Final Exam. May 8, 2001. Question 3 -- Answers and Grading

10 points total

Answer

A. False. This is, frankly, theoretically impossible, since if someone steals the key, they can steal the text. It is the other way around: After sending you the public key, you can send me securely encrypted text.

B. The access rights of a file in Unix is held in the file's inode, and is a combination of the protection bits, the user id, and the group id. There are ways to change each of these:

chmod, to change the user, group, and world protection bits
chgrp, to change the group id of the file.
chown, to change the owner of the file.

Additionally, there is a fourth way -- the superuser can change the composition of groups by editing the /etc/groups file.

Some of you said that changing the setuid bit is another way, but that is a subset of chmod. If you didn't put chmod, then you got half credit (.3 points) for that answer. If you did put chmod, then you did not receive credit for mentioning the setuid bit.

C. Thrashing occurs when the combined size of the working sets of all resident programs is greater than the size of physical memory.

D. True: LRU is a ``stack'' algorithm, and does not exhibit Belady's anomaly.

E. False: public key encryption is based on the assumption that given those numbers d and n, it is very hard to discover e. Remember how d and e are defined: N is the product of two large primes p and q, and d and e are defined such that d*e mod (p-1)(q-1) equals one. So public key encryption is based on the fact that it is extremely hard to factor a product of two large primes.

You could have figured this out without worrying about the mechanics of public-key encryption if you knew that the running time of finding a greatest common denominator of m and n is O(log m) (where m is greater than n).

Grading

Two points per question. If you answered one of the true/falses wrong, but then gave me a good definition that meant that you should have gotten it correct, I gave you half credit.