Collectively Witnessing Log Servers in CT

This document proposes a backward-compatible extension to CT enabling log servers to obtain compact collective signatures from any number of well-known "witness" servers, which clients can check without gossip to verify that log server records have been widely witnessed. Collective signatures proactively protect clients from man-in-the- middle attackers who may have stolen the private keys of one or more log servers, even if the attacker controls the client's network access, the client is unwilling to gossip for privacy reasons, or the client does not wish to incur the network bandwidth and/or latency costs of gossip.