Past exam of the computer science course of the University of Oxford / 2015 / Computer security / 2 / c / Solution

A denial-of-service attack can exploit both an honest relay and replay at Tim. The adversary repeatedly sends cheap unauthenticated message-1 requests naming Alice and Bob. Bob then performs the public-key encryption in message 2, and Tim performs a private-key decryption, generates a session key, looks up the users' records and performs two public-key encryptions. The adversary's first messages contain no cryptographic work.

Alternatively, after observing one valid message 2, the adversary submits copies directly to Tim. It remains well formed; the printed protocol has no request-replay cache. Tim's promise to reject malformed requests does not imply duplicate suppression. Each replay can provoke the expensive operations again. If such a cache is added, the repeated-message-1 method still induces distinct requests through Bob unless admission is controlled.