Transcript CCC Module PH19510
Chaos, Communication and Consciousness Module PH19510
Lecture 10 Cryptography - The Science of Secret Writing
Overview of Lecture
Ways of keeping information secret Steganography Ceasar’s cipher The Vigenere square Mechanical Cryptography
The Code Book
Simon Singh Fourth Estate ISBN 1-85702-889-9 £9.99
http://www.simonsingh.com
Ways of keeping information secret Steganography
steganos
– covered
graphein
– to write Hidden Message Crytopgraphy
kryptos
– hidden
graphein
– to write Hidden Meaning
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 1 st century AD. – Invisible ink from variety of organic fluids 16 th Century Italy – Write on shell of egg in alum solution. Message appears on egg inside when boiled.
2 nd World war – Microfilm, text shrunk to full stop size.
Now: Hide inside music file/image ?
Cryptography
2 main options Substitution Letters retain position Identity of letters substituted Transposition Letters retain identity Position of letters scrambled
Transposition ciphers
Re-arrange letters of message Need pre-arranged method, otherwise one long anagram (possibly many solutions) eg. Rail fence code Scytale
General Cipher Process
Plain Text Key Algorithm Cipher Text Key Algorithm Plain Text
Encryption Decryption
Substitution Cipher
Algorithm substitute letters Key cipher alphabet
A I Q P F C W O H J T N U L B M E V S G Z D X K Y R Simple cipher alphabet based of pairs of letters Plain Text Cipher Text
a t t a c k t h e c a s t l e a t d a w n L K K L S T K D P S L C K A P L K H L G Y
Caesar Cipher
Shift alphabet along by n places n is key eg n=3 a b c d e f g h i j k l m n o p q r s t u v w x y z D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
Monoalphabet Substitution Ciphers How many different cipher alphabets ?
26 × 25 × 24 × ….. × 1 = 26! ≈ 4 x 10 26 Seems difficult to break Good until ≈ 850AD
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
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
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
Le chiffre indéchiffrable
The Vigen ère Cipher Belaso 1553 Vigen ère C19 th Polyalphabetic substitution Key word/phrase Key: PlainText: thelordoftherings CipherText:
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
Making the Vigen ère 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
Mechanical Encryption
Automate encryption process Freedom from mistakes Possible to use complicated algorithms Speed
The Enigma Machine
Patented 1921 by Arthur Scherbius Used in WWII Input via Keyboard Output via Lamps Plugboard fixed substitution Lamps Rotors substitution changes every character Plugboard Rotors Keys
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
The Enigma Keys
Arrangement of rotors 3 rotors, 6 possible arrangements Total 17,576x6x 100,391,791,500 ≈10 16 ≈53 bits =6 Start position for rotors 3 rotors, 26 start positions=26x26x26=17,576 Plugboard swap 6 from 26 =100,391,791,500
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
Bletchley Park & ULTRA
UK codebreakers Station X Bletchley Park Alan Turing Product known as ULTRA Shortened WWII by 2 years
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
Cracking Enigma #2
Look for loops in crib/ciphertext Separate effect of plugboard & rotors Use machine to test possibilities bombe
Review of Lecture
Ways of keeping information secret Steganography Ceasar’s cipher The Vigenere square Mechanical Cryptography The Enigma Machine