# 03-09-2010-answers - CS 161 Computer Security Spring 2010...

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Unformatted text preview: CS 161 Computer Security Spring 2010 Paxson/Wagner Discussion 6 March 09, 2010 1. One-Time Pads Recall how a one-time pad works. Alice shares a stream of random bits with Bob, and she encrypts a message of length n for Bob by XORing the next n bits of this stream with the message. Bob decrypts by XORing the ciphertext with the same n bits from the stream of random bits. (a) Does this scheme work if we replace XOR with OR? How about with AND? (b) Suppose you want to encrypt a message M ∈ { , 1 , 2 } using a shared random key K ∈ { , 1 , 2 } . Sup- pose you do this by representing K and M using two bits (00, 01, or 10), and then XORing the two representations. Does this scheme have the same security guarantees of the one-time pad? Explain. (c) Give an alternate encryption algorithm for carrying out the above task that does provide strong security guarantees. Note: You must not change the message space { , 1 , 2 } or the key space { , 1 , 2 } . Instead, we want you to design an encryption algorithm E ( · , · ) so that E ( K , M ) is a secure encryption of M , when K and M are distributed as above. Answer: (a) No, it doesn’t work with either OR or AND. First of all, correctness is broken; it is not true that ( m ∨ k ) ∨ k = m for all choices of m and k (where m and k are each a single bit). Similarly, ( m ∧ k ) ∧ k 6 = m for all choices of m and k . This means that you can’t actually decrypt an encrypted message. Security is also broken. For OR, consider what an eavesdropper learns when she sees a 0 bit in the ciphertext. The only way this can happen is if both the key bit k and the message bit m are 0. For AND, when an eavesdropper sees a 1 bit in the ciphertext, she knows that both k and m are 1. Both OR and AND leak information. (b) No, this scheme does not have the security guarantees of a one-time pad. The table below lists the resulting encrypted messages using this scheme. We can see that some outcomes exclude certain inputs. For example, given E ( K , M ) = 11 an attacker knows that the sent message M is not 0. K M E ( K , M ) 00 00 00 01 00 01 10 00 10 00 01 01 01 01 00 10 01 11 00 10 10 01 10 11 10 10 00 CS 161, Spring 2010, Discussion 6 1 (c) We wish to design a new encryption algorithm E * ( · , · ) that has the security guarantees of the one-time pad. We require that given E * ( K , M ) , an attacker should get no information about M . This property is satisfied for any...
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## This note was uploaded on 04/14/2010 for the course CS 161 taught by Professor Wagner during the Spring '10 term at University of Central Arkansas.

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03-09-2010-answers - CS 161 Computer Security Spring 2010...

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