import struct


def drkey_signing_input_req(isd_as, prefetch, timestamp):
    """
    Pack the input such that it can be signed.

    :param ISD_AS isd_as: the ISD-AS.
    :param Bool prefetch: indicator if prefetch (True) or not (False).
    :param int timestamp: signature creation time (format: drkey_time()).
    :returns: the packed signature input.
    :rtype: bytes
    """
    fetch = struct.pack("!B", prefetch)
    ts = struct.pack("!Q", timestamp)
    return b"".join([isd_as.pack(), fetch, ts])