CS513 Homework 2: Least Privilege and Authentication

General Instructions. You are expected to work alone on this assignment.

Due Date: Thursday, Feb 12, 2004. In class. No late assignments will be accepted.

Please fit your solution on at most one page (use both sides, if necessary), typeset, with 10 point or larger font.

Problem 1:

(a) In what sense is the following exhortation an instance of the Principle of Least Privilege?
Principal of Least Privilege:
Each task should be assigned to the principal that (i) has the least set of privileges and (ii) is capable of accomplishing the task.

(b) In what sense are the two different? Illustrate by examples.

Problem 2:

Below is an authentication protocol similar to what we discussed in class. There, k is a key shared only by principals A and B. This protocol is subject to a reflection attack: 
1.  B --> A:  B, r
2.  A --> B:  A, {A, r}k
3.  B --> A:  B, {B, r}k

(a) One way to eliminate reflection attacks is by insisting that the challenge/response for the initiator is different from that employed the responder. Since the response r+1 is different from r-1, we might modify the protocol as follows. 

1.  B --> A:  B, r
2.  A --> B:  A, {A, r+1}k
3.  B --> A:  B, {B, r-1}k

Has this eliminated the reflection attack? Either argue that a reflection attack is no longer possible or exhibit the interleaved protocol steps the constitute an attack.

(b) More generally, suppose we postulate functions G and H and rewrite the protocol in terms of them: 

1.  B --> A:  B, r
2.  A --> B:  A, {A, G(A,r)}k
3.  B --> A:  B, {B, H(B,r)}k
What properties (if any) of G and H would make this protocol immune to reflection attacks? If no G and H would work, then list the interleaved protocol steps (in terms of G and H).