Fishy Codes: Bletchley's Other Secret - Computerphile

TL;DR

Bletchley Park’s “fishy codes” were mysterious cipher communications, including Tunny traffic, that helped shift its work beyond the Enigma story toward the beginnings of computer science. Y stations recorded Tunny’s unusual waveform, experts identified it as broadcast teleprinter traffic, and analysts considered whether Germany used a machine-generated Vernam-style key with five-bit patterns and exclusive OR. Read on to understand the interception, cipher theory, and synchronization problem.

Transcript

some of you will have seen Shawn and me going wild around Bletchley Park and will deduce from that that we paid a visit to BP as they call it in the trade it's time now for me to move off from the enigma story which I've done two or three videos on already and tell a story this in many ways is even more relevant to the start of computer science tha... Read More

Key Insights

• 🥌 Tunney Traffic was a type of cipher communication intercepted by Bletchley Park, resembling touch-tone telephone signals.
• 🤙 Bletchley Park developed a user-friendly cipher machine, called British Honey, to decrypt Tunney Traffic.
• 🐵 Tape synchronization and identifying matching keys were challenges faced in deciphering Tunney Traffic.
• 💁 The indicator in Tunney Traffic messages provided important information about the encryption settings used.
• 💁 Deciphering Tunney Traffic provided valuable information for the Allies during World War II.
• ❓ The XOR additive cipher used in Tunney Traffic was vulnerable to certain weaknesses, but it provided a basis for decryption efforts.
• ❓ The deciphered Tunney Traffic messages were compressed using Huffman encoding for easier analysis.

Questions & Answers

Q: What were Bletchley Park’s “fishy codes”?

Bletchley Park gave fish names to mysterious types of intercepted traffic. These included Tunny, Sturgeon, and Halbert, with the discussion focusing on Tunny traffic.

Q: What was Tunny traffic?

Tunny traffic was a mysterious cipher communication detected by British listening stations and sent to Bletchley Park for analysis. Experts examining its recorded waveform identified it as broadcast teleprinter traffic rather than Morse code.

Q: Why was Tunny traffic broadcast instead of sent over landlines?

Teleprinter traffic commonly traveled over landlines, but the speaker suggests that some routes were too long, giving Greece and Germany as an example. Broadcasting allowed the teleprinter communication to cover such distances.

Q: How did British Y stations detect Tunny traffic?

Y stations around the UK listened for Enigma traffic and anything else unusual. They recorded Tunny as a black waveform trace on paper, resembling a mixture of sine and square waves.

Q: Why was Tunny traffic important to the beginnings of computer science?

The speaker describes its story as even more relevant to the start of computer science than Enigma. Bill Tutte, Max Newman, and Tommy Flowers are named as figures connected with this cipher traffic.

Q: What is a Vernam cipher in this account?

The Vernam approach encrypts teleprinter transmissions by applying exclusive OR between two five-bit patterns. One pattern represents a plaintext character, while the other supplies a key character, producing ciphertext character by character.

Q: Why was the two-tape Vernam system difficult to operate?

Its plaintext and key tapes had to remain exactly synchronized while moving through the equipment. Differential stretching, torn sprocket holes, and slippage could disrupt synchronization, especially when operators increased the speed.

Q: What did Bletchley Park suspect about the German cipher machine?

Analysts considered whether the Germans had avoided the troublesome second key tape by generating key characters with a machine. They envisioned an add-on to a teleprinter that would use wheels or rotors to produce the key characters.

Summary & Key Takeaways

• Bletchley Park encounters Tunney Traffic, a mysterious type of cipher communication.

• The traffic is encrypted teleprinter transmissions, resembling touch-tone telephone signals.

• Bletchley Park develops a new cipher machine, called British Honey, to decipher Tunney Traffic.