Exercise 2. CPA Encryption [20 points] In this question you will break CPA security of two...
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Exercise 2. CPA Encryption [20 points] In this question you will break CPA security of two encryption schemes (the first build with PRGs and the second build with PRFs). 1. State the CPA security game. Consider the following encryption scheme: Let G() : {0,1}” → {0,1}² be a secure PRG. Gen (1) Output a key k of size n bits selected uniformly at random from {0,1}n Enc(k, m) : c = m + G(k) + 1|2n| (where 1n is a string of all 1's of size n) Dec(k, c) m = c +G(k) + 1|2n| Show that the above construction is not CPA-secure. 1. Describe an adversary that can break CPA security of above construction. 2. Explicitly state the probability of success of this adversary. Now consider another construction: Let F be a fixed-length PRF. Gen (1) k{0,1}n : Enc(k, m) F(On) + m Dec(k, c) Fk (0) + c Show that the above construction is not CPA-secure. 1. Describe an adversary that can break CPA security of the above construction. 2. Explicitly state the probability of success of this adversary. Exercise 2. CPA Encryption [20 points] In this question you will break CPA security of two encryption schemes (the first build with PRGs and the second build with PRFs). 1. State the CPA security game. Consider the following encryption scheme: Let G() : {0,1}” → {0,1}² be a secure PRG. Gen (1) Output a key k of size n bits selected uniformly at random from {0,1}n Enc(k, m) : c = m + G(k) + 1|2n| (where 1n is a string of all 1's of size n) Dec(k, c) m = c +G(k) + 1|2n| Show that the above construction is not CPA-secure. 1. Describe an adversary that can break CPA security of above construction. 2. Explicitly state the probability of success of this adversary. Now consider another construction: Let F be a fixed-length PRF. Gen (1) k{0,1}n : Enc(k, m) F(On) + m Dec(k, c) Fk (0) + c Show that the above construction is not CPA-secure. 1. Describe an adversary that can break CPA security of the above construction. 2. Explicitly state the probability of success of this adversary.
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CPA Security Game The CPA security game is a method to evaluate the security of an encryption scheme It involves an adversary who has the capability to obtain the ciphertexts of plaintexts of their ch... View the full answer
Related Book For
Income Tax Fundamentals 2013
ISBN: 9781285586618
31st Edition
Authors: Gerald E. Whittenburg, Martha Altus Buller, Steven L Gill
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