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Unformatted text preview: CS361: Introduction to Computer Security Cryptography I Dr. Bill Young Department of Computer Sciences University of Texas at Austin Last updated: October 5, 2010 at 13:15 CS361 Slideset 5: 1 Cryptography I Elementary Cryptography This is not a course in cryptography. The department offers one and you are advised to take it, if you plan to work in the security field. Our point here will be to give some intuitions about: what are the key concepts of cryptography; how is it used as a tool for security; how effective is it in that regard. CS361 Slideset 5: 2 Cryptography I A Thought Experiment Suppose youre confronted with a text that you believe to be the encryption of some message. Youd like to apply your cryptanalytic skills. How do you get started? What questions should you ask? CS361 Slideset 5: 3 Cryptography I A Thought Experiment Suppose youre confronted with a text that you believe to be the encryption of some message. Youd like to apply your cryptanalytic skills. How do you get started? What questions should you ask? What is the likely underlying language of the plaintext? What characteristics of the probable source text are relevant? What characteristics of the source language are relevant? Have any transformations/compressions been applied prior to encryption? What is the likely nature/complexity of the encryption algorithm? Anything else? CS361 Slideset 5: 4 Cryptography I Thought Experiment: The Gold Bug The setting: In the early 1800s, a man named William Legrand finds a scrap of parchment on a South Carolina beach. The parchment appears blank, but when he holds it close to a candle flame to examine it, a strange encoded message appears. In the upper corner is a drawing of a goats head. Legrand wonders if the message could be directions to the location of a treasure buried by the infamous pirate Captain Kidd. CS361 Slideset 5: 5 Cryptography I The Ciphertext 53++!305))6*;4826)4+.)4+);806*;48!8 P60))85;1+(;:+*8!83(88)5*!;46(;88*96 *?;8)*+(;485);5*!2:*+(;4956*2(5*-4)8 P8*;4069285);)6!8)4++;1(+9;48081;8:8+ 1;48!85;4)485!528806*81(+9;48;(88;4 (+?34;48)4+;161;:188;+; CS361 Slideset 5: 6 Cryptography I An Aside: Talk Like a Pirate A useful Pirate to English translator can be found at: http://www.talklikeapirate.com/translator.html CS361 Slideset 5: 7 Cryptography I Information Theory and Cryptography Information theory vitally informs cryptography in a number of ways: What effect does encoding a message have on the information content of the file? An attempt to decrypt a message is really an attempt to recover a message from a (systematically) noisy channel. How can redundancy in the source give clues to the decoding process? Is a perfect encryption possible (i.e., one that is theoretically unbreakable) ?...
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slides5-cryptography - CS361: Introduction to Computer...

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