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# ch02 - Classical Encryption Techniques Chapter 2 2 Outline...

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1 Classical Encryption Techniques Chapter 2 Internet Security 2 EE 282 ° classical cipher techniques and terminology ° monoalphabetic substitution ciphers ° cryptanalysis using letter frequencies ° Playfair ciphers ° polyalphabetic ciphers ° transposition ciphers ° product ciphers and rotor machines ° stenography Outline Please also read Chapter2, – you can skip section on “Hill Cipher”

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2 Internet Security 3 EE 282 Introduction: Symmetric Encryption p All traditional schemes are symmetric / single key / private-key encryption algorithms, p with a single key , used for both encryption and decryption, p both sender and receiver are equivalent, either can encrypt or decrypt messages using that common key. p was only type of encryption scheme prior to invention of public-key in 1970’s Internet Security 4 EE 282 Basic Terminology The following is a brief review of some of terminology used throughout the course. p plaintext - the original message p ciphertext - the coded message p cipher - algorithm for transforming plaintext to ciphertext p key - info used in cipher known only to sender/receiver p encipher (encrypt) - converting plaintext to ciphertext p decipher (decrypt) - recovering plaintext from ciphertext p cryptography - study of encryption principles/methods p cryptanalysis (codebreaking) - the study of principles/ methods of deciphering ciphertext without knowing key p cryptology - the field of both cryptography and cryptanalysis
3 Internet Security 5 EE 282 Symmetric Cipher Model Symmetric cipher model consists of 5 components (figure below): •plaintext •encryption algorithm – performs substitutions/transformations on plaintext •secret key – for performing substitutions/transformations needed in encryption algorithm •ciphertext, ? decryption algorithm Internet Security 6 EE 282 Requirements p two requirements for secure use of symmetric encryption: ° a strong encryption algorithm ° a secret key known only to the sender & receiver p For a plaintext X , its cipher text Y , and a key K , the encryption transformation E K is denoted by: Y = E(K, X ), or E K ( X ) “Y is an encryption of message X under key K Similarly, the decryption algorithm D K is denoted by: X = D(K, Y), or D K ( Y ) “X is the decryption of Y under key K”

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