Obtain few piece of plaintext that Alice encrypts

Assignment Help Basic Computer Science

Reference no: EM133196322 , Length: 2 Pages

1. In an attack scenario, we assume that the attacker, Oscar, somehow manages to obtain a few piece of plaintext that Alice encrypts. Show how Oscar can break the affine cipher by using two pairs of plaintext-ciphertext (x1, y1) and (x2, y2). What is the condition for choosing x1 and x2?

2. A stream cipher is given, which uses a single LFSR as key stream generator. The LFSR has a degree of 256.

a. How many plaintext/ciphertext bit pairs are needed to launch a successful attack?

b. Describe all steps of the attack in detail and develop the formulae that need to be solved.

3. Assume we have a stream cipher whose period is quite short. We happen to know that the period is 150~200 bit in length. We assume that we do not know anything else about the internals of the stream cipher. In particular, we should not assume that it is a simple LFSR. Describe in detail how such a cipher can be attacked and specify what Oscar has to know in terms of plaintext/ciphertext and how he can decrypt all ciphertext.

Reference no: EM133196322

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