Test 2 Review. (b) Give one significant advantage of a nonce over a timestamp.
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1 Test 2 Review Name Student ID number Notation: {X} Bob Apply Bob s public key to X [Y ] Bob Apply Bob s private key to Y E(P, K) Encrypt P with symmetric key K D(C, K) Decrypt C with symmetric key K h(x) Apply the cryptographic hash function h to x Directions: Read each problem carefully and provide complete but concise answers. Note that when analyzing protocols, we assume that the cryptography is secure. 1. (10 points) Timestamps and nonces are both used in security protocols to prevent replay attacks. (a) Give one significant advantage of a timestamp over a nonce. Fewer messages (or, more efficient) (b) Give one significant advantage of a nonce over a timestamp. Time is not a security critical parameter (or, clock skew is not an issue) 2. (10 points) Consider passwords that are stored in a file. (a) What is a salt? A non-secret value that is appended to a password before hashing (b) How and why is a salt used when hashing passwords? A salt makes dictionary attacks more difficult Trudy cannot use pre-computed password hashes
2 3. (10 points) (a) Methods used to prevent covert channels are inherently weak. Is it better to use such weak methods or to do nothing at all? It is better to use such methods (b) Methods used for inference control are inherently weak. Is it better to use such weak methods or to do nothing at all? Same answer as (a) (c) Suppose that the only cryptosystem you have access to is known to be weak. Is it better to use this weak system to encrypt your data or to do nothing at all? In this case, it s generally better to do nothing 4. (10 points) In term of Lampson s access control matrix (a) What are capabilities (C-lists)? Capabilities correspond to the rows in Lampson s access control matrix (b) What are access control lists (ACLs)? ACLs correspond to the columns in Lampson s access control matrix 5. (10 points) This problem deals with biometrics. (a) What are the most significant differences between the authentication problem and the identification problem? For authentication, you have one-to-one comparisons and cooperative subjects, whereas for identification you have many-to-one comparisons and subjects are often uncooperative. This latter issue often results in poor enrollment data in the identification case. (b) Which is the easier problem, authentication or identification? Why? Authentication is easier, since it s a one-to-one comparison. The more comparisons, the greater the chance of an error.
3 6. (10 points) Suppose that passwords are stored as follows, where there are 128 possible choices for each character: If a password exceeds 16 characters, it is truncated to 16 characters. If a password is less than 16 characters, it is padded with A until it is exactly 16 characters. The resulting 16-character password is split into two parts, X 0 and X 1, where X 0 consists of the first 6 characters and X 1 consists of the last 10 characters. Then we compute and store Y 0 = h(x 0 ) and Y 1 = h(x 1 ), which are used for password verification. (a) What is the expected work for an exhaustive search to recover one specific password? The work is ( )/2 = (b) How would you attack a password in a way that would, in general, provide a significant shortcut as compared to an exhaustive search or a standard dictionary attack? Do an exhaustive search on Y 0 and use the result to simplify the dictionary attack on Y 1 by restricting the dictionary to passwords that start with X (10 points) Conceptually, at which layer of the protocol stack does each of the following types of firewalls operate? (a) Packet filter Network layer (b) Stateful packet filter Transport layer (c) Application proxy Application layer
4 8. (10 points) Consider the following 2-message protocol, which is based on public key cryptography. Alice Message 1 [T + 1] Bob Bob For each part below, answer the following questions: Is the protocol practical, given that Bob is a server who must deal with many clients? Who is securely authenticated and who is not? Is the session key K secure? (a) Message 1: {[T, K] Alice } Bob Not practical (Bob does not know which public key to use), mutual authentication, and K is secure (b) Message 1: { Alice, [T, K] Alice } Bob Practical, mutual authentication, and K is secure (c) Message 1: Alice, {[T, K] Alice } Bob Same as part (b). (d) Message 1: T, Alice, {[K] Alice } Bob Practical, Alice is not authenticated, Bob is authenticated, and K is secure (e) Message 1: Alice, {[T ] Alice } Bob and let K = h(t ) Practical, mutual authentication, K is not secure
5 9. (10 points) Consider the following 3-message protocol, based on the shared symmetric key K AB. Message 1 Alice Message 2 Bob R B For each part below, answer the following questions: Is the protocol practical, given that Bob is a server who must deal with many clients? Who is securely authenticated and who is not? Is the session key K secure? (a) Message 1: E( Alice, K, R A, K AB ) Message 2: R A, E(R B, K AB ) Not practical, mutual authentication, K is secure (b) Message 1: Alice, E(K, R A, K AB ) Message 2: R A, E(R B, K) Practical, mutual authentication, and the session key K is secure (c) Message 1: Alice, E(K, R A, K AB ) Message 2: R A, E(R B, K AB ) Same as (b) (d) Message 1: Alice, E(K, R A, K) Message 2: R A, E(R B, K AB ) Not practical, since Bob would need to know K in advance. Assuming Bob does know K, then it would provide mutual authentication. (e) Message 1: Alice, R A Message 2: E(K, R A, R B, K AB ) Same as (b)
6 10. (10 points) The Fiat-Shamir zero knowledge protocol is illustrated below. x = r 2 mod N Alice e {0, 1} Bob y = r S e mod N Suppose that N = 55 and v = 34. (a) In the first iteration of the protocol, Alice sends x = 4 in message one, Bob sends e = 1 in message two and Alice sends y = 24 in message three. Does Bob accept this iteration of the protocol? Show your work. Yes. We must show y 2 = x v e mod N. In this case, we have y 2 = 24 2 = 26 mod 55, and x v e = = 26 mod 55 (b) In the second iteration of the protocol, Alice sends x = 4, Bob sends e = 0, and Alice sends y = 13. Find S. Hint: 13 1 mod 55 = 17. Since x = 4 in both iterations, it s likely that Alice chose the same r. If we assume this is the case, from (a) we have r S = 24 mod 55 and from (b) we have r = 13 mod 55. Therefore, S = r 1 r S = = = 23 mod 55. Note that you can verify the answer is correct since v = S 2 = 23 2 = 34 mod 55 Extra Credit: TBD
Test 2 Review. 1. (10 points) Timestamps and nonces are both used in security protocols to prevent replay attacks.
Test 2 Review Name Student ID number Notation: {X} Bob Apply Bob s public key to X [Y ] Bob Apply Bob s private key to Y E(P, K) Encrypt P with symmetric key K D(C, K) Decrypt C with symmetric key K h(x)
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