1. 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.
2. 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
3. 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.
4. List three reasons people might be reluctant to use biometrics for authentication. Can you think of ways to counter those objections?
5. If you forget your password for a website and you click [Forgot my password], sometimes the company sends you a new password by email but sometimes it sends you your old password by email. Compare these two cases in terms of vulnerability of the website owner.
6. Defeating authentication follows the method–opportunity–motive paradigm described in Chapter 1. Discuss how these three factors apply to an attack on authentication.
7. Suggest a source of some very long unpredictable numbers. Your source must be something that both the sender and receiver can readily access but that is not obvious to outsiders and not transmitted directly from sender to receiver.
8. Humans are said to be the weakest link in any security system. Give an example for each of the following:
(a) a situation in which human failure could lead to a compromise of encrypted data
(b) a situation in which human failure could lead to a compromise of identification and authentication
(c) a situation in which human failure could lead to a compromise of access control.
9. Explain why hash collisions occur. That is, why must there always be two different plaintexts that have the same hash value?
10. What property of a hash function means that collisions are not a security problem? That is, why can an attacker not capitalize on collisions and change the underlying plaintext to another form whose value collides with the hash value of the original plaintext?
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