Given a set of ciphertext bundles (C, X, Z) and their associated uuid,
compute a fixed-length set of “challenges” >= the number of SeverMessageStore entries.
A challenge is returned as a tuple of DH agreement outputs (or random data tuples of the same length).
For benchmarking purposes, supply the rng as a separable parameter, and allow the total number of expected responses to be specified as a paremeter (worst case performance
when the number of items in the server store approaches num total_responses.)