A Colossal Secret
Maximum PC|May 2021
Ian Evenden discovers how British WWII braniacs, faced with an impossible task, helped save millions of lives and invented the computer as we know it
Por Ian Evenden

Your country is at war, and you start picking up directional radio signals that sound like nothing you’ve ever heard before. The enemy is highly mechanized, makes use of the latest technology, and has a record of using complex codes and ciphers in its transmissions. This is something new, however. Instead of the dots and dashes of the Morse code you’re used to intercepting, this sounds like a harsh wailing.

These days, we’d probably equate the sound with a 56K modem, but back in the 1940s, it was known as teleprinter code. Nazi high command, not content with the “unbreakable” Enigma cipher it used to spread orders among its companies and brigades, was also using a machine for higher-level communications called the Lorenz SZ that British codebreakers had never seen, and would not see until the end of World War II. Despite this, they were able to deduce how it worked, and crack the encipherment.

This is the story of a country house in England full of mathematicians, crossword enthusiasts, the occasional genius, and a man from the postal service more used to creating automated telephone exchanges. Together, they read German military communications and shortened WWII by up to four years, saved possibly millions of lives, and created Colossus, the first programmable electronic computer, in the process.

This is a complicated story, and we’ve had to leave bits out, but bear with us, there’s still a lot of backstory. The tale of how the British, Alan Turing in particular, cracked Enigma, opening up Nazi communications, and making it possible to anticipate their every move is already fairly well known, but it has some parallels with the decryption of the Lorenz machine that can’t be ignored. The Nazis had mechanized the sending of secret messages, so a mechanized approach was needed for reading them. Lorenz had similar weaknesses to the Enigma, especially if you could be confident of the content of certain parts of messages, such as ending everything with “Heil Hitler,” for example.

Enigma was an electro-mechanical machine with three, sometimes four, rotor wheels and a plugboard. Input came from a keyboard, and its output was a lamp that lit up behind a grid of letters. The rotors moved every time a key was pressed, one full rotation triggering the next one to move, then the next. The plugboard swapped letters over, so connecting A to S always switched those letters. With three rotors from a set of five in use, and 10 connections on the plugboard, the machine was capable of nearly 159 quintillion (18 zeroes) combinations. Adding a fourth rotor, as the German Navy did, just made things worse.

Lorenz had 12 rotors, and acted as an add-on to a teleprinter machine (you type at one end, the message comes out printed on paper tape at the other), but otherwise operated in a similar way. Once the machine’s settings for the day were entered, however, it was much more automatic. The enciphered text was encoded as a 5-bit Baudot code (International Telegraph Alphabet No. 2) for transmission, which made use of a modulated on/off signal— binary, in other words—that was the source of the modem-like screeching the British listening stations were picking up.

“It was a binary machine, but not understood as a binary machine,” says Paul Gannon, author of Colossus: Bletchley Park’s Greatest Secret. “They used a different terminology at the time—mark and space. They didn’t think of it as zero and one; that’s a backward projection from what we know now.”

The Lorenz machine then encrypted the Baudot further with a Verman stream cipher. Ten of the 12 rotors in the machine generated a keystream in two groups of five, which was combined with the plaintext using a logical operation known as Exclusive Or (XOR—see table on page 51). The remaining two wheels added a pseudorandom “stutter” to the keystream, complicating it further. Each rotor had a different number of cams on it, which could be raised or lowered to create the wheel settings. The total number of settings is a number so large it would be followed by 150 zeroes (there’s 18 in a quintillion, remember).

“There’s no reason why it had to be like this,” says Gannon, “but with Enigma they would write the message down on a piece of paper, one guy would then read out the message, another would type in each letter of the message and read off the encoded letter, then write it down. Then somebody else would transmit that using Morse code. At the other end, they would read the Morse code, and reverse the message with an Enigma set up with the same settings. But the teleprinter was a central communications tool within Germany, and especially with the Nazis when they took over, and so they wanted to carry on using it. First of all they transmitted from teleprinter to teleprinter over cables, and then using a wireless link.”

Gannon continues: “Your message was automatically encrypted and at the other end automatically decrypted, so all you needed was one typist at each end, rather than three people writing things down. Nobody saw the encrypted version of the message. The teleprinter was the key to communications within the German army, and this suited the British at the time because they had cracked the German naval and air force Enigma, but hadn’t cracked the army Enigma. So when they started intercepting these weird sounds, which they described as being like a cow wailing, it took some time to work out what they were.”

The first transmissions were intercepted in 1941, a link between Berlin, Germany, and Athens, Greece. Used to dealing with Morse code, the British classified these as NoMo, or nonMorse, and the Berlin-Athens link picked up the code name Tunny (tuna fish). Tunny would become the code name for the Lorenz machine itself (Geheimschreiber, or secret writer, in German) after the link closed in 1942, while the teleprinter operation as a whole became Fish. As new links sprang up, they attracted new species of sea life: Berlin to Tunis was Herring, Berlin to Paris Jellyfish, and Berlin to Oslo was given the code name Mullet. This was a recurring theme—three-rotor Enigma transmissions were code-named Dolphin, while four-rotor transmissions were Shark. The intelligence gathered by codebreaking was known as Ultra, and was an incredibly closely guarded secret.

Bletchley Park, home of the innocuously named Government Code and Cipher School (think today’s NSA), is a country house in the middle of England, built in the 1880s. These days, it’s a museum, but during WWII, it was a branch of the British intelligence services. Its activities were so highly classified that it wasn’t until 1974, with the publication of a book, The Ultra Secret, by F. W. Winterbotham, that details began to trickle out.

What took slightly longer to emerge, however, were the details of what they’d built there. The Enigma cipher was broken through human ingenuity (a lot of early work had been done in Poland during the 1930s, and transferred to Britain at the outbreak of war), and a series of electromechanical devices known as Bombes, which simulated the rotors of the Enigma machine and could sift through possible settings to crack that day’s wheel order and plugboard connections—effectively a brute force attack but using human brainpower, some wartime heroics (such as snatching a codebook from a sinking submarine), and user errors, such as transmitting two identical messages with different settings, to lower the number of permutations necessary. Lorenz, being far more complex, required a different approach.

At this point, Thomas H. Flowers, known as Tommy, the top man in research and development at the General Post Office Research Station at Dollis Hill, London, became involved. He was the son of a bricklayer, in contrast to the middle or upper-class types usually found at Bletchley Park, and had earned a degree in electrical engineering by taking evening classes. He’d been exploring the use of electronics in telephone exchanges—usually powered by young women before this point in time—and was convinced he could build an all-electric one.

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