Cryptography and Network Security Chapter 2Chapter 2 – Classical Encryption TechniquesSymmetric EncryptionSome Basic TerminologySymmetric Cipher ModelRequirementsCryptographyCryptanalysisCryptanalytic AttacksMore DefinitionsBrute Force SearchClassical Substitution CiphersCaesar CipherSlide 14Cryptanalysis of Caesar CipherMonoalphabetic CipherMonoalphabetic Cipher SecurityLanguage Redundancy and CryptanalysisEnglish Letter FrequenciesUse in CryptanalysisExample CryptanalysisPlayfair CipherPlayfair Key MatrixEncrypting and DecryptingSecurity of Playfair CipherPolyalphabetic CiphersVigenère CipherExample of Vigenère CipherAidsSecurity of Vigenère CiphersKasiski MethodAutokey CipherVernam CipherOne-Time PadTransposition CiphersRail Fence cipherRow Transposition CiphersProduct CiphersRotor MachinesHagelin Rotor MachineRotor Machine PrinciplesSteganographySummaryCryptography and Cryptography and Network SecurityNetwork SecurityChapter 2Chapter 2Fifth EditionFifth Editionby William Stallingsby William StallingsLecture slides by Lawrie BrownLecture slides by Lawrie BrownChapter 2 – Chapter 2 – Classical EncryptionClassical EncryptionTechniquesTechniques"I am fairly familiar with all the forms of secret "I am fairly familiar with all the forms of secret writings, and am myself the author of a trifling writings, and am myself the author of a trifling monograph upon the subject, in which I analyze monograph upon the subject, in which I analyze one hundred and sixty separate ciphers," said one hundred and sixty separate ciphers," said Holmes.Holmes.. . ——The Adventure of the Dancing MenThe Adventure of the Dancing Men, Sir Arthur , Sir Arthur Conan DoyleConan DoyleSymmetric EncryptionSymmetric Encryptionor conventional / or conventional / private-keyprivate-key / single-key / single-keysender and recipient share a common keysender and recipient share a common keyall classical encryption algorithms are all classical encryption algorithms are private-keyprivate-keywas only type prior to invention of public-was only type prior to invention of public-key in 1970’skey in 1970’sand by far most widely usedand by far most widely usedSome Basic TerminologySome Basic Terminologyplaintextplaintext - original message - original message ciphertextciphertext - coded message - coded message ciphercipher - algorithm for transforming plaintext to ciphertext - algorithm for transforming plaintext to ciphertext keykey - info used in cipher known only to sender/receiver - info used in cipher known only to sender/receiver encipher (encrypt)encipher (encrypt) - converting plaintext to ciphertext - converting plaintext to ciphertext decipher (decrypt)decipher (decrypt) - recovering ciphertext from plaintext - recovering ciphertext from plaintextcryptographycryptography - study of encryption principles/methods - study of encryption principles/methodscryptanalysis (codebreaking)cryptanalysis (codebreaking) - study of principles/ - study of principles/ methods of deciphering ciphertext methods of deciphering ciphertext withoutwithout knowing key knowing keycryptologycryptology - field of both cryptography and cryptanalysis - field of both cryptography and cryptanalysisSymmetric Cipher ModelSymmetric Cipher ModelRequirementsRequirementstwo requirements for secure use of two requirements for secure use of symmetric encryption:symmetric encryption:a strong encryption algorithma strong encryption algorithma secret key known only to sender / receivera secret key known only to sender / receivermathematically have:mathematically have:Y Y = E(K, = E(K, XX))X X = D(K, = D(K, YY))assume encryption algorithm is knownassume encryption algorithm is knownimplies a secure channel to distribute keyimplies a secure channel to distribute keyCryptographyCryptographycan characterize cryptographic system by:can characterize cryptographic system by:type of encryption operations usedtype of encryption operations used•substitutionsubstitution•transpositiontransposition•productproductnumber of keys usednumber of keys used•single-key or privatesingle-key or private•two-key or publictwo-key or publicway in which plaintext is processedway in which plaintext is processed•blockblock•streamstreamCryptanalysisCryptanalysisobjective to recover key not just messageobjective to recover key not just messagegeneral approaches:general approaches:cryptanalytic attackcryptanalytic attackbrute-force attackbrute-force attackif either succeed all key use compromisedif either succeed all key use compromisedCryptanalytic AttacksCryptanalytic Attacksciphertext onlyciphertext only only know algorithm & ciphertext, is statistical, only know algorithm & ciphertext, is statistical, know or can identify plaintext know or can identify plaintext known plaintextknown plaintext know/suspect plaintext & ciphertextknow/suspect plaintext & ciphertextchosen plaintextchosen plaintext select plaintext and obtain ciphertextselect plaintext and obtain ciphertextchosen ciphertextchosen ciphertext select ciphertext and obtain plaintextselect ciphertext and obtain plaintextchosen textchosen text select plaintext or ciphertext to en/decryptselect plaintext or ciphertext to en/decryptMore DefinitionsMore Definitionsunconditional securityunconditional security no matter how much computer power or time no matter how much computer power or time is available, the cipher cannot be broken is available, the cipher cannot be broken since the ciphertext provides insufficient since the ciphertext provides insufficient information to uniquely determine the information to uniquely determine the corresponding plaintext corresponding plaintext computational securitycomputational security given limited computing resources (eg time given limited computing resources (eg time needed for calculations is greater than age of needed for calculations is greater than age of universe), the cipher cannot be broken universe), the cipher cannot be brokenBrute Force SearchBrute Force Searchalways possible to simply try every key always possible to simply try every key most basic attack, proportional to key size most basic attack, proportional to key size assume either know / recognise plaintextassume either know / recognise plaintextKey Size (bits) Number of Alternative KeysTime required at 1 decryption/µsTime required at 106