I am a student taking a cryptography course so forgive me if this comes off as a silly question.
This is an assignment question:
Alice ------R--------> Bob Alice <-----[R]B------ Bob Alice ------[R+1]A---> Bob
Why doesn't this protocol provide mutual authentication?
Here's a plausible scenario Alice sends R trudy intercepts it and sends it to bob impersonating alice. Bob sends back R signed by him to trudy. Trudy now impersonates Bob and sends R with Bob's signature to Alice impersonating Bob. alice sends [R+1] signed by alice which trudy can now use to impersonate as Alice to Bob.
If that is a valid attack. Wouldn't this work with every protocol? After all we're merely just relaying message back and forth among the two parties.