Variations/ extensions One-way vs mutual authentication Guaranteeing freshness Timestamps: simple, but requires clock synchronisation Nonces: requires more messages but no nchronised clocks Counters: extra state has to be kept14 Variations / Extensions • One-way vs mutual authentication • Guaranteeing freshness: – Timestamps: simple, but requires clock synchronisation – Nonces: requires more messages, but no synchronised clocks – Counters: extra state has to be kept