lec-03-des - 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec...

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Unformatted text preview: 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.1 CSE565: S. Upadhyaya Lec 3.1 CSE565: Computer Security Lectures 3 & 4 Block Ciphers and Data Encryption Standard Shambhu Upadhyaya Computer Science & Eng. University at Buffalo Buffalo, New York 14260 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.2 CSE565: S. Upadhyaya Lec 3.2 Overview Block Ciphers 10 minutes Description of DES 55 minutes Encryption Algorithm (Initial Permutation, Key Transformation, Expansion Permutation, S-Box, P- Box, Final Permutation) Decryption Algorithm Security of DES 10 minutes Cryptanalysis Linear, Differential & Related key 10 minutes DES Variants 5 minutes (some slides adapted from Lawrie Brown) 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.3 CSE565: S. Upadhyaya Lec 3.3 Modern Block Ciphers Study of modern block ciphers One of the most widely used types of cryptographic algorithms In particular will introduce DES (Data Encryption Standard) We will study AES later 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.4 CSE565: S. Upadhyaya Lec 3.4 S-Permutation Ciphers Shannon observed that two weak but complementary ciphers can be made more secure by applying them together Product cipher (Apply E1 and then E2) In 1949 Shannon introduced idea of substitution-permutation (S- P) networks modern substitution-transposition product cipher This forms the basis of modern block ciphers S-P networks are based on the two primitive cryptographic operations we have seen before: substitution (S-box) permutation (P-box) Provide confusion and diffusion of message 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.5 CSE565: S. Upadhyaya Lec 3.5 Confusion and Diffusion Cipher needs to completely obscure statistical properties of original message A one-time pad does this More practically Shannon suggested combining elements to obtain: Diffusion dissipates statistical structure of plaintext over bulk of ciphertext Confusion makes relationship between ciphertext and key as complex as possible 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.6 CSE565: S. Upadhyaya Lec 3.6 Feistel Cipher Structure Horst Feistel devised the Feistel cipher based on concept of invertible product cipher Partitions input block into two halves process through multiple rounds perform a substitution on left data half this is based on applying a round function to the right half & subkey and take the Ex-OR then have permutation swapping halves Implements Shannons substitution-permutation network concept 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.7 CSE565: S. Upadhyaya Lec 3.7 Feistel Cipher Structure 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec 3.8 CSE565: S. Upadhyaya Lec 3.8 Feistel Cipher Design Principles Block size increasing size improves security, but slows cipher Key size increasing size improves security, makes exhaustive key searching...
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This note was uploaded on 10/12/2010 for the course CSE 565 taught by Professor Shambhu during the Fall '09 term at SUNY Buffalo.

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lec-03-des - 9/07/10 UB Fall 2010 CSE565: S. Upadhyaya Lec...

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