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link a fishy name: Berlin–Paris was ‘Jellyfish’, Berlin–Rome was ‘Bream’, Berlin–Copenhagen
was ‘Turbot’, and so on.
The two central exchanges for Tunny traffic were at Strausberg (near Berlin) for the western
links, and Königsberg for the eastern links into Russia. In July 1944 the Königsberg exchange
closed down and a new hub was established for the eastern links at Golssen, about 20 miles
from the Wehrmacht’s underground command headquarters south of Berlin. During the final
stages of the war the Tunny network became increasingly disorganized, and by the time of
the German surrender the remaining central exchange had been transported from Berlin to
Salzburg in Austria.
There were also fixed exchanges at some other large centres, such as Paris. The distant ends
of the links, usually close to the fighting, were mobile. Each mobile Tunny unit consisted of
two trucks. One carried the radio equipment, which needed to be kept well away from the
teletype equipment because of interference. The other truck carried two Tunny machines, one
for sending and one for receiving. Sometimes a landline was used instead of radio and the truck
carrying the Tunnies was connected up directly to the telephone system. Only Tunny traffic
sent by radio could be intercepted in Britain.
How Tunny encrypts
As with Enigma, the heart of the Tunny machine was a system of wheels. Some or all of the
wheels moved each time the operator typed a character at the teleprinter keyboard (or, in the
case of an ‘auto’ transmission, each time a new letter was read in from the pre-punched tape).
There were twelve wheels in all. They stood side by side in a single row, like plates in a dish rack.
The rim of each wheel was marked with numbers (01, 02, 03, etc.), and these were visible to the
operator through a small window.
From October 1942 the following operating procedure was used. Before starting to send
a message, the operator would use his thumb or fingers to turn the twelve wheels to a given
combination, such as 02 14 21 16 03 36 21 16 43 21 50 26: he obtained this combination from
a codebook. This codebook, known as a ‘QEP’ book, contained a hundred or more combina-
tions. At Bletchley Park, the selected combination was called the ‘setting’ for that particular
message. The wheels were supposed to be turned to a new setting at the start of each new
message, although because of operator error this did not always happen. The operator at
the receiving end, who had the same QEP book, set the wheels of his Tunny machine to the
same combination. This enabled the machine to decrypt the message automatically as it was
received.
The Tunny machine encrypted each letter of the message by adding another letter to it, so
masking the identity of the original letter. Letters were added by means of adding the individual
dots and crosses that composed them. The rules the Tunny’s designers selected for dot-and-
cross addition are simple: dot plus dot is dot; cross plus cross is dot; dot plus cross is cross; and
cross plus dot is also cross:
- • = • x + x = • • + x = x x + • = x
In short, adding two instances of the same thing produces a dot, whereas adding a mixed pair
produces a cross. (Computer techies will recognize Tunny addition as Boolean XOR.)