Security in Computing (3rd Edition)

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CS 161 Computer Security Fall 2005 Joseph/Tygar/Vazirani/Wagner Final P RINT your name: , (last) (first) S IGN your name: P RINT your Unix account name: P RINT your TA’s name: You may consult any books, notes, or other paper-based inanimate objects available to you. Calculators and computers are not permitted. Please write your answers in the spaces provided in the test; in particular, we will not grade anything on the back of an exam page unless we are clearly told on the front of the page to look there. Please be concise. You have 3 hours. There are 10 questions, of varying credit (100 points total), not necessarily in order of difficulty. The questions are of varying difficulty, so avoid spending too long on any one question. Do not turn this page until your proctor tells you to do so. Problem 1 Problem 2 Problem 3 Problem 4 Problem 5 Problem 6 Problem 7 Problem 8 Problem 9 Problem 10 Total CS 161, Fall 2005, Final 1
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Problem 1. [Defaults] (9 points) Short answer: At most one sentence of explanation. (a) Which is generally safer (from a security point of view), a firewall with a “default deny” policy or a firewall with a “default allow” policy? Why? (b) Many spam filters can be configured to use either a whitelist or a blacklist. Name one advantage of using a whitelist (instead of a blacklist) for your spam filter. (c) Name one disadvantage of using a whitelist (compared to a blacklist) for your spam filter. Problem 2. [Authentication] (8 points) Describe two fundamentally different conceptual approaches that can be used for user authentication. Be concise: One sentence should suffice. Scheme #1: Scheme #2: CS 161, Fall 2005, Final 2
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Problem 3. [Intrusion Response] (6 points) The software company Snoracle (slogan: “Unwakeable”) is selling a new defense against DDoS attacks. Their software looks at the source IP address on all incoming packets, and if it finds any IP address that accounts for more than 1% of traffic over the last hour, it installs an entry in the router that blocks all packets from that address for the next 24 hours. Their marketing folks are claiming that this will stop all DDoS attacks cold in the water. Is this a good solution to the problem? Give one reason why or why not. Problem 4. [Hardware Support for Dual-Mode Operation] (6 points) Early Intel processors (e.g., the 8086) did not provide hardware support for dual-mode operation (i.e., sup- port for a separate user mode and kernel mode). As a result, most of the systems implemented on these processors did not support multi-user operation. List and explain one potential problem associated with supporting multi-user operation without hardware support for dual-mode operation. Be concise: one or two sentences should suffice.
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