Secret Life of Bletchley Park - McKay Sinclair [26]
Indeed, Welchman wasted no time at all during his exile. Mathematician John Herivel recalls one of the key innovations. Welchman was quick to form a strong working relationship with the most secret listening station – ‘an old fort’ at Chatham, Kent, on the Thames Estuary. It was through this establishment that, at that time, much of the German signalling traffic came.
Once the signals were picked up, the officers at Chatham would have bundles of intercepted coded messages sent to Welchman. These bundles would be driven from the coast to Bletchley, through the middle of the night and at harum-scarum speeds, often in the most atrocious weather conditions, by special motorcycle couriers.
The German rule was that no message should be more than 250 letters in length; if it was necessary to send a longer message, it should be split into multiple parts. This was designed to make life more difficult for codebreakers: the longer the message, the easier it might be for such a person to see patterns of letters forming among the apparent chaos.
But each Enigma message had a preamble and some operators used different discriminants (that is, groups of characters indicating the code set-up and reciphering key, and distinguishing the section of traffic) for the different parts. As Herivel noted, it was thus possible to work out any ‘given key’. ‘In this way,’ he wrote, ‘all the different German keys coming from Chatham – and later from France – could be identified, and every day, traffic could be divided into different bundles for different keys.’
The keys were then given different colours, to separate them out in the most direct visual way. To start with, the colours used were yellow, green, red and blue, respectively signifying the German Norway campaign, the army and air force codes, and separate air force training codes. The time and the frequency of each message would be noted down on vast sheets of paper, leading to an immediate strain on the Park’s supply of coloured pencils. But the colour keys were a stroke of organisational genius – a vivid signifier that allowed everyone to identify each key with ease. Indeed, Welchman recalled in his memoir that one element of recruitment to Bletchley Park involved asking candidates ‘if they were colour-blind’.
In our world of ubiquitous touch-screen technology, such a system smells of pencil shavings and glue and bits of string. But it worked. The avalanche of thousands upon thousands of intercepts – the numbers of which would multiply dramatically as the Phoney War ended and the conflict intensified – had order imposed upon it. It could be deduced from which part of the German military they were being sent, and to which.
It soon became necessary to develop subdivisions for the keys (such as SS messages, and messages to do with German railways); before long every colour in the rainbow was deployed. When all available colours had been used, keys were named after marine life: birds; then elephants; and insects …
There were many occasions when a message could not be broken in its entirety, because words were missing or incomplete. As a result, a comment would be added with the words ‘strong indications’, ‘fair indications’ or ‘slight indications’ to convey the varying degrees of the decrypt’s reliability. But it was through the work of Chatham – and later other intercept stations such as Denmark Hill in south London – that Welchman began to understand about the different call-signs used by different operators, and what signs would be used for what sorts of messages.
Dilly Knox was also using the period of the Phoney War to get himself and his team ready for the onslaught of work that was to come. The difference between Knox and Welchman was Knox’s apparent taste for pulchritude on his team, although now Mavis Batey laughs off – with a very faint edge of annoyance – any idea that she was recruited chiefly for reasons of glamour: ‘Dilly was very firm. He said he did not want any debutantes who had