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# SHW2 - IS2150/TEL2810 Introduction to Security Homework 2...

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IS2150/TEL2810 Introduction to Security Homework 2 Total Points: 50 1) Exercise on Propositional/Predicate logic [25 Points] (a) Prove that A B (A ∧￢ B) ( A B) (you can use the truth table) (5 points) (b) Express the following sentences in propositional/ first order logic. Be sure to define all propositional components (e.g., predicate function, constants, and variables). i) If it does not rain we will go to the Steeler’s game. (3 points) R = it rains S = go to Steelers game R → S ii) If a subject has Secret clearance then he/she is allowed to write to Secret and Top Secret files (3 points) SecretSubject(x) = subject x has secret clearance TopSecretFile(f) = file f is top secret SecretFile(f) = file f is secret allowWrite(x, y) = subject x is allowed to write to file y x f [ SecretSubject(x) ((SecretFile(f) TopSecretFile(f)) canWrite(x,f) ) ] iii) A person can approve a check or cash it but cannot do both. (3 points) canApprove(x, c) = x can approve check c canCash(x, c) = x can cash check c x, c [ (canApprove (x, c) canCash(x, c)) (canCash(x, c) canApprove(x, c)) ] iv) A directory is older than the directories and the files that it contains. (4 points) isOlder(x, y) = x is older than y isDirectory(x) = x is a directory isFile(x) = x is a file Contains(x, y) = x contains y

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SHW2 - IS2150/TEL2810 Introduction to Security Homework 2...

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