Read The Secrets of Station X Online
Authors: Michael Smith
Similarly, those in Hut 6 knew nothing of what went on
elsewhere
in the Park. âIt was a very curious organisation,' said Susan Wenham, one of the Hut 6 codebreakers.
We were very, very departmentalised. You never discussed your work with anyone except your little group that you worked with. I hadn't a clue what was going on in the rest of the Park and nobody else had a clue what we were doing, except the real high-ups. It was a curious world of its own.
At the outset of war, Bletchley Park was at the centre of a web of intercept sites around the country where wireless operators carefully logged all German radio messages before sending them to the codebreakers by teleprinter, or initially at least from some of the smaller stations by motorcycle courier. These âY Service' stations were operated by the Army, Navy and Air Force; the Foreign Office; the Post Office; and the Metropolitan Police.
The main Navy and Army intercept sites were as they had been for much of the inter-war period with the Navy sites
at Scarborough and Flowerdown and the Army site at Fort Bridgewoods, near Chatham. The RAF Y Service site had moved from Waddington to Cheadle in 1938. In the months leading up to the war, the Post Office had been building a number of other intercept sites for diplomatic traffic to allow the armed forces to concentrate on military and naval traffic and one of these, at Sandridge, near St Albans, was already in place working directly to Bletchley Park. Two more would shortly be opened in Scotland at Cupar and Brora.
âGC&CS had always tended to take too little interest in the radio by which they lived,' Cooper recalled. Similarly the three services had little understanding of the work of the codebreakers and believed that their intercept sites produced sufficient
intelligence
simply by analysing the activities of the radio networks they were monitoring, a process known as âlog-reading', or later âTraffic Analysis'. But without the service, Post Office and Metropolitan Police intercept operators, Bletchley would have had no Enigma traffic to work on in the first place.
Joan Nicholls talked her way into the female equivalent of the Army, the Auxiliary Territorial Service (ATS), at the age of fifteen by pretending she was two years older. She was posted to one of the intercept sites where she was taught to operate a radio set and how to transcribe German messages sent out in Morse code. âWhen there was a lot of excitement the wires would be absolutely humming with Morse,' she said.
They would be transmitting all over the place and we would really have cramp in our fingers sometimes trying to write it down non-stop, because you only had one chance to get it.
We had to get the preamble and the first three blocks absolutely accurate because that was the key to decoding the message. If we missed a letter we had to know exactly what position it was in and if the signal faded and we lost
information
, we had to know how many blocks we missed. Every one of our sets had access to three aerials. We had what looked like
a little switchboard in the set room and, if the reception wasn't good from one aerial, we could switch over to another. It could be quite exciting but sometimes we had a very quiet shift and not an awful lot happened. Nights were very boring, hour after hour of listening for the station to open. It was a little bit like a cat sitting outside a mousehole waiting patiently for the mouse to appear and that's how we were. We'd be sitting there listening, waiting for them to come to life. Then suddenly we'd hear the signal of them coming back, transmitting again and then all your adrenaline was running and it didn't matter how tired you were, how sleepy or bored you felt, the minute that station came alive again, you would be alive too, tearing pieces of paper off the pad and scribbling away like mad.
The intercept operators had several additional weapons in their armoury that would assist in the identification of stations and therefore in building up the intelligence picture. The first was direction-finding (DF), a technique in use since the First World War in which a directional aerial or an array of aerials was set up in such a way as to produce a bearing to the enemy transmitter and by using two or more âDF stations' against the same target, a number of different bearings could be plotted on a map using triangulation to determine the enemy location. The second was identification of the specific wireless transmitter in use, on the basis that every transmitter contained vagaries that could be identified from analysis of its pattern of transmission in a process known as radio finger-printing (RFP). The last was the operator's âhand', the way in which the German operator sent his Morse code messages, known as TINA.
âIt is like a voice,' said Joan Nicholls.
When a member of your family comes through the door and says something to you, you don't have to look to see who it is because you know the voice. You recognise the voice of your friends and family on the telephone because the minute they
speak you know it's them and it's the same with the operators. They each had their own particular sound, that lilt or staccato way of sending Morse dots and dashes. So if they changed frequency and we lost them, we would go looking for them and we would listen first of all for the sound of the transmitter and then we would tune in to that transmitter and listen for the operator and the minute we found him, that was him, there was no question of âWe think we have him'. As soon as we heard the sound of our man, the way he sent the letters, he was our man.
The Army site at Chatham had been intercepting most of the Enigma traffic, including both the
Red
and the
Green
cyphers. It was not clear when the first breaks occurred in early 1940 that the
Red
, rather than the
Green
, would become the most
important
of the keys. But what was clear was that the
Red
was not, as had always been assumed until it was actually broken, an Army cypher. When it first appeared, shortly after the formation of the
Wehrmacht
in March 1935, it was clearly passing traffic between a number of ground stations. The British military intelligence section that controlled the Army Y station at Chatham, MI8, ruled that since the messages were between land-based stations they must be military rather than air force and therefore the responsibility of Army intercept operators rather than RAF operators. But once the codebreakers managed to crack it they soon discovered it was being used to encypher communications between various
Luftwaffe
headquarters.
Since the end of 1938, the
Red
Enigma traffic had been the Army's main priority. It came as something of a shock to MI8 to discover that for more than a year it had been
funding
an expensive operation which should have been carried out by the RAF. But trying to persuade Air Ministry that it should be intercepted by the RAF site at Cheadle rather than by Army operators at Chatham was a difficult task and Group Captain Lyster Blandy, who was in charge of the RAF intercept
operation, but unaware of the potential importance of the Enigma traffic, refused to allow his men to take encyphered messages, Cooper recalled. âEven after Enigma had begun to be read, early in 1940, when I suggested to Blandy that Air Ministry ought to take a hand in intercepting it, he replied: “My Y Service exists to produce intelligence, not to provide stuff for people at Bletchley to fool about with”.' The Army was equally adamant that its men should not be tied up with intercepting
Luftwaffe
traffic and the row continued for several months at the beginning of 1940.
The RAF eventually decided to allocate twenty sets to the
Red
traffic but this was not enough to provide sufficient messages and the operators at Cheadle did not have the same appreciation of the level of accuracy that was required in taking down the five-letter Enigma traffic. As a result, Chatham had to continue taking it as well.
A section which could liaise with Chatham, Cheadle and the other intercept sites taking German Army or
Luftwaffe
messages was set up in Hut 6 with Travis ringing round the top London banks and begging them to loan him their brightest young men to coordinate between the academics who broke the cyphers and the service Y stations. This liaison section, known as Bletchley Park Control, was to be manned twenty-four hours a day and to stay in constant touch with the intercept sites to ensure that their coverage of radio frequencies and networks was coordinated and that as little as possible was missed. Where an important station was difficult to hear, it was to be âdouble-banked', taken by two different stations so that the chances of picking up a false letter that might throw a spanner into the works were cut down and that the material that came to Hut 6 could be worked on with some degree of confidence by the Hut 6 codebreakers.
âWe were told what we would cover and that came from Station X, the intercept control there would tell us what to cover that day with what priority,' Joan Nicholls recalled.
They would tell us if they wanted them double-banked, two
people to take them, or if one good quality operator would be sufficient. We didn't know that Station X was Bletchley Park. We never knew where it was. You were only told what you needed to know and we just needed to know that Station X was controlling what we actually monitored.
Many of the messages themselves arrived from the outstations by motorcycle courier. But Traffic Registers giving the
preambles
and first six groups of the messages intercepted by the outstations were sent by teleprinter to the Hut 6 Registration Room. Here a number of female graduates recruited by
Milner-Barry
from Newnham College, where his sister had been
vice-principal
, tried to establish the specific Enigma cypher in use from the preambles, carefully examining them to see if there was any intelligence that could be garnered before the
codebreakers
got to work. A description of each message,
containing
the frequency and callsigns; the number; whether or not it was urgent; and the first two groups, was carefully logged on so-called B-Lists. These became known colloquially as Blists and the female graduates were dubbed the âBlisters'.
At this stage, the Bombes had not yet been built and Enigma was being broken purely by hand, a difficult, if not virtually impossible, task with a machine cypher. The first step in
breaking
any cypher is to try to find features which correspond to the original plain text. Whereas codes substitute groups of letters or figures for words, phrases or even complete concepts, cyphers replace every individual letter of every word with another letter. They therefore tend to reflect the characteristics of the language of the original text. This makes them vulnerable to studies of letter frequency; for example the most common letters in English are E, T, A, O and N. If a reasonable amount, or âdepth', of English text encyphered in the same simple cypher were studied for âletter frequency', the letter that came up most often would represent E. The second most common letter would be T and so on. By working this out and filling in the letters, some will form
obvious words with letters missing, allowing the codebreaker to fill in the gaps and recover those letters as well.
Another basic weapon used by the codebreaker, âcontact
analysis
', takes this principle a step further. Some letters will appear frequently alongside each other. The most obvious example in the English language is TH as in âthe' or âthis'. So by combining these two weapons, the codebreaker could make a reasonable guess that where a single letter appeared repeatedly after the T which had already been recovered from letter frequency, the unknown letter was probably H, particularly if the next letter had already been recovered as E. In that case, he might conclude that the letter after the E was probably the start of a new word and so the process of building up the message would go on.
Machine cyphers like Enigma were developed to try to protect against these tell-tale frequencies and letter pairings, which is why the wheels of the Enigma machine were designed to move around one step after a number of key strokes. By doing this, the Germans hoped to ensure that no original letter was ever represented by the same encyphered letter often enough to allow the codebreakers to build up sufficient depth to break the keys. But it still left open a few chinks of light that would permit the British codebreakers to attack it. They made the assumption, correct far more often than not, that in the part of the message being studied the right-hand wheel would not have had the opportunity to move the middle wheel on a notch. This reduced the odds to a more manageable proportion. They were shortened still further by the Enigma machine's great drawback. No letter could ever be represented by itself. This was of great assistance in using cribs, pieces of plain text that were thought likely to appear in an Enigma message. This might be because it was in a common proforma, or because there was an obvious word or phrase it was expected to contain. Sometimes it was even possible to predict that a message passed at a lower level, on a system that had already been broken, would be repeated on a radio link using the Enigma cypher. If the two identical
messages could be matched up, in what was known as a âkiss', it would provide an easy method of breaking the keys.
The Germans, with their liking for order, were particularly prone to providing the British with potential cribs. The same words were frequently used at the start of the message to give the address of the recipient, a popular opening being
An die Gruppe
(To the group). Later in the war, there were a number of lazy operators in underemployed backwaters whose situation reports regularly read simply:
Keine besondere Ereignisse
, or ânothing to report', said Peter Twinn, one of the leading Bletchley codebreakers.
âYou can guess sometimes how messages are started even though you haven't seen the German text,' Twinn said.