cryptanalysis 4

cryptanalysis 4 - Algorithms and Mechanisms Cryptography is...

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Algorithms and Mechanisms Cryptography is nothing more than a mathematical framework for discussing the implications of various paranoid delusions Don Alvarez Historical Ciphers Non-standard hieroglyphics, 1900BC Atbash cipher (Old Testament, reversed Hebrew alphabet, 600BC) Caesar cipher: letter = letter + 3 ‘fish’ ilvk rot13: Add 13/swap alphabet halves Usenet convention used to hide possibly offensive jokes Applying it twice restores the original text
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Substitution Ciphers Simple substitution cipher: a = p, b = m, c = f, . .. Break via letter frequency analysis Polyalphabetic substitution cipher 1. a = p, b = m, c = f, . .. 2. a = l, b = t, c = a, . .. 3. a = f, b = x, c = p, . .. Break by decomposing into individual alphabets, then solve as simple substitution One-time Pad (1917) OTP is unbreakable provided Pad is never reused (VENONA) Unpredictable random numbers are used (physical sources, e.g. radioactive decay) x x c d m g 24 24 3 4 13 7 5 19 12 1 8 +15 OTP 19 5 17 3 5 18 t e r c e s Message
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One-time Pad (ctd) Used by Russian spies The Washington- Moscow “hot line” CIA covert operations Many snake oil algorithms claim unbreakability by claiming to be a OTP Pseudo-OTPs give pseudo-security Cipher machines attempted to create approximations to OTPs, first mechanically, then electronically Cipher Machines (~1920) 1. Basic component = wired rotor Simple substitution 2. Step the rotor after each letter Polyalphabetic substitution, period = 26
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Cipher Machines (ctd) 3. Chain multiple rotors Each rotor steps the next one when a full turn is complete Cipher Machines (ctd) Two rotors, period = 26 26 = 676 Three rotors, period = 26 26 26 = 17,576 Rotor sizes are chosen to be relatively prime to give maximum-length sequence Key = rotor wiring, rotor start position
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Cipher Machines (ctd) Famous rotor machines US: Converter M-209 UK: TYPEX Japan: Red, Purple Germany: Enigma Many books on Enigma Kahn, Seizing the Enigma Levin, Ultra Goes to War Welchman, The Hut Six Story Winterbotham, The Ultra Secret “It would have been secure if used properly” Use of predictable openings: Mein Fuehrer! . ..” “Nothing to report” Use of the same key over an extended period Encryption of the same message with old (compromised) and new keys Post-war KW-26 common fill device shredded the key card when the cover was opened to prevent this Device treated as a magic black box, a mistake still made today Inventors believed it was infallible, " " " " "
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Cipher Machines (ctd) Various kludges were made to try to improve security none worked Enigmas were sold to friendly nations after the war Improved rotor machines were used into the 70’s and 80’s Further reading: Kahn, The Codebreakers Cryptologia, quarterly journal Stream Ciphers Binary pad (keystream), use XOR instead of addition Plaintext = original, unencrypted data Ciphertext = encrypted data Two XORs with the same data always cancel out 1 1 0 1 0 0 1 Plaintext 1 0 1 1 0 1 0 XOR Keystream 0 1 1 0 0 1 1 Ciphertext 1 0 1 1 0 1 0 XOR Keystream 1 1 0 1 0 0 1 Plaintext
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Stream Ciphers (ctd) Using the keystream and ciphertext, we can recover the
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This note was uploaded on 03/22/2012 for the course CSE 5345 taught by Professor Youngheliu during the Spring '12 term at UT Arlington.

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cryptanalysis 4 - Algorithms and Mechanisms Cryptography is...

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