The following protocol is used to authenticate the user A to server B. But not vice versa, i.e. we..

The following protocol is used to authenticate the user A to server B. But not vice versa, i.e. we assume that the identity of the server is checked by some means (e.g. certificate). 1. A ? B : A 2. B ? A : NB 3. A ? B : EKAB (NB) a) The assumption is that the user and server shares the same encryption (symmetric) key KAB. Explain why it is important that NB is not repeated. What kind of attack is applicable when the same NB is reused ?