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 users identi?er IDA, the users public key, PUa, and the corresponding private key PRa. This private key is encrypted using DES with a key derived from the users 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?