CHAPTER I THE MACHINE AND THE TRAFFIC
If the history of Hut 8 is to be understood, it is essential to understand roughly how the machine works and thus obtain some idea of the problem which had to be tackled. Developments which have taken place during the war have complicated the problem but have left the machine fundamentally the same.
The process of cyphering is simple and quick. The message is "typed" on a normal keyboard and as each letter is pressed, another letter is illuminated on a lampboard containing the 26 letters of the alphabet. The series of letters illuminated on the lampboard form the cypher text and the recipient of the cypher message, in possession of an identical machine, types out the cypher text and the decoded message appears on the lamp board.
The main scrambler unit consists of 3 (later 4) wheels and an Umkehrwalze which I shall refer to henceforth as the Reflector - an admirable American translation. These wheels have on each side 26 contacts which we will for convenience label A to Z and the contacts on the one side are wired in an arbitrary and haphazard fashion to the contacts on the other. Each wheel is of course, wired differently. The reflector has 26 contacts which are wired together arbitrarily in pairs. What happens when one of the letters of the keyboard is pressed may be seen from the following diagram.
The current in this example enters the right hand wheel at A and leaves it at M, A being wired to M in this wheel: it enters the middle wheel at M and leaves it at Q and so on until it reaches the reflector where it turns around and returns through the wheels in a similar fashion, eventually leaving the right hand wheel at position N and lighting the appropriate lamp in the lampboard. Pressing a key may light up any bulb except that which is the same as the key pressed for a letter to light up itself it would be necessary for the current to return through the wheels by the same route as it entered and, from the nature of the reflector, this is clearly impossible. This inability of the machine to encypher a letter as itself is a vital factor in the breaking of Enigma.