5 - Lecture #5 SS G513 Network Security Chapter 6...

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Lecture #5 SS G513 Network Security
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Chapter 6 – Contemporary Symmetric Ciphers "I am fairly familiar with all the forms of secret writings, and am myself the author of a trifling monograph upon the subject, in which I analyze one hundred and sixty separate ciphers," said Holmes. The Adventure of the Dancing Men, Sir Arthur Conan Doyle
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Triple DES clear a replacement for DES was needed theoretical attacks that can break it demonstrated exhaustive key search attacks AES is a new cipher alternative prior to this alternative was to use multiple encryption with DES implementations Triple-DES is the chosen form
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Why Triple-DES? why not Double-DES? NOT same as some other single-DES use, but have meet-in-the-middle attack works whenever use a cipher twice – since X = E K1 [P] = D K2 [C] attack by encrypting P with all keys and store then decrypt C with keys and match X value can show takes O(2 56 ) steps
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Triple-DES with Two-Keys hence must use 3 encryptions would seem to need 3 distinct keys but can use 2 keys with E-D-E sequence C = E K1 [D K2 [E K1 [P]]] if K1=K2 then can work with single DES no current known practical attacks
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Triple-DES with Three-Keys although are no practical attacks on two- key Triple-DES have some indications can use Triple-DES with Three-Keys to avoid even these C = E K3 [D K2 [E K1 [P]]] has been adopted by some Internet applications, eg PGP, S/MIME
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Block Cipher Characteristics features seen in modern block ciphers are: variable key length / block size / no rounds mixed operators, data/key dependent rotation key dependent S-boxes more complex key scheduling operation of full data in each round varying non-linear functions
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Stream Ciphers process the message bit by bit (as a stream) typically have a (pseudo) random stream key combined (XOR) with plaintext bit by bit randomness of stream key completely destroys any statistically properties in the message C i = M i XOR StreamKey i what could be simpler!!!! but must never reuse stream key otherwise can remove effect and recover messages
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Stream Cipher Properties some design considerations are: long period with no repetitions statistically random depends on large enough key large linear complexity correlation immunity confusion diffusion use of highly non-linear boolean functions
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This note was uploaded on 03/14/2010 for the course CSE SS ZG513 taught by Professor Sundarb during the Summer '10 term at Birla Institute of Technology & Science.

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5 - Lecture #5 SS G513 Network Security Chapter 6...

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