Because of the known risks of the unix password system, the sunos-4.0 documentation recommends that the password ? le be removed and replaced with a publicly readable ? le called /etc/publickey. an entry in the ? le for user a consists of a user’s identi? er ida, the user’s public key, pua, and the corresponding private key pra. this private key is encrypted using des with a key derived from the user’s login password pa. when a logs in, the system decrypts e(pa, pra)to obtain pra. (a) the system then veri? es that pa was correctly supplied. how? (b) how can an opponent attack this system?