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Unformatted text preview: ICS 201: Cryptography and Communication Security 10/22/2007 Homework 2 Due Monday, Oct 29, in class Problem 1 and 2 Do exercises 3.2 and 3.4. Problem 3 Consider a stateful encryption scheme, e.g. a stream cipher. Here both the encryption and the decyrption algorithms take a state p as an additional input. The scheme also requires a stateupdate procedure, p Update ( p,m ) for the encryptor (and a similar procedure for the decryptor). Lets assume that the initial state p is set to 0. Consider a Chosen Plaintext Attack on a stateful encryption algorithm, with the attacker algorithm denoted A , running on input 1 n : 1. Key k is chosen by the key generation procedure on 1 n . 2. Let p 1 = 0 and i = 1. The following loop is repeated as long as A wants: A makes any query m i to the encryption algorithm, which returns c i = Enc k ( m ; p i ), i.e. the encryption of m i under k given state p i . The encryptors state is updated, p i +1 Update ( p i ,m i ), and we increase the i counter, i i + 1. 3. A outputs two messages m (0) ,m (1) of equal length (we call these challenge messages ), bit b is chosen at random, and adversary is given c = Enc k ( m ( b ) ; p i ). The encryptors state is updated, p i +1 Update ( p i ,m ( b ) )....
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 Fall '07
 Jarecki

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