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Unformatted text preview: SS ZG513 Cryptography and Network Security Third Edition by William Stallings Lecture slides by Lawrie Brown Chapter 2 – Classical Encryption Techniques Many savages at the present day regard their names as vital parts of themselves, and therefore take great pains to conceal their real names, lest these should give to evildisposed persons a handle by which to injure their owners. — The Golden Bough, Sir James George Frazer Symmetric Encryption • or conventional / privatekey / singlekey • sender and recipient share a common key • all classical encryption algorithms are privatekey • was only type prior to invention of public key in 1970’s Basic Terminology • plaintext the original message • ciphertext the coded message • cipher algorithm for transforming plaintext to ciphertext • key info used in cipher known only to sender/receiver • encipher (encrypt) converting plaintext to ciphertext • decipher (decrypt) recovering ciphertext from plaintext • cryptography study of encryption principles/methods • cryptanalysis (codebreaking) the study of principles/ methods of deciphering ciphertext without knowing key • cryptology the field of both cryptography and cryptanalysis Symmetric Cipher Model Requirements • two requirements for secure use of symmetric encryption: – a strong encryption algorithm – a secret key known only to sender / receiver Y = E K ( X ) X = D K ( Y ) • assume encryption algorithm is known • implies a secure channel to distribute key Cryptography • can characterize by: – type of encryption operations used • substitution / transposition / product – number of keys used • singlekey or private / twokey or public – way in which plaintext is processed • block / stream Types of Cryptanalytic Attacks • ciphertext only – only know algorithm / ciphertext, statistical, can identify plaintext • known plaintext – know/suspect plaintext & ciphertext to attack cipher • chosen plaintext – select plaintext and obtain ciphertext to attack cipher • chosen ciphertext – select ciphertext and obtain plaintext to attack cipher • chosen text – select either plaintext or ciphertext to en/decrypt to attack cipher Brute Force Search • always possible to simply try every key • most basic attack, proportional to key size • assume either know / recognise plaintext More Definitions • unconditional security – no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext • computational security – given limited computing resources (eg time needed for calculations is greater than age of universe), the cipher cannot be broken Classical Substitution Ciphers • where letters of plaintext are replaced by other letters or by numbers or symbols • or if plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns Caesar Cipher...
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This note was uploaded on 03/14/2010 for the course CSE SS ZG513 taught by Professor Sundarb during the Summer '10 term at Birla Institute of Technology & Science.
 Summer '10
 SundarB

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