Homework 4

Points: 100	Due Date: Monday, December 6, 1999 at 11:59PM



1.	(25 points) Chapter 7, exercise 2

2.	(10 points) Chapter 7, supplementary exercise 2

3.	(15 points) The relations certified (see ER1) and allowed (see ER2) can be collapsed into a single relation. Why 
doesn't the Clark-Wilson Model do this?

4.	(25 points) Consider the following protection mechanism. Each object and each process is assigned a number. A 
process can only access an object if the object has a higher number than the process. Which of the mechanisms 
discussed in class (ACL, capabilities, lock-and-key, the MULTICS ring protection scheme) does this most 
closely resemble, and why? In what essential way does it differ from the scheme discussed in class?

5.	(25 points) Assume the Clark-Wilson model is implemented on a computer system. Could a computer virus that 
scrambled constrained data items be introduced into the system? Why or why not? Specifically, if not, identify 
the precise control that would prevent it from being intorduced, and say why it would prevent the virus from 
being introduced; if yes, identify the specific control or controls that allow it to be introduced and say why they 
fail to keep it out.

Extra Credit

6.	(10 points) Chapter 7, exercise 4

7.	(5 points) Chapter 7, supplementary exercise 1