We pass a timestamp to `.validate`, as in case of a timeout
        #       condition we'll have to validate against it

        # cryptoconditions makes no assumptions of the encoding of the
        # message to sign or verify. It only accepts bytestrings
        ffill_valid = parsed_ffill.validate(message=message.digest())
        return output_valid and ffill_valid
Validates a single Input against a single Output.

Note:
In case of a `CREATE` Transaction, this method
does not validate against `output_condition_uri`.

Args:
input_ (:class:`~bigchaindb.common.transaction.
Input`) The Input to be signed.
operation (str): The type of Transaction.
message (str): The fulfillment message.
output_condition_uri (str, optional): An Output to check the
Input against.

Returns:
bool: If the Input is valid.
bigchaindb/common/transaction.py