The shooting party

Having briefly reviewed the contributions made by the Polish cryptographers and French intelligence service to the cracking of German military codes yesterday – and let us reiterate how very important both were – we can move on to the stars of both fiction and non-fiction, the codebreakers of Bletchley Park.

Bletchley Manor. Photo by Matt Crypto

Bletchley was a backwater with no significant military targets, a distinct advantage for the GC&CS, the so-called Government Code and Cypher School.  Bletchley Manor was within walking distance of the train station and the train itself connected to the line that ran between Oxford and Cambridge, the chief sources of cryptanalysts.

The Manor had been built by a Victorian financier, but by the 1930s it had seen better days and was scheduled for demolition.  At that point, in 1938, the head of MI6, Sir Hugh Sinclair, bought it and the first cohort of codebreakers and administrators moved in.

As cover, the earliest arrivals announced themselves as ‘Captain Ridley’s shooting party.’

One year later, the French provided the manuals and key lists for the Enigma machine and Polish mathematicians provided the formulae for decryption.

From 500 people at the start of the war, Bletchley grew to number 9,000 by the end; about 80% of those were women, mostly Wrens and Waafs.

On day one, staff signed the Official Secrets Act – GC&CS members did not talk about their work to friends or family. Or each other. The rules were
Do not talk at meals …
Do not talk in the transport …
Do not talk travelling …
Do not talk in the billet …
Do not talk by your own fireside …
Be careful even in your Hut …

People who worked at Bletchley did not mention it to their families until some 30 years after the war. Even today,  many refuse to discuss the details.

And no action based on code-cracking was ever taken without a red herring provided as a source of intel – in most cases a reconnaissance plane was sent out.  For obvious reasons, it was critical that the Germans never suspect that the Allies were reading their signals.

Less obviously, ‘Both of the two German electro-mechanical rotor-machines whose signals were decrypted at Bletchley Park, Enigma and the Lorenz Cipher, were virtually unbreakable if properly used. It was poor operational procedures and sloppy operator behaviour that allowed the GC&CS cryptanalysts to find ways to read them.’

In short, without human error, there’d have been no intel – and no way to track down the U-boats in the Atlantic, no way to stop Rommel at Tobruk.

No one had ever seen the Lorenz cipher teleprinter, but analysts reverse-engineered one from the logic of its signals  – the Colossus. The world’s first programmable digital electronic computer, it worked.

If you would like to know all the details about Bletchley, you can read about it in Harry Hinsley’s Codebreakers: The Inside Story of Bletchley Park or Britain’s Secret War by Michael Smith, or any number of other publications.  Just check the bibliography  here.

But you can also  listen to actual codebreakers describe life at Bletchely via interviews at a recent Bletchley gathering:

Sir Arthur Bonsall describes how he was recruited, the delightful Margaret Francis tells the story of how it was that her husband – serving in Italy – was actually the recipient of her signals, though neither knew it at the time, and finally, Giles Sandeman-Allen talks about his grandfather, mother and aunt, all of whom were at Bletchley in various jobs.

There are more interviews at the site – look in the column on the right.  What a joy to hear the actual participants describe history.

Reaping secrets

In May of 1919, the Cipher Section of the Polish Army was created – it soon proved its value in the ensuing Polish-Russian border war, the result of some imprecise wording in the Treaty of Paris.  (Poland had regained its independence after WWI and was eager to extend its borders.)

A military enigma machine. Photo by Karsten Sperling

The decrypting of more than a hundred separate Russian ciphers and thus thousands of signal messages gave the Poles a definite tactical advantage. By the winter of 1927-28, the Section had become the Cipher Bureau, expanded its staff and in July of 1928 it tackled the biggest job it would ever have.

It started with a package gone astray.  Officials at the Warsaw Customs House were holding a box that according to its declaration was radio equipment.  But before it could go through customs, a representative of the German shipper appeared, said a mistake had been made and demanded that it be returned to Germany.

Classic.  No one was paying much attention until the fuss started, but then they did.  Someone remembered that the Army’s Cipher Bureau was always interested in new radio technology and so they were called in. When the Bureau experts opened the box, it was found to contain a mass market cipher machine intended for commercial use.  It had been named by its inventor – Artur Scherbius – the Enigma machine.

Marian Rejewski, probably 1932.
Photo courtesy of Janina Sylwestrzak, Rejewski's daughter.

The German Navy had adopted an Enigma almost two years earlier.  Soon, the Army would as well.

During the summer following a careful examination of the Enigma machine, the Poles intercepted the first of many German machine-encypted signals.  But even with the Enigma machine on the shelf, they were unable to crack the code.

The Enigma machine was – and continued to be for many years – a near-perfect encryption device. A detailed description of why it works so well is here. No one would ever have been able to decipher German signals if not for a number of events.

First, the French had a mole inside the German Cipher Office and a resident spy who got the information from the mole to the Intelligence service.

Second, in 1932, the Poles hired a young cryptanalyst right out of university, a mathematician named Marian Rejewski.

French Intelligence came to the Poles in 1932 and offered to share their info – during the previous four years they’d gotten copies of the Enigma machine’s instruction manual, operating procedures, and lists of key settings, but they still weren’t able to decrypt signals.

Using the information from the French, Rejewski turned to a branch of pure mathematics called the ‘theory of permutations and groups’ and very soon the first decryptions began to emerge.  Over the next six years the Polish Cipher Bureau decrypted upwards of 75% of the German signal traffic.

Then, in 1939, the Poles shared their work with the Allies and the spotlight shifted to England and ‘Captain Ridley’s shooting party.’

Next, Bletchley Park.