The Bletchley Park Codebreakers (21 page)

I will return to the history later. First, there is much more to be said about the circumstances and the methods of the cryptanalysis.

The enciphered messages came to us from two sources. Motorcycle despatch riders, who are an abiding memory for anyone who worked at the Park, brought bags of it two or three times daily from Flowerdown, who also teleprinted it. These were current messages, but constituted only some 15 per cent of the total available. Much bigger deliveries came by bag two or three times a week from Washington, taking us to some 85 per cent of the total available. But the Washington material was two or three weeks old when it reached us: a serious handicap we were never to overcome.

On arrival the traffic went first to the Processing Party which served all the Japanese Naval parties. This was led by Mrs E. Parsons and it moved in September 1943 from Hut 7 to occupy the lower floor of Block B. At its peak it had some 200 staff, three-quarters of them Wrens. The Processing Party registered all incoming messages, sorted them by type and distributed them to where they were needed. I return later to their other functions and ways in which we worked with them.

Our share of the incoming messages went first to the Big Room where they were individually registered with details of their date, time and call signs of origin and address. As the indicator system was known, new messages could be placed immediately at their correct starting points on the additive table. They were written on to ‘depth sheets' (also called ‘Tiltman sheets') prepared for the purpose. These were something like 50 inches wide, ruled vertically into 100 columns so that each sheet corresponded to all ten lines and ten columns of one page of the additive table, and each additive position was identified by its page, row and column numbers. The sheets were deep enough to provide for (I guess) some 30 messages to be written one below the other, with room for another couple of lines left blank for working below each message. As new messages were added, each from its own starting-point, they created ‘depth' where they overlapped: because all the enciphered groups in a column had been enciphered by the same additive. Depth was the life-blood of the additive recovery process. Providing on the sheets for up to 30 messages in depth was an aspiration and an act of faith. Most often we had to work on depths of as little as five: a depth of eight or ten was luxury. To fit the 5-digit groups within the columns called for a small neat hand, so those without that talent were politely excused that task.

The routine attack on a column, given a depth of some five or more, was through ‘differencing'. This is explained in detail in Appendix VI. To summarise, differencing down a column of enciphered groups would hopefully produce one or more speculative additives for it. Subtracting each of these in turn down the column – ‘stripping' it – gave speculative deciphered codegroups, to be tested first to see if they scanned and – if they did – for good groups. A sufficiency of good groups (I return later to what ‘sufficiency' meant) would confirm that speculative additive as probably correct. To make this practicable we relied heavily on the ‘Freebornery' to provide us with Indexes of Differences and of Good Groups. This was the massive Hollerith installation in Block C, serving the whole of Bletchley Park and named after its head ‘Mr Freeborn'.

Once a first break had been made on a stretch of additive, usually by differencing, and several more or less adjoining additives recovered, the cryptanalysts moved on to a subtler approach. This was based on study of the good groups, their behaviour and the company they kept: did they have favourite positions in the message, did they tend to recur, were they regularly associated with other good groups preceding or following them? For some of these the bookbuilders, whose job was to work out the textual meaning of each codegroup, would have worked out the meaning, for others not. For the cryptanalysts the meaning was useful but not essential. A good group, though only a five-digit number, could become a good friend.

For instance, the feature of a pair of codegroups regularly appearing together with a few others in between could be produced by meanings as different as ‘(….)' and ‘today… (weather)… tomorrow… (weather)'. Such a feature could be exploited irrespective of meaning: if one of the pair appeared, the other was searched for. Of course if some of the meanings were known, the search was better informed. When one of the codegroups for
Maru
(ship) appeared it would very probably be preceded by a ship-name, flanked by the ‘arrive' or ‘depart' codegroups and accompanied by the names of the ports identified by the message's callsigns. If any of these details were known, that was a ‘crib' worth trying out.

Each possibility produced by this kind of reasoning had then to be followed up by inserting the suggested new codegroup into its adjoining column, deriving the speculative additive it implied there by subtracting it from the corresponding enciphered group and testing
that speculative additive as before. This procedure, called ‘horizontal stripping' to distinguish it from differencing, was more art than arithmetic and was capable of limitless improvement in knowledge and technique. In the internal history of the work on JN-25 which I compiled in August 1945, I wrote:

The Big Room covered all the functions from the clerical routine writing of new messages on depth sheets and preparation of material to go to the Freebornery to the cryptanalysis itself. Its members did not divide into servicers and strippers by rank or experience, and everyone had to share the clerical work when the pressure required it, with Adelaide Dickens steering them when necessary to ensure a satisfactory spread of effort across all the tasks. Those with the aptitude and the opportunity for stripping had a degree of flexibility as to how they went about it. Racks held the depth sheets in order, offering a choice: whether to have first go at a depth newly enhanced by new messages or to persevere with a stubborn one, now dog-eared, which really ought to be made to yield. Copies of the Indexes of Good Groups and of Differences, and of the codebook as so far recovered, were all around for reference. And when a certain point was reached – which could be either success or the need to get help – the stripper would move around to deliver or to consult. It was essentially tedious work but performed with great spirit and good humour, encouraged always by the prospect that the next column might be the one that would come out sensationally right.

If the work of the Big Room was spread fairly thinly over very many depth sheets, that of the group called GEN was quite the opposite: concentrated attention to individual problems. It had begun with Gladys Gibson working on routine messages such as fuel returns or weather reports. Our break into J-30 had indeed come through a routine, not through differencing. Later she led GEN comprising
Henry Reed, Commander Belfield and Olive Thorogood to provide in-team intelligence, starting with little more than the Rose Innes and Kenkyusha Japanese dictionaries and a map. Their job was to develop knowledge and techniques, contexts and cribs, to find a way round when the frontal attack on a column had got stuck. A Big Room stripper with a possible but still dodgy speculative additive which had thrown up some familiar good groups could walk across the corridor and ask GEN for suggestions for extending the text to right or to left. Each new codegroup that GEN suggested was taken back to the depth sheet and used to produce more speculative additives to be tested as before. If one of them proved good, that was both a new recovered additive and confirmation of the dodgy one that had led to it.

Assembling and organising the rich mixture of crib and context information and devising means of getting into it quickly called for a certain ingenuity, and it was skilfully done. Of course formal cataloguing and indexing were only part of it; as with skilled stripping, acute observation, a good memory, lateral thinking and imaginative hunches founded in experience often found the way in. This small team contributed substantially to the overall success and patently enjoyed doing it.

At one stage when we were struggling to keep up with the increasing quantity of incoming traffic Washington sent us a dozen or so calculating machines made by the National Cash Register Company. We called them the ‘fruit machines'. They were big, perhaps four feet tall, and floor-standing; mechanical, not electrical. They were set up to do non-carrying addition and subtraction of five-digit numbers. For example, all the enciphered groups of a column of depth could be entered and then a speculative additive placed at their head: one pull of the handle stripped the additive from all the groups simultaneously to reveal all the speculative codegroups, which could be tested immediately for scanning. The enciphered groups stayed on the machine until all the speculative additives for that column had been tried. At first and at their best the machines were a great help, faster than manual stripping and easier on the eyes. The trouble was that they proved prone to mechanical breakdown and, as there seemed to be few technicians around capable of mending them, they were out of action for long periods. Confidence in them waned and eventually we gave them up altogether.

In discussing differencing above I mentioned ‘a sufficiency of good groups' but slid by the question that this begs. It was not just the
number of good groups that carried conviction: some were patently better evidence than others. How did a speculative additive producing one very frequently-used codegroup rate against another which produced a string of middling ones? As more and more Wrens joined the Big Room and there were big differences in their experience, there was an evident need to simplify the answering of that question, to standardise the judgements that had to be made right across the team and to enable new recruits to be initiated more quickly.

The answer that the mathematicians, with Ian Cassels in the lead, came up with applied Bayes' Theorem to combine the weights of the evidence that each speculative codegroup provided, then used logarithms to substitute addition for multiplication and finally rounded the logarithms to two-digit scores which could be added mentally or by pencil jotting. This is explained more fully in Appendix VII.

The system performed very successfully its intended purpose of quickly and systematically testing the masses of speculative additives that differencing produced, and picking out those that were probably true. It was never seen as doing the whole job. It could not help with horizontal stripping. Borderline cases, or messages of particular concern, could be handed over for more intensive study elsewhere in the party. And, as always, the imaginative hunch grounded in experience could often find significance where the arithmetic alone failed to.

Our party's output of recovered additives went daily to the Processing Party. For them it was but a supplement to the much greater number of additives they received from Washington. Their central task was to apply all the known additives to all the messages enciphered by them, deciphering them to their codegroups, and to write in the codegroup meanings where known to reproduce the original message texts. They sent this deciphered material in two directions: by teleprinter to Bletchley Park's outstations at Colombo and elsewhere, and to our book-builders and translators in Block B. They fulfilled these functions for other Japanese systems, and after September 1943 for all of them, not just JN-25.

The relationship between our cryptanalytic party and the Processing Party was essentially a logistical one, keeping the paper flowing smoothly. They did not concern themselves with the cryptanalysis, nor we with their routine deciphering, decoding and teleprinting. We supported each other well, however, in staffing, each
recognising that we shared a chronic understaffing problem. There were occasions when we lent strippers (whom we could ill spare) to help the Processing Party to keep abreast of their essential work when the pressure was extreme, and other occasions when they could spare time to help us. Mrs Parsons was happy to recommend some of her particularly able Wrens (whom she had no means of promoting) for a trial run on stripping with us; and they always made good.

On the other hand, our relationship with the bookbuilding team, led by Michael Loewe, was close. There was to-and-fro consultation at all levels without inhibition or formality, depending only on the problem that had to be discussed: it could be Michael Loewe studying with the newest Wren the depth she was working on, to see what insight they could together find. By contrast we compared notes with no-one outside our JN-25 work area, not even with those on JN-11 who were our close neighbours both in task and on the corridor. Commentators now refer with a certain awe to Bletchley Park's success in keeping secrets: it seemed only natural to us at the time.

It is clear in Appendix VI how heavily we relied on the services of the ‘Freebornery', particularly in providing successive editions of our Indexes of Good Groups and of Differences. Almost nothing has been published about Block C, but an account by Mr Freeborn's Deputy, Ronald Whelan, has been released. It contains some vivid recollections (for example, about Winston Churchill's visit) as well as technical information about the machines; and it strengthens the view that the Freebornery's contribution to Bletchley Park's overall success is still inadequately recognised.

Referring to an earlier era, when the Freebornery was in Hut 7, Gordon Welchman commented: