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quiz8_sol - given only the ciphertext This is usually the...

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CIS 4360 Introduction to Computer Security QUIZ 8, Fall 2011 (5 minutes only) — with answers This quiz concerns cipher systems. 1. Ceasar’s cipher is a substitution cipher in which each one of the 26 letters of the alphabet is substituted by a letter obtained by shifted it K places forward, where K is the key. For example, if the key is K = 3, then the word CEASAR is encrypted as: FHDV DU because if we shift the letters of the alphabet three places forward we get: C D E F E F G H A B C D S T U V A B C D and R S T U Letters are rolled over when reaching Z . So Y Z A B . What is the plaintext if the ciphertext is: ( Hint: Shift all the letters one-at-a-time) E W I J K P W D W Y G A N F X J K L Q X E X Z H B O G Y K L M R Y F Y A I C P H Z L M N S Z G Z B J D Q I A M N O T A H A C K E R (The English alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ) 2. In the previous example you found the plaintext
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Unformatted text preview: given only the ciphertext . This is usually the hardest task a cryptanalyst must do—of course Ceaser’s cipher is a trivial cipher. We discussed “Attacks on Cryptosystems” in class and gave a special name to this “attack”. What was it called? A ciphertext only attack. 3. The One-Time Pad. This is an encryption system that o±ers perfect secrecy . This means that (True or False): • given a plaintext, the corresponding ciphertext is random. TRUE • given a ciphertext, the corresponding plaintext is random. FALSE • the plaintext is random. FALSE • the ciphertext is random. TRUE Mike Burmester...
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