CIS 4360 Introduction to Computer Security

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CIS 4360 Introduction to Computer Security Home Assignment 9, Fall 2010 – with answers Due: Thu 12:30pm, 23/11/2010 This assignment concerns Key Establishment and Authentication. Examples taken from: Dieter Gollmann, Computer Security, 2nd Edition, John Wiley. 1. In the HTTP basic authentication protocol, analyze the security gains (if any) when the client sends a hash of the password instead of a base64 encoding of the password. Answer. No real gain. The attacker just intercepts the hash of the password and provides the hash later to imper- sonate the user. The protocol has to use some kind of nonce to avoid such replay attacks. N.B. In contrast, with the Digest Access Authentication the Server sends a nonce, which the client must include in the hash: request−digest = h(h(username||realm||password)||nonce||h(method||digest−uri)), and even though h(username||realm||password), h(method||digest−uri) are the same, the value of nonce (which the Server picks) changes. However this scheme can be broken with a dictionary attack. The dictionary will give the attacker a value for h(username||realm||password)||nonce||h(method||digest−uri), and if the attacker replaces the value of nonce, the hash of the new string is likely to have the correct value. 2. Justify that the AKEP2 protocol provides (a) mutual entity authentication and (b) implicit key authentication. Answer. (a) Mutual entity authentication. A gets a reply hK (B, A, nA, nB) to its challenge nA that can only come from B (assuming the long term key K is not compromised). B gets a reply hK (A, nB) to its challenge nB that can only come from A. (b) Implicit key authentication. The session key k = hK (nB) is computed from the shared secret K and the nonce nB; thus, it can only be computed by A and B, and it cannot be the replay of a previously compromised session key. 3. Consider this simple password-based challenge-response protocol run between a user A and a server S. PA denotes A’s password, x is a random nonce generated by the server, and h is a known cryptographic hash function. (a) S → A : ePA(x) (b) A → S : ePA(h(x)) [ePA(x) is the encryption of x with password PA]. Show that this protocol is vulnerable to an oﬀ-line password guessing attack. Answer. The attacker intercepts the two messages and guesses a password, and gets two values, x and y, by decrypting both messages. If y = h(x), the guessed password is likely to be correct. Mike Burmester 1.

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© Jones & Bartlett Learning,© Jones LLC & Bartlett Learning LLC, an Ascend Learning© Jones Company. & NOT Bartlett FOR SALE Learning, OR DISTRIBUTION. LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC © Digital_Art/ShutterstockNOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION CHAPTER 6 © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FORAuthenticating SALE OR DISTRIBUTION PeopleNOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION ▸ About This Chapter In this chapter, we look at techniques to accurately associate individuals with user identities inside a computer. The discussion will focus on passwords with an intro- duction to© other Jones methods. & Bartlett The chapter Learning, examines the LLC following topics: © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION ■ The three authentication factors: what you know, you have, and you are ■ Passwords, password bias, and search space calculations ■ Cryptographic building blocks: random choice, one-way hash ■ Authentication devices: personal tokens and biometrics ■ © Jones &Basic Bartlett issues in Learning, authentication LLC policy © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION ▸ 6.1 Unlocking a Door We use padlocks, deadbolts, and safes to protect unattended things. We arrange it so that only authorized users may open these locks. We either restrict copies of © Jones & Bartlett keysLearning, to the trusted LLC people, or we share lock combinations© Jones with& Bartlett only those Learning, trusted LLC NOT FOR SALE ORfew. DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Bob shares his computer with his employees and with those of Alice’s Arts.
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