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Computer System Security CSE 5339/7339

Computer System Security CSE 5339/7339

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Computer System Security CSE 5339/7339

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

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

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

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

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

  6. OS -- Review Process Memory Management

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

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

  9. 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)

  10. Encryption Original plaintext ciphertext plaintext Encryption Decryption

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

  12. 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!

  13. 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)

  14. Exercise wklv phvvdjh lv qrw wrr kdug wr euhdn

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

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

  17. 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

  18. Example • ABCDEFGHIJKLMNOPQRSTUVWXYZ • wordabcefghijklmnpqstuvxyz • ABCDEFGHIJKLMNOPQRSTUVWXYZ • profesinalbcdghjkmqtuvwxyz

  19. 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

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

  21. 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

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

  23. 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