Let f(x) be a One-Way Homomorphic Function where f(kp+1)=f(1) for any integer k and a bigprime p. Given the value f(1) and the ElGamal ciphertext CT=(f(r), f(r*b)?M) where pk=f(b)and sk=b. Here, r is...

Let f(x) be a One-Way Homomorphic Function where f(kp+1)=f(1) for any integer k and a bigprime p. Given the value f(1) and the ElGamal ciphertext CT=(f(r), f(r*b)?M) where pk=f(b)and sk=b. Here, r is a random number chosen by the encryptor and f(r*b) is a bit string.Q1: Show how toquicklycompute f(111)step by step.Q2: Suppose that the output f(x) for all x has the problem that the first bit (MSB) is equalto 1 with probability 99.9999%. Show how to break the ElGamal ciphertext in the IND-CPAsecurity model.

May 02, 2022
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