The department assigns each of them an account based on the students first initial and last name. For the rest of this question, assume that the first security flaw is not exploited. This attack is an example of a problem of freshness. We saw how authentication protocols insure that messages are fresh. It's not a problem in public key signed e-mail because the private part of the public key pair is kept secret to Alice.
Nonces insure freshness.
To implement nonces in the voicepritn system, when Alice wants to log in the system will generate a random phrase and force Alice to say it back. Show each step in your proof, clearly indicating what you know before and after and what rule you're using. I.e., the first message is irrelevant. In the second, for simplicity we ignore the duration and will treat the timestamp like a second nonce. TABS is a transaction system that cooperated successfully with an operating system.