Authentication Using Symmetric Keys Assumption -Without Trusted Third Party:each pair of parties share a secret key K -With Trusted Third Party:each party shares a secret key K with the 3rd party Threat model: -Message injection Inject a new message into a channel -Message modification Modify a message in a channel Message loss Delete a message in a channel -Message replay ●Replay an old message 22 Authentication Using Symmetric Keys  Assumption ─ Without Trusted Third Party: each pair of parties share a secret key K ─ With Trusted Third Party: each party shares a secret key K with the 3rd party  Threat model: ─ Message injection ● Inject a new message into a channel ─ Message modification ● Modify a message in a channel ─ Message loss ● Delete a message in a channel ─ Message replay ● Replay an old message