How to Safely Close a Discussion

In the secure communication problem, we focus on safe termination. In applications such as electronic transactions, we want each party to be ensured that both sides agree on the same state: success or failure. This problem is equivalent to the well known coordinated attack problem. Solutions exist. They however concentrate on the probability of disagreement, and attack incentives have been overlooked so far. Furthermore, they focus on a notion of round and are not optimal in terms of communication complexity. To solve the safe termination problem, we revisit the Keep-in-Touch protocol that we introduced in 2003. Considering the communication complexity, the probability of unsafe termination, and the attack incentive, we prove that the Keep-in-Touch protocol is optimal.