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Before starting to send a message, the operator would use his thumb to turn the wheels to a combination that he looked up in a codebook containing one hundred or more combinations (known as the QEP book). 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 occur).The operator at the receiving end, who had the same QEP book, set the wheels of his Tunny machine to the same combination, enabling his machine to decrypt the message automatically as it was received.Sometimes a land line was used in preference to radio.
This truck also carried a device for punching tapes for auto transmission.After a year-long struggle with the new cipher, Bletchley Park first read current Tunny traffic in July 1942.Tunny decrypts contained intelligence that changed the course of the war in Europe, saving an incalculable number of lives.(Only Tunny traffic sent by radio was intercepted by the British.) As with the Enigma, the heart of the Tunny machine was a system of wheels (see right-hand column).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 from a pre-punched tape, each time a new letter was read in from the tape). They stood side by side in a single row, like plates in a dish rack.
Once all the combinations in a QEP book had been used it was replaced by a new one. In short, adding two sames produces dot, and adding a mixed pair produces cross.