Chaos, Communication and Consciousness Module PH19510

1. Chaos, Communication and ConsciousnessModule PH19510 Lecture 10 Cryptography - The Science of Secret Writing

2. Overview of Lecture • Ways of keeping information secret • Steganography • Ceasar’s cipher • The Vigenere square • Mechanical Cryptography

3. The Code Book • Simon Singh • Fourth Estate • ISBN • 1-85702-889-9 • £9.99 • http://www.simonsingh.com

4. Steganography steganos – covered graphein – to write Hidden Message Crytopgraphy kryptos – hidden graphein – to write Hidden Meaning Ways of keeping information secret

5. Steganography • Ancient Greece – write message on shaved head of slave, allow hair to grow back • Ancient China – write message on fine silk, roll into ball, cover in wax, swallow • 1st century AD. – Invisible ink from variety of organic fluids • 16th Century Italy – Write on shell of egg in alum solution. Message appears on egg inside when boiled. • 2nd World war – Microfilm, text shrunk to full stop size. • Now: Hide inside music file/image ?

6. Cryptography • 2 main options • Substitution • Letters retain position • Identity of letters substituted • Transposition • Letters retain identity • Position of letters scrambled

7. Transposition ciphers • Re-arrange letters of message • Need pre-arranged method, otherwise one long anagram (possibly many solutions) • eg. Rail fence code • Scytale

8. General Cipher Process Key Key PlainText CipherText PlainText Algorithm Algorithm Decryption Encryption

9. Substitution Cipher • Algorithm  substitute letters • Key  cipher alphabet Simple cipher alphabet based of pairs of letters Plain Text Cipher Text

10. Caesar Cipher • Shift alphabet along by n places • n is key • eg n=3

11. Monoalphabet Substitution Ciphers • How many different cipher alphabets ? • 26 × 25 × 24 × ….. × 1 = 26! ≈ 4 x 1026 • Seems difficult to break • Good until ≈ 850AD

12. Cryptanalysis - Code breaking • Al-Kindi • 800 – 873 AD • Analysis of text • frequency of letters • double letters (ee, oo, mm, tt …) • adjacent letters • single letter words • common words

13. The Renaissance – Code makers trying to stay ahead • Addition to ciphers to make frequency analysis more difficult: • Nulls – meaningless symbols or letters • Misspellings – DISTAWT PHREKWENCYS • Code words/symbols for common words

14. Mary Queen of Scots • Plot to assassinate Queen Elizabeth • Messages hidden in bung of barrel – steganography • Substitution cipher • Nulls • Codewords • Broken by Walsingham • Mary executed 1587

15. Key: PlainText: CipherText: Le chiffre indéchiffrableThe Vigenère Cipher • Belaso 1553 • Vigenère C19th • Polyalphabetic substitution • Key word/phrase PIG PIGPIGPIGPIGPIGPI thelordoftherings I IP IPKAWXSWLIPKGQTVA

16. Cracking le chiffre indéchiffrable • Look for repeated groups in cipher text Example:IPKAWXSWLIPKGQTVA • Result of repeat of key with same plaintext • Distance between repeats is multiple of key length • Possible to guess length of key • Split problem into several monoalphabet ciphers • Apply frequency analysis to each in turn

17. Making the Vigenère cipher unbreakable • Security increases with key length • Unbreakable if key: • truly random (radioactive decay, electronic noise, quantum effect) • doesn’t repeat • One-time pad • But … key distribution problem

18. Mechanical Encryption • Automate encryption process • Freedom from mistakes • Possible to use complicated algorithms • Speed

19. The Enigma Machine • Patented 1921 by Arthur Scherbius • Used in WWII • Input via Keyboard • Output via Lamps • Plugboard • fixed substitution • Rotors • substitution • changes every character Rotors Lamps Keys Plugboard

20. The Enigma rotors • Rotor • 26 way substitution • 3 rotors • Reflector • Rotors advance every keystroke & change substitutions • Middle Rotor advances for every complete turn of Right • Ditto for left rotor & reflector

21. The Enigma Keys • Arrangement of rotors • 3 rotors,  6 possible arrangements =6 • Start position for rotors • 3 rotors, 26 start positions=26x26x26=17,576 • Plugboard • swap 6 from 26 =100,391,791,500 • Total • 17,576x6x 100,391,791,500≈1016≈53 bits

22. Day Keys & Message Keys • Don’t send too much information with same key • Generate ‘random’ key for each message • Message Key • Use day key (from codebook) to encrypt message key, put at start of message. • Encrypt rest of message with message key • Day key only used to encrypt message keys

23. Bletchley Park & ULTRA • UK codebreakers • Station X • Bletchley Park • Alan Turing • Product known as ULTRA • Shortened WWII by 2 years

24. Cracking Enigma • Captured/stolen machines & codebooks • Known/guessed plaintext (Cribs) • weather station reports in fixed format • ‘planted’ information • Operator & Systematic weaknesses • ‘easy’ message keys (‘cillies’) • Message key sent twice • Restrictions on plugboard & rotor settings

25. Cracking Enigma #2 • Look for loops in crib/ciphertext • Separate effect of plugboard & rotors • Use machine to test possibilities - bombe

26. Review of Lecture • Ways of keeping information secret • Steganography • Ceasar’s cipher • The Vigenere square • Mechanical Cryptography • The Enigma Machine