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# DES - CSE 425 Introduction to Computer Security Chapter 5...

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CSE 425, Introduction to Computer Security Chapter 5. Cryptography. Data Encryption Standard (DES) © 2007 by Wayne R. Dyksen. All Rights Reserved. 5-1 Michigan State University Department of Computer Science and Engineering Cryptography and Network Security Chapter 3. Block Ciphers and the Data Encryption Standard by William Stallings Modern Cryptography Chapter 7.6. The Data Encryption Standard by Wenbo Mao Professor Wayne Dyksen Department of Computer Science and Engineering Michigan State University Spring 2007 CSE 425, Introduction to Computer Security 5-2 Cryptographic Systems Three Characteristics Types of (E/D) Operations Number of Keys Number of Elements Per (E/D) Operation (E/D = Encryption/Decryption) (Element = Letter, Bit, Byte, Etc…) 5-3 Types of (E/D) Operations Two Fundamental Operations Substitution Map One Element Into Another E.g., a B, b C, c → D, etc… Cryptoquote Permutation (Transposition) Rearrange Elements E.g., 0 7, 1 3, 2 5, 3 0, 4 2, 5 6, 6 1, 7 4 Word Scramble Product Systems Combination of Substitutions and Transpositions Really “Composition” Systems 5-4 Substitution Map One Element Into Another E.g., a → B, b → C, c → D, etc… Cryptoquote • Often… Implemented as Table Lookup Table Called “S - Box” (“Substitution - Box”) 1 0 14 4 13 1 2 15 11 8 3 10 6 12 5 9 0 7 0 1 1 0 0 0 1 0 15 7 4 14 2 13 1 10 6 12 11 9 5 3 8 0 0 0 Row 2 4 1 14 8 13 6 2 11 15 12 9 7 3 10 5 0 1 1 0 0 12 Column 3 15 12 8 2 4 9 1 7 5 11 3 14 10 0 6 13 5 S Box 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0101 Bits 1 to 6 S Box 5-5 Number of Keys One Key Same Key for Encryption and Decryption AKA Symmetric Single-Key Secret-Key Two Keys Separate Keys for Encryption and Decryption AKA Asymmetric Two-Key Public-Key 5-6 # of Elements Per (E/D) Operation Stream Cipher One Element Letter Bit Byte • Etc… Processed As a “Stream” Block Cipher N Elements At A Time Letter Bit Byte • Etc… Processed In “Blocks”

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