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CS6674
Homework 1
#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?
(This attack scenario is denoted as a chosen plaintext attack.)
#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.
* On the top of your homework, please don’t forget to include your Name.
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