lec-03 - CSE565: Computer Security Lectures 3 & 4 Block...

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1 9/08/09 UB Fall 2009 CSE565: S. Upadhyaya Lec 3.1 CSE565: Computer Security Block Ciphers and Data Encryption Standard Shambhu Upadhyaya Computer Science & Eng. University at Buffalo Buffalo, New York 14260 9/08/09 UB Fall 2009 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)
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2 9/08/09 UB Fall 2009 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 9/08/09 UB Fall 2009 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
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3 9/08/09 UB Fall 2009 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 9/08/09 UB Fall 2009 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 Shannon’s substitution- permutation network concept
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4 9/08/09 UB Fall 2009 CSE565: S. Upadhyaya Lec 3.7 Feistel Cipher Structure 9/08/09 UB Fall 2009 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 harder, but may slow cipher Number of rounds increasing number improves security, but slows cipher Subkey generation greater complexity can make analysis harder, but slows cipher Round function
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lec-03 - CSE565: Computer Security Lectures 3 & 4 Block...

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