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?
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
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
(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.
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