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Lecture notes on Computer and network security: Lecture 2 - Avinash Kak

Lecture 2, classical encryption techniques. This chapter includes contents: Basic vocabulary of encryption and decryption, building blocks of classical encryption techniques, caesar cipher, the swahili angle, monoalphabetic ciphers, the all-fearsome statistical attack, multiple-character encryption to mask plaintext structure, another multi-letter cipher, polyalphabetic ciphers,. | Lecture 2: Classical Encryption Techniques Lecture Notes on “Computer and Network Security” by Avi Kak (kak@purdue.edu) January 15, 2016 12:25am c 2016 Avinash Kak, Purdue University Goals: • To introduce the rudiments of encryption/decryption vocabulary. • To trace the history of some early approaches to cryptography and to show through this history a common failing of humans to get carried away by the technological and scientific hubris of the moment. • Simple Python and Perl scripts that give you pretty good security for confidential communications. Only good for fun, though. CONTENTS Section Title Page 2.1 Basic Vocabulary of Encryption and Decryption 3 2.2 Building Blocks of Classical Encryption Techniques 8 2.3 Caesar Cipher 9 2.4 The Swahili Angle . 11 2.5 Monoalphabetic Ciphers 13 2.5.1 2.6.1 2.7 15 The All-Fearsome Statistical Attack 2.6 A Very Large Key Space But 16 Comparing the Statistics for Digrams and Trigrams Multiple-Character Encryption to Mask Plaintext Structure: The Playfair Cipher 18 20 2.7.1 Constructing the Matrix for Pairwise Substitutions in the Playfair Cipher 21 2.7.2 Substitution Rules for Pairs of Characters in the Playfair Cipher 22 2.7.3 How Secure Is the Playfair Cipher? 24 2.8 Another Multi-Letter Cipher: The Hill Cipher 2.8.1 2.9 How Secure Is the Hill Cipher? Polyalphabetic Ciphers: The Vigenere Cipher 2.9.1 How Secure Is the Vigenere Cipher? 27 29 30 31 2.10 Transposition Techniques 33 2.11 Establishing Secure Communications for Fun (But Not for Profit) 36 2.12 Homework Problems 49 2 Computer and Network Security by Avi Kak Lecture 2 2.1: BASIC VOCABULARY OF ENCRYPTION AND DECRYPTION plaintext: This is what you want to encrypt ciphertext: The encrypted output enciphering or encryption: The process by which plaintext is converted into ciphertext encryption algorithm: The sequence of data processing steps that go into transforming plaintext into ciphertext. Various .