3 Digital signatures a. Digital signatures are sometimes used for authenticating users in network protocols. SSH and TLS, for example, can both use such a echanism. Consider the following protocol . The server sends Alice a randomly chosen number Alice digitally signs the number and sends the signature back to the server The server checks the signature using Alice's public key. If the signature is valid, then the server accepts Alice's connection request Suppose that Alice uses the same private key to log in to a server and sign her emails. Show how a server could forge emails from Alice. For this exercise, assume that the authentication mechanism signs the bare challenge without hashing, while for emails Alice signs the hash of the message. 2 marks. b. Suggest a modification to the authentication protocol which would defeat this attack. 2 marks