General Report on Tunny

14A Page 28



(a) General position.

In order that information sent out by the Germans in Tunny messages might become available to Allied authorities, four types of organisation had to be built up. These were all under the direction of G.C. and C.S. and concerned Interception, Cryptography, Traffic Analysis and Intelligence.

This report is concerned only with the Cryptographic work on Tunny, and the sections at Station X concerned with this occupied an intermediate position between
         GCWS KNOCKHOLT and ancilliary non-morse interception stations working on Fish Traffic, and
         Intelligence sections at Station X to which Tunny decodes passed (Hut 3, Naval Section, 180S).

Traffic Analysis - undertaken by Sixta (Non-morse) - was often of cryptographic value, and several references to Sixta's work will be found in the chapters that follow.

(b) Three periods.

The history of cryptographic work on Tunny can be suitably divided into three periods - the Research period, the Testery period, and the combined period.

Tunny traffic was tackled by the Research section shortly after the first link was set up in June, 1941. The Research period lasted until July, 1942, by which time a stretch of key had been obtained from a depth (August, 1941), the workings of the machine deduced (January, 1942), and various hand methods of wheel-breaking and setting on the basis of the indicating system, depths, near depths and short cribs devised and used with success on the traffic of March to July, 1942. In July, current traffic was read for the first time.

In July, 1942 Major R.P. Tester formed a Tunny section (the "Testery" - consisting mainly of ex-members of the Research section) to tackle Tunny on an operational basis, and from July to October, 1942 nearly every message was read. In October, 1942, the expansion of the Tunny system started and the QKP system was introduced. After this, operational activity was restricted to wheel-breaking and setting from depths. Depths were frequent and produced many sets of wheels but covered only a fringe of the setting problem.

The Research section again set to work on Tunny and devised statistical and mechanical methods of setting which did not depend on depth. Mr. M.H.A. Newman was given the job of developing these operationally in December, 1942, and his section (the "Newmanry") with its first two machines was founded in June, 1943. The section was at first regarded by members of Major Tester's section with some amusement, but by October, techniques were improved and operational work had started.

With the introduction of P5 limitation in December, 1943, depths disappeared. Mr. Newman's section became essential to all Tunny work and a new division of labour was effected. The section became responsible for chi-breaking and setting (which had to be done mechanically), and Major Tester's section for psis and motors which could be broken or set by hand. More and better machines were ordered, so that, when the

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