When using replicated log as the storage back-end for registrar, we currently rely on ZooKeeper to discover replicas (see ZooKeeperNetwork in src/log/network.hpp). We simply broadcast Paxos messages to all replicas in the ZooKeeperNetwork.
There is a security concern using this approach. For example, say initially there are 3 masters and the quorum size is 2. Now, if a 4th master is accidentally added and joined the ZooKeeperNetwork, we will then operate at 4 replicas with quorum size 2. This could lead to inconsistency in the replicated log (and thus registrar).
The idea here is to introduce a whitelist for masters. We still use ZooKeeperNetwork to discover replicas. However, when broadcasting Paxos messages in the replicated log, we check the whitelist and make sure we don't send Paxos messages to a master that is not in this whitelist.