Good afternoon,
I will take as example the Homework 8, "Providing all three properties", question 1.
If Alice sends a message Ks{m} to Bob. Let's assume that Persa is in the path between Alice and Bob. Persa could modify the msg arbitrarily and send it to Bob.
So Bob will receive two messages : one from Alice and one from Persa.
My question : How can Persa block Alice's msg, so only one reaches Bob ?