Computer Science and Engineering Computer System Security CSE 5339/7339 Lecture 3 August 26, 2004

Computer Science and Engineering Contents  Algorithms (Revisited)  Operating Systems Review  Students Topics for Presentation  Encryption  Substitution and Transposition Ciphers

Computer Science and Engineering Algorithms -- revisited Hashing - Why? Hash Tables Hash Functions Insert Lookup

Computer Science and Engineering Applications OS -- Review OS – a program that acts as an intermediary between a user of a computer and the computer hardware. OS Hardware Users

Computer Science and Engineering OS -- Review OS Services Program Execution I/O Operation File System manipulation Communications Error detection Resource Allocation Accounting Protection

Computer Science and Engineering OS -- Review Process Memory Management

Computer Science and Engineering Student Presentations (15 minutes) 8/31 9/2 9/7 9/9

Computer Science and Engineering Main Components in Sending Messages senderreceiverMedium Intruder Block it Intercept it Modify it Fabricate an authentic looking message

Computer Science and Engineering Cryptography nSecret writing nDisguised data cannot be read, modified, or fabricated easily nEncryption : encoding (encipher) n plaintext  cipher text P =  C = C = E(P) (E = encryption rule) nDecryption : decoding (decipher) n Cipher text  plaintext C =  P = P = D(C) (D = decryption rule)

Computer Science and Engineering Encryption Decryption plaintext Original plaintext ciphertext

Computer Science and Engineering Encryption Decryption plaintext Original plaintext ciphertext EncryptionDecryption plaintext Original plaintext ciphertext Symmetric Cryptosystem Asymmetric Cryptosystem key KEKE KDKD

Computer Science and Engineering Cryptanalysis nHow to break an encryption! nCryptanalyst n Deduce the original meaning of the ciphertext n Determine the decryption algorithm that matches the encryption one used Breakable Encryption!

Computer Science and Engineering Ciphers  Substitution Ciphers Substitute a character or a symbol for each character of the original message  Transposition Ciphers The order of letters is rearranged (Uppercase – plaintext, lowercase – ciphertext)

Computer Science and Engineering Exercise wklv phvvdjh lv qrw wrr kdug wr euhdn

Computer Science and Engineering The Caesar Cipher -- Substitution  C i = p i + 3 A  d B  e C  f … X  a Y  b Z  c  Time complexity  table search  ??

Computer Science and Engineering Cryptanalysis of the Caesar Cipher  TREATY IMPOSSIBLE  wuhdwb lpsrvvleoh  Break is preserved  Double letters are preserved  Repeated letters

Computer Science and Engineering Other Substitutions  Permutation – Alphabet is scrambled, each plaintext letter maps to a unique ciphertext letter For example 1, 2, 3, 4, 5, 6, 7, 8, 9, 10  1 = 1, 3, 5, 7, 9, 8, 6, 4, 2  1 (1) = 1,  1 (2) = 3,  1 (3) = 5,  1 (4) = 7, etc.  Key can be used to control the permutation used to

Computer Science and Engineering Example  ABCDEFGHIJKLMNOPQRSTUVWXYZ  wordabcefghijklmnpqstuvxyz  ABCDEFGHIJKLMNOPQRSTUVWXYZ  profesinalbcdghjkmqtuvwxyz

Computer Science and Engineering Cryptanalysis of substitution ciphers  Clues  Short words  Words with repeated patterns  Common initial and final letters  ….  Brute force attack (could be impossible – more than 1000 years)  Knowledge of language may simplify it  English E, T, O, A occur far more than J, Q, X, Z  Context

Computer Science and Engineering Vernam Cipher EncryptionDecryption plaintextOriginal plaintext ciphertext Non-repeating series of numbers

Computer Science and Engineering Example Plaintext  V E R N A M C I P H E R  Random numbers  Sum  Sum mod 26  Ciphertext  t a h r s p i t x m a b

Computer Science and Engineering Transposition  The letters of the message are rearranged  Columnar transposition  Example: THIS IS A MESSAGE TO SHOW HOW A COLMUNAR TRANSPOSITION WORKS

Computer Science and Engineering T H I S I S A M E S S A G E T O S H O W H O W A C O L M U N A R T R A N S P O S I T I O N W O R K S tssoh oaniw haaso lrsto imghw utpir seeoa mrook istwc nasna