ps3 - Computer Science and Engineering, UCSD CSE 207:...

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Computer Science and Engineering, UCSD Spring 11 CSE 207: Modern Cryptography Instructor: Mihir Bellare Problem Set 3 October 13, 2011 Problem Set 3 Due: Wednesday April 20, 2011, in class. Problem 1. [80 points] Let E : { 0 , 1 } k ×{ 0 , 1 } n → { 0 , 1 } n be a block cipher and let algorithm K return K $ ← { 0 , 1 } k . Assume messages to be encrypted have length ‘ < n . Let E be the following encryption algorithm: algorithm E K ( M ) if | M | 6 = then return // Only encrypts -bit messages R $ ← { 0 , 1 } n - C E K ( R k M ) return C Above, “ x k y ” denotes the concatenation of strings x and y . 1. [10 points] Specify a decryption algorithm D such that SE = ( K , E , D ) is a symmetric encryption scheme providing correct decryption. 2. [30 points] Give the best attack you can on this scheme. Given an even number q , your attack should take the form of an ind-cpa adversary A that makes q oracle queries and has running time around that for O ( q ) applications of E
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This note was uploaded on 08/31/2011 for the course CSE 207 taught by Professor Daniele during the Winter '08 term at UCSD.

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ps3 - Computer Science and Engineering, UCSD CSE 207:...

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