Computer System Security CSE 5339/7339

Computer System SecurityCSE 5339/7339 Lecture 3 August 26, 2004

Contents • Algorithms (Revisited) • Operating Systems Review • Students Topics for Presentation • Encryption • Substitution and Transposition Ciphers

Algorithms -- revisited Hashing - Why? Hash Tables Hash Functions Insert Lookup

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

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

OS -- Review Process Memory Management

Student Presentations (15 minutes) 8/31 9/2 9/7 9/9

Main Components in Sending Messages sender Medium receiver Block it Intercept it Modify it Fabricate an authentic looking message Intruder

Cryptography • Secret writing • Disguised data cannot be read, modified, or fabricated easily • Encryption : encoding (encipher) • plaintext  cipher text P = <p1, p2, p3, .., pn>  C = <c1, c2, c3, .., cm> C = E(P) (E = encryption rule) • Decryption : decoding (decipher) • Cipher text  plaintext C = <c1, c2, c3, .., cm>  P = <p1, p2, p3, .., pn> P = D(C) (D = decryption rule)

Encryption Original plaintext ciphertext plaintext Encryption Decryption

Encryption key Original plaintext plaintext ciphertext Encryption Decryption Symmetric Cryptosystem KE KD Original plaintext plaintext ciphertext Encryption Decryption Asymmetric Cryptosystem

Cryptanalysis • How to break an encryption! • Cryptanalyst • Deduce the original meaning of the ciphertext • Determine the decryption algorithm that matches the encryption one used Breakable Encryption!

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)

Exercise wklv phvvdjh lv qrw wrr kdug wr euhdn

The Caesar Cipher -- Substitution • Ci = pi + 3 A  d B  e C  f … X  a Y  b Z  c • Time complexity  table search  ??

Cryptanalysis of the Caesar Cipher • TREATY IMPOSSIBLE  wuhdwb lpsrvvleoh • Break is preserved • Double letters are preserved • Repeated letters

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 p1 = 1, 3, 5, 7, 9, 8, 6, 4, 2 p1(1) = 1, p1(2) = 3, p1(3) = 5, p1(4) = 7, etc. • Key can be used to control the permutation used to

Example • ABCDEFGHIJKLMNOPQRSTUVWXYZ • wordabcefghijklmnpqstuvxyz • ABCDEFGHIJKLMNOPQRSTUVWXYZ • profesinalbcdghjkmqtuvwxyz

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

Vernam Cipher Non-repeating series of numbers Encryption Decryption ciphertext plaintext Original plaintext

Example Plaintext • V E R N A M C I P H E R • 21 4 17 13 0 12 2 8 15 7 4 17 Random numbers • 76 48 16 82 44 3 58 11 60 5 48 88 Sum • 97 52 33 95 44 15 60 19 75 12 52 105 Sum mod 26 • 19 0 7 17 18 15 8 19 23 12 0 1 Ciphertext • t a h r s p i t x m a b

Transposition • The letters of the message are rearranged • Columnar transposition • Example: THIS IS A MESSAGE TO SHOW HOW A COLMUNAR TRANSPOSITION WORKS

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

Computer System Security CSE 5339/7339