How They "Cracked" the Enigma Machine...

Overview

Bletchley Park, Britain’s wartime code breaking centre, achieved one of the greatest cryptanalytic feats of all time: breaking the German military's Enigma cipher. The core method involved guessing likely plaintext phrases (called cribs) and testing them using mechanical decryption machines called Bombes.

Assumptions and Background

• Enigma used 3 rotors and a plugboard (Steckerbrett).
• Settings changed daily; code breakers had to deduce each day’s key.
• Messages were intercepted by Allied listening stations and sent to Bletchley Park.

Step 1: Intercepting Messages

Thousands of encrypted messages were intercepted daily. Each message consisted of a header and a body of cipher-text produced by the day’s Enigma settings.

Step 2: Identifying a Crib

A crib is a guessed section of plaintext, typically a phrase the Germans were known to use repeatedly. Common examples included:

• “WETTERBERICHT” (weather report)
• “HEIL HITLER” (a common sign-off)
• Standard formats like dates, salutations, or location names

Cribs had to be carefully chosen to avoid logical contradictions and maximize their effectiveness.

Step 3: Constructing a Menu

Once a crib was matched to part of the cipher-text, code breakers created a menu — a logical map of how letters in the crib may have been encrypted. For example:

Cipher-text: Y M U P W X
Plaintext: H E I L H I

This established pairings like Y→H, M→E, etc., forming a logic puzzle that could be tested mechanically.

Step 4: Entering the Menu into the Bombe

The Bombe machine was designed by Alan Turing and Gordon Welchman to test thousands of rotor positions using the menu logic. It worked by:

• Simulating multiple Enigma machines
• Running the menu logic across 17,576 rotor settings (26 × 26 × 26)
• Detecting logical contradictions — e.g., a wire trying to connect to two outputs
• Flagging “non-contradictory” settings for further human checking

Step 5: Human Verification

Candidate settings from the Bombe were tested manually on real Enigma replicas by skilled checkers. If they produced legible plaintext from cipher-text, the day’s settings were confirmed — and all other messages could then be decrypted.

Why It Worked

• Enigma never encrypted a letter as itself.
• German operators reused formats and greetings.
• The plugboard and rotor logic introduced patterns that could be detected with mechanical analysis.
• Human habits created enough cracks in the armour to exploit daily.

Technical Tidbit: Bombe Stats

Conclusion

The brilliance of the Bletchley Park team lay not just in mathematics or engineering, but in combining intuition, logic, and mechanical innovation. Cribs gave them an entry point. The Bombe machines did the heavy lifting. And thanks to this partnership, the Allies gained insights that likely shortened the war by years.