Suppose there is a publicly readable file called /etc/publickey. An entry in the file for user A consists of a user's identifier IDa, the user's public key KUa, and the corresponding private key KRa. 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 Epa[KRa] to obtain KRa. How does the system then verify that Pa was correctly supplied? How could an opponent successfully attack this system?