Homework 2
Due: October 22, 2025
Points: 100
Short Answer
Answer these questions in one or two sentences.
- (5 points) What is the difference in the goals of the Bell-LaPadula model and the Biba model?
- (5 points) What is the key difference between symmetric and public key cryptosystems?
- (5 points) What is the difference between a digital signature and a digitized signature?
Longer Answer
You can use more than 1 or 2 sentences to answer these. Remember to write clearly (if you need help, go to the Writing Center on campus) and justify your answers!
- (21 points) An affine cipher has the form c = (am + b) mod n. Suppose m is an integer between 0 and 25, each integer representing a letter.
- Let n = 26, a = 3, and b = 19. What is the ciphertext corresponding to the phrase THIS IS A CIPHER MESSAGE.
- A requirement for a cipher is that every plaintext letter correspond to a different ciphertext letter. If a is not relatively prime to n, does the affine cipher meet this property? Either prove it does or present a counterexample.
- If a is relatively prime to n but b is not relatively prime to n, does the affine cipher meet the property in part (b)? Either prove it does or present a counterexample.
- (20 points) Consider the Otway-Rees protocol. Assume that each enciphered message is simply the bits corresponding to the components of the message concatenated together. So, for example, in the first message, one must know the names “Alice” and “Bob”, and the length of the random numbers r1 and n, to be able to parse the portion of the first message that is enciphered with kAlice. The separate parts of the enciphered message have no indicators; the recipient is expected to determine them.
- Consider Alice when all 4 steps of the protocol have been completed.
How does Alice know that steps 2 and 3 have taken place?
- Massicotte asks us to assume that an adversary Edgar is impersonating Bob, and has sufficient control over the exchange
so that he receives the messages intended for Bob. Bob never sees them. What components of the protocol does Edgar know —
that is, does he know r1, r2, n, or ksession, or the names of “Alice” and “Bob”? How?
- Given this, in step 4 of the protocol, how might Edgar provide Alice with a session key that he knows?
- How might someone fix this?
- (20 points) Suppose a user wishes to edit the file xyzzy in a capability-based system. How can he be sure that the editor cannot access any other file? Could this be done in an ACL-based system? If so, how? If not, why not?
- (24 points) Consider Multics procedures p and q. Procedure p is executing and needs to invoke procedure q. Procedure q’s access bracket is (5, 6) and its call bracket is (6, 9). Assume that q’s access control list gives p full (read, write, append, and execute) rights to q. In which ring(s) must p execute for the following to happen?
- p can invoke q, but a ring-crossing fault occurs.
- p can invoke q provided that a valid gate is used as an entry point.
- p cannot invoke q.
- p can invoke q without any ring-crossing fault occurring, but not necessarily through a valid gate.
