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to eavesdrop on lengthy back-and-forth communications between the grand architects of
Germany’s battle plans. Tunny leaked detailed information about German strategy, tactical
planning, and military strengths and weaknesses.
The Berlin engineering company C. Lorenz AG manufactured the Tunny machine. The first
model bore the designation SZ40, ‘SZ’ standing for ‘Schlüsselzusatz’ (‘cipher attachment’). A
later version, the SZ42A, was introduced in February 1943, followed by the SZ42B in June
1944 (‘40’ and ‘42’ refer to years, as in Windows 98). Tunny was one of a family of three new
German cipher machines, manufactured by different companies. Bletchley Park gave all three
the general cover name ‘Fish’. The Fish family’s other members were codenamed Sturgeon and
Thrasher; Thrasher had only limited use, while Sturgeon provided nothing more than a trickle
of intelligence compared with the deluge produced by Tunny.
‘Listening stations’ in the south of England first intercepted Tunny radio messages in June
- After a year-long struggle with the new supercode, Tunny messages were being broken
in quantity by the middle of 1942. Naturally, Turing played a key role in the attack on Tunny.
The Tunny system
The Tunny machine, measuring 19 ̋ × 15½ ̋ × 17 ̋ high, was a ‘cipher attachment’: attached to a
teleprinter or teletypewriter, it automatically encrypted the outgoing stream of pulses produced
by the teletype equipment. It also automatically decrypted incoming messages, enabling the
teletype equipment to print out the unenciphered message.
At the sending end of a Tunny link the operator typed plain German (the ‘plaintext’ or ‘clear
text’ of the message) at the teletype keyboard, and at the receiving end the plaintext was printed
out automatically, usually onto a paper strip as in old-fashioned telegrams. With the Tunny
machine in ‘auto’ mode, many long messages could be sent at high speed, one after another.
When the transmitting machine was operating in auto mode, the plaintext spooled into the
teletype equipment on pre-punched paper tape.
Morse-based Enigma was clumsy by comparison. With Tunny, the German operators at the
sending and receiving ends of the radio link did not even see the ciphertext (the encrypted form
of the message). With Enigma, on the other hand, the ciphertext appeared at the machine’s
lampboard, letter by letter. As the keyboard operator typed the plaintext at the machine’s key-
board, an assistant laboriously noted down the ciphertext letters as they appeared one by one
at the Enigma’s lampboard; the radio operator then transmitted the ciphertext in Morse code.
Tunny was faster, could handle much longer messages, and required two people (rather than
six) to send and receive a message. Morse was not needed in the Tunny system: the Tunny
machine’s output, encrypted teleprinter code, went directly to air.
International teleprinter code—which was at the time in widespread use around the globe,
and remains so today—assigns a pattern of five zeros and ones to each letter and keyboard
character. In this system ‘1’ represents a pulse and ‘0’ represents the absence of a pulse. Using
Bletchley Park’s now antiquated convention of representing a pulse by a cross and the absence
of a pulse by a dot: the letter C, for example, is •xxx• (no-pulse, pulse, pulse, pulse, no-pulse).
Today we write 0 instead of • and 1 instead of x, so C is 01110. To give some other examples: O
is •••xx, L is •x••x, U is xxx••, and S is x•x••. Figure 14.2 shows a wartime memory aid that lists
the pattern of dots and crosses associated with each keyboard character.