This, for sure, is one of my favorite questions during my university year.
Q: A local cable company is improving the security of their method for distribution of “pay per view” movies. Each subscriber is given a secret key Ki at subscription time; this is programmed into that users black box on top of their TV, and kept in the company’s secured database. A movie M, which consists of a string of many gigabits
of data, is delivered to a subscriber’s home by cable. A user’s request for a movie is authenticated by a data value which is a function of Ki, sent from the access box to the cable company. Ki is also used to recover a special “movie key” as described below.
• Scheme I. The movie M is encrypted under Ki for each user Ui that requested to view
the movie, and such user Ui is sent the movie as a different string of bits EKi(M).
