whoami

• Freelance PHP developer and Linux system administrator
• Specialise in legacy code/systems and financial services
• I like trains, politics and technology (@pwaring)

Background

• World War 2: 1939 - 1945
• Germany + others (Axis) vs Britain + others (Allies)
• Enigma used pre- and post-war (e.g. British colonies)
• Other crytographical systems, e.g. Lorenz, Purple

Problem

• Need to transmit messages across long distances
• Sender and recipient are not in fixed locations
• Messages are time sensitive
• Line of sight not guaranteed
• Cables take time to run and can be cut
• Only available technology: radio

Radio

• Radio signals are broadcast
• Anyone can listen
• Anyone can broadcast
• Ideally want authentication, integrity and encryption
• Will focus mainly on encryption

Enigma as an example

• Demonstrates flaws in design
• Demonstrates flaws in operation
• State vs state - significant resources available
• Highest stakes possible
• Probably the biggest 'hack' in history
• War shortened by 1-2 years, millions of lives saved

The machine

Enigma Machine

Operation

• Effectively a polyalphabetic substitution cipher
• Plugboard: Swaps pairs of letters
• Rotors: Position and starting letter
• Press plaintext on keyboard, ciphertext lamp lights up
• Reversible - same settings for decipherment

Key distribution and exchange

• Key was plugboard and rotor settings
• Monthly book which contained settings for each day
• Day key used to exchange a message key
• Message key transmitted twice
• Message key had same rotor order and plugboard as day key

Traffic analysis

• Can triangulate radio signals to get rough location
• Track operators by fingerprint
• Useful even if message text is unknown
• Sometimes gives clues for codebreaking
• Similar to metadata from emails, websites etc.

Design

• Enigma could never encrypt a character as itself
• Helps eliminate potential keys
• Plaintext guessing using cribs

Operational rules

• Rule against having rotors in same position for more than 2 days
• No swapping plugboard letters with neighbours
• Preventing 'obvious' combinations reduced key space

Plaintext knowledge

• Knowing (or guessing) some of the plaintext helps with codebreaking
• Knowing the position of plaintext is even better (can be guessed)
• Army reports tend to be structured and regular (e.g. weather)
• 'Nothing to report' reports
• Intentionally cause known messages to be sent ('gardening')

Compromised keys

• Capture a U-boat and you have the keys
• Germans did not always revoke keys after capture (expensive)
• Reliance on U-boat crew destroying keys
• Extremely dangerous operation, but rewarding

U-33

• Sent to lay mines in Scottish waters
• Normal procedure: no Enigma machines or codes on mine-layers
• Procedure ignored, crew and parts of Enigma machine captured

Per-message keys

• Choose a new key per message, encrypted with the day book setting
• Reduces the amount of ciphertext using the day book setting
• Send key twice to avoid transmission problems (initially)
• Obtain the day key and you have the message keys

Guessable message keys

• People can't choose truly random sequences
• Operators choose their/partner's initials etc. ('cillies')
• Operators re-use keys
• Actual key space much lower than theorectical key space
• Similar to card PINs today

Faith in security

• Enigma assumed to be unbreakable
• Reasonable assumption if used correctly
• Occasionally not using decrypted messages
• Send spotter planes to known U-boat locations
• Assumption of spies etc. instead of code breakers
• Zimmerman telegram (WWI)

Lessons learned

• Key space can be reduced by flaws in design and operation
• Reduced key space + automation = brute force can work
• Repetition, re-use and regularity undermines cryptography
• Traffic analysis can sidestep encryption and aid codebreaking
• If keys are lost, assume they have been compromised
• No system survives first contact with users

Thanks for listening

• Questions?
• Slides at: talks.phpdeveloper.org.uk
• Bletchley Park trip mailing list: lists.xk7.net

Further reading

• The Code Book (Simon Singh)
• Enigma: The Battle for the Code (Hugh Sebag-Montefiore)
• Visit Bletchley Park and The National Museum of Computing