Practical and Organisational Factors in the Development History of the Typex Cipher Machine and its Use at Bletchley Park

The Typex was Britain’s main cipher machine during the Second World War. The best-described Typex models are the Mark II and the compact Marks III and VI. However, there remain gaps in the Typex ‘family tree’. This paper reviews the development history of Typex and describes several previously unknown models of Typex based on documents produced by the British Signals Intelligence agency, the Government Code and Cypher School, a major user of Typex while based at Bletchley Park during the Second World War. Although these models were not brought into widespread service, the documentation sheds useful light on the design process. The design of successive models of Typex, and their adoption or rejection, had less to do with cryptographic considerations than the various mechanical and practical problems involved in designing a reliable cipher machine compatible with the communications systems used by the British state and armed forces.