Chapter 1

1. Distinguish between vulnerability, threat, and control.

2. Theft usually results in some kind of harm. For example, if someone steals your car, you may suffer financial loss, inconvenience (by losing your mode of transportation), and emotional upset (because of invasion of your personal property and space). List three kinds of harm a company might experience from theft of computer equipment.

3. List at least three kinds of harm a company could experience from electronic espionage or unauthorized viewing of confidential company materials.

4. List at least three kinds of damage a company could suffer when the integrity of a program or company data is compromised.

5. List at least three kinds of harm a company could encounter from loss of service, that is, failure of availability. List the product or capability to which access is lost, and explain how this loss hurts the company.

6. Describe a situation in which you have experienced harm as a consequence of a failure of computer security. Was the failure malicious or not? Did the attack target you specifically or was it general and you were the unfortunate victim?

Chapter 2

1. Describe each of the following four kinds of access control mechanisms in terms of (a) ease of determining authorized access during execution, (b) ease of adding access for a new subject, (c) ease of deleting access by a subject, and (d) ease of creating a new object to which all subjects by default have access.

• per-subject access control list (that is, one list for each subject tells all the objects to which that subject has access)
• per-object access control list (that is, one list for each object tells all the subjects who have access to that object)
• access control matrix
• capability

2. Suppose a per-subject access control list is used. Deleting an object in such a system is inconvenient because all changes must be made to the control lists of all subjects who did have access to the object. Suggest an alternative, less costly means of handling deletion.

3. File access control relates largely to the secrecy dimension of security. What is the relationship between an access control matrix and the integrity of the objects to which access is being controlled?

4. One feature of a capability-based protection system is the ability of one process to transfer a copy of a capability to another process. Describe a situation in which one process should be able to transfer a capability to another.

5. Suggest an efficient scheme for maintaining a per-user protection scheme. That is, the system maintains one directory per user, and that directory lists all the objects to which the user is allowed access. Your design should address the needs of a system with 1000 users, of whom no more than 20 are active at any time. Each user has an average of 200 permitted objects; there are 50,000 total objects in the system.

6. Calculate the timing of password-guessing attacks:

(a) If passwords are three uppercase alphabetic characters long, how much time would it take to determine a particular password, assuming that testing an individual password requires 5 seconds? How much time if testing requires 0.001 seconds?

(b) Argue for a particular amount of time as the starting point for "secure." That is, suppose an attacker plans to use a brute-force attack to determine a password. For what value of x (the total amount of time to try as many passwords as necessary) would the attacker find this attack prohibitively long?

(c) If the cutoff between "insecure" and "secure" were x amount of time, how long would a secure password have to be? State and justify your assumptions regarding the character set from which the password is selected and the amount of time required to test a single password.

7. Design a protocol by which two mutually suspicious parties can authenticate each other. Your protocol should be usable the first time these parties try to authenticate each other.

8. List three reasons people might be reluctant to use biometrics for authentication. Can you think of ways to counter those objections?

9. False positive and false negative rates can be adjusted, and they are often complementary: Lowering one raises the other. List two situations in which false negatives are significantly more serious than false positives.

10. In a typical office, biometric authentication might be used to control access to employees and registered visitors only. We know the system will have some false negatives, some employees falsely denied access, so we need a human override, someone who can examine the employee and allow access in spite of the failed authentication. Thus, we need a human guard at the door to handle problems, as well as the authentication device; without biometrics we would have had just the guard.

Consequently, we have the same number of personnel with or without biometrics, plus we have the added cost to acquire and maintain the biometrics system. Explain the security advantage in this situation that justifies the extra expense.