Security Lessons from Bletchley Park and Enigma

Franklin Heath Ltd28 May 2013
Security Lessons from Bletchley Park and Enigma
Image: Bletchley Park Mansion by Antoine Taveneaux

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Topics
 How the Enigma machine works
 How Bletchley Park exploited German mistakes
 Five lessons we can draw from this
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© Franklin Heath Ltd

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The Enigma Machine
 Invented by Arthur Scherbius in 1918
 Commercially available from 1923
 Adopted by German military from 1927
 Several variants, notably:
 Enigma I, German army 1932
 Enigma M4, German U-boats 1941
 Principally mechanical
 Battery is used only to illuminate the output letter
 Used throughout WWII by German military + agencies
 Estimated 100,000 machines produced
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Enigma Machine Components
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Scrambler
Rotors and reflector
Output
Battery-powered lamps
Input
Keys (switch and lever)
Plug Board
Static, swaps letters

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Enigma Machine Components
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Scrambler
Rotors and reflector
Output
Battery-powered lamps
Input
Keys (switch and lever)

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Fully Functional Paper Model
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Example Enigma Settings Sheet
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Enigma Simulator
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Enigma Cipher Characteristics
 26-letter alphabet
 Numbers typically spelled out
 Reciprocal substitution cipher
 Operation is its own inverse
 Independent of preceding text
 Message key sets start “state”
 Never encrypts a letter as itself
 Keys are SPDT switches selecting
input or output
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Enigma Machine Key Length
 4-rotor Enigma M4
 2 possible reflectors
 672 possible rotor choices
 676 possible notch positions
 532,985,208,200,576 possible combinations of plugs
 456,976 possible starting positions
 = 221,286,292,668,406,558,235,295,744 possible keys
 Log2 gives equivalent binary key length: ~88 bits
 Still export-controlled today!
 Yet it could be broken with 70-year old mechanical technology
 Key length isn’t the most important characteristic
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Bletchley Park’s “Wicked Uncles”
 Senior codebreakers recruited in 1939
 Introduced mathematical and mechanised methods
 1941 memo delivered to P.M Winston Churchill
 Response: “Make sure they have all they want on extreme
priority and report to me that this had been done.”
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Alan Turing 1912-1954
Gordon Welchman 1906-1985Hugh Alexander 1909-1974Stuart Milner-Barry 1906-1995

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Types of Breaks into Enigma
 Polish Cipher Bureau, 1932 onwards
 Common start positions (mitigated 1938)
 Repeated message key (mitigated 1940)
 UK GC&CS, 1937 onwards
 “Rodding” using cribs (mitigated by plug board)
 Herivel tip, to deduce ring settings
 Cillies, to deduce message keys
 Banburismus, to identify likely rotor orders
 Bombe menus from cribs, to test rotor orders
 EINS catalogue, to deduce message keys and bigram tables
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The Turing-Welchman Bombe
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Images Credit: Antoine Taveneaux

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Aside: What is This?
 Part of the Turing exhibit at the Science Museum
 “a cryptographic aid used at Bletchley Park”
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Lesson 1:
Cryptosystems have Subtle Flaws
 Long keys do not alone make a strong cryptosystem
 Stream ciphers can have unfortunate interactions with
themselves (especially reciprocal synchronous ones)
 Attackers can take advantage of predictable plain text or even
predictable repetitions in otherwise unknown plaintext
 Best practice for modern systems seems to be to use
block ciphers like AES with chaining modes
 2001 break of WEP (“Wired Equivalent Privacy”) exploited use
of duplicate initialisation vectors with RC4 stream cipher
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Lesson 2:
Plan for Key Compromise
 “Pinches” provided a way into new Enigma networks
 1940 HMS Gleaner: rotors VI and VII from U-33
 1940 HMS Griffin: settings and cribs from armed trawler Polares
 1941 HMS Tartar: code books from weather ship Lauenberg
 1941 HMS Somali: rotors and code books from armed trawler Krebs
 1941 HMS Somali: code books from weather ship München
 1941 HMS Bulldog: machine and code books from U-110
 1942 HMS Petard: machine and code books from U-559
 They had emergency procedures to switch to other settings
 Modern security systems need to have “renewability” too
 for recovery from “class breaks” like the DVD CSS key breach in 1999
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Lesson 3:
Users Pick Poor Passwords
 Many Enigma messages were read by guessing the
message key that the operator chose (“Cillies”)
 AAA BBB, QWE ASD, BER LIN, etc.
 This was addressed later in the war by operational
procedures
 Daily settings used as a pseudo-random generator
 Cryptographic keys need more entropy than users can
supply in the form of a password
 Salts, nonces, initialisation vectors, etc.
 You can crack many unsalted MD5 passwords just with Google
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Lesson 4:
Pick a Good RNG and Trust It
 Don’t be tempted to interfere to make it look random
 German cipher staff had rules for not repeating rotor
order and not plugging adjacent letters
 This significantly reduced the number of possible settings that
needed to be tried on the Bombe
 Many security vulnerabilities in modern systems are
due to poor randomness
 e.g. Debian OpenSSL vulnerability in 2008
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Lesson 5:
Don’t Underestimate the Enemy
 German high command told Enigma was “unbreakable”
 German cryptographers knew it was theoretically
breakable, but thought no one would put in that much
effort
 Bletchley Park’s mathematical approach and production
line methods led to industrial-scale cryptanalysis
 Modern example:
 2009 breaking of GSM A5/1 using precomputed rainbow tables
 Used GPUs in a distributed collaborative project
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Summary
 The Enigma machine cipher is elegant, efficient and has
few significant inherent flaws
 Bletchley Park benefited greatly from weaknesses in the
key establishment procedures and from analysis of traffic
for which keys had been compromised
 Five lessons:
 Cryptosystems have subtle flaws
 Plan for key compromise
 Users pick poor passwords
 Pick a good RNG and trust it
 Don’t underestimate the enemy
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Security Lessons from Bletchley Park and Enigma

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