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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _sign_simple_signature_fulfillment(cls, input_, message, key_pairs):
 """Signs a Ed25519Fulfillment. 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. """

        # NOTE: To eliminate the dangers of accidentally signing a condition by
        #       reference, we remove the reference of input_ here
        #       intentionally. If the user of this class knows how to use it,
        #       this should never happen, but then again, never say never.
        input_ = deepcopy(input_)
        public_key = input_.owners_before[0]
        message = sha3_256(message.encode())
        if input_.fulfills:
            message.update('{}{}'.format(
                input_.fulfills.txid, input_.fulfills.output).encode())

        try:
            # cryptoconditions makes no assumptions of the encoding of the
            # message to sign or verify. It only accepts bytestrings
            input_.fulfillment.sign(
                message.digest(), base58.b58decode(key_pairs[public_key].encode()))
        except KeyError:
            raise KeypairMismatchException('Public key {} is not a pair to '
                                           'any of the private keys'
                                           .format(public_key))
        return input_