Home Up April to May RAF ULTRA Field Hospital


ULTRA was the codename for the information that came from the code-breakers at Bletchley Park.  This information was so secret that it was many years after the war before any of those who had been involved spoke of their part in it.
It is not my intention to try to write a comprehensive account of Ultra, or such essential elements as the Enigma machine or Bletchley Park, Station X, where the teams worked.  I will give an overview of them, as they were important for events on Crete.  Anyone wishing to know more might try the suggested reading, below.  
As mentioned at the top of this page, ULTRA was the codeword for the information which came from Bletchley Park (BP).  The information was relied on  very heavily by Churchill because of its source and its proven accuracy.  The information was coded by the Germans using their Enigma machines, the signals were then intercepted, fed to Bletchley Park, and then decoded using the specialised teams and equipment at BP, also known as Station X.  

I will give a little of the history of the Enigma machine, an insight (not in depth) into how it worked, and a little on how BP was set up and operated.  The information obtained was, by its very nature, 'aged' by the time that it was translated into English 'clear text'.  It's value lay in the way it indicated plans and intentions, rather than immediate current events.  This was particularly the case with information relevant to the Balkans, Greece and the invasion of Crete.

Ultra goes to war

This book deals with the use of Ultra during the war.

Seizing the Enigma

This one is primarily about the capture of the Enigma machine from a U-boat.

At the heart of the German secret communications was the Enigma machine.  Looking a little like a cumbersome type-writer, with small lights shining through letters on the top of the machine, this was the means of enciphering the messages.  The Germans believed that the output of the machine could not be broken by enemy code breakers, and this belief together with some complacency, was their undoing.  They did not believe that their messages had been broken, and many operators were lazy in their procedures.  Given the difficulty of decoding an Enigma message, it is as well for BP that they had these little strokes of good luck.
The Enigma machine in its original form had been around for some years, and a patent had been filed with the British Patent Office in 1927.  Nobody took much notice.  The Germans started to adopt it throughout their armed forces; the Navy in 1926, the Army in 1928 and the Air Force in 1933.  Luckily for the Allies the Poles had taken a very real interest.  They, historically, had needed to take notice of what happened in Germany, and wanted to be able to intercept and read German communications traffic.  Their experience with Enigma, and their skill in code-breaking would be invaluable to the British when they finally woke up to the same need.  The Poles had been successfully deciphering traffic from three rotor machines (more on this below), up to as much as 75% by 1938.  However, this stopped when the Germans introduced four and five rotor machines.  By July/August of 1939 the Poles had opened up their knowledge to the British, giving them an Enigma machine and drawings of their Bomba (an electro-mechanical device for assisting with decryption).
The communications that we are talking about are primarily radio messages between different German units, it was these that the British listening stations could tune into and take down. The German army tended to use land lines for a large part of their communications traffic, but the Navy and the Air Force were heavily reliant on radio communications.  The British listening posts could not pick up the land line transmissions, and the Navy communications were very secure, it was mid 1941 before BP were able to break the Naval Enigma traffic.  I hope to cover the capture of the naval Enigma machine in a later update. For these reasons, and because the Luftwaffe were the most lax in their signal discipline, much of the early traffic for BP was related to Luftwaffe activity.  This was to be of particular significance for operations on Crete.

A 3 rotor Enigma machine

The photo on the right shows a 3 rotor Army Enigma machine.  Working from the bottom of the photo; the front flap is open showing a 'plug board' with some 18 'jacks' in use.  Above that is a keyboard with 3 rows of keys.  Directly above these are 26 light bulbs which will shine through little windows in the open lid of the box.  These will light up a letter on the lid.  Above the bulbs are the rotors, here there are 3 on a barrel.  The rotors have a sleeve alongside them, on the left of the wheel, which has letters all the way round.  In the lid of the machine there are 3 slits that allow the rotors to stand 'proud' through the lid, and on the left of each slit is a small window allowing the operator to see one of the letters on the sleeve alongside each rotor.  That, in essence, is the Enigma encoding machine.  But its internals, and the way it is setup each day are the complicating factors for the people in Bletchley Park
The photo on the left shows two spare rotors in their box, the circle of studs are electrical contacts.  Photo taken in IWM.  In simple terms the machine works by translating any one key depression on the keyboard into a letter which is lit up on the lid of the machine by one of the 26 light bulbs.  If that's all there was to it there would have been no great problem for BP.  If the letter 'A' were pressed, it might produce a 'G' on the lid, but if the same key were pressed again, it would not produce a 'G' this time, but some other letter.  Each time a key is pressed the rotors move round 1/26 of a revolution, so going on by the equivalent of one letter in the alphabet.

The photo above was taken in the Imperial War Museum by myself.

When a key is pressed an electrical signal is sent off on a complicated route from the key, through contacts on the three rotors to a 'reflector' on the end of the rotor barrel, it is then sent back through the rotors, but via a different route and then to the light bulb.  The rotors themselves can be put in any of the three rotor positions, and to make matters a little more complicated, they can also start the day (the start of day setting) with any one of the 26 letters on the rotor sleeve showing through its little window in the lid.  At the start of the day then, any units on a particular communications 'net' would need to know the starting setup for the day, without that a message from one unit could not be decoded at the other end, they both needed to start from the same base line.  So, at start of day the operator would need to know which rotor went in which of the three positions, as well as which letter should show through the lid against each rotor.  These initial settings are obviously a vital element for BP, the first thing they need to do each day is to try to decipher the 'daily setting' if they are to decode any of the transmissions.
On the machine above there are 18 'jacks' on the front of the machine.  A further complication.  As well as the signal from the key depression going the route described above, it also goes via these 'jacks', and because these are plugable they can be plugged in a variety of combinations, making life at BP very difficult!  It is estimated that a 3 rotor machine gave some 3 x 1018  combinations, this obviously went up enormously as fourth and fifth rotors were added later.  It was this mind boggling number of combinations that caused many operators to become lazy and complacent in their radio discipline, and would allow BP to crack the code.  As an example of the problem, the Poles, trying to crack the code of a 5 rotor machine, established the key settings on the 17th Jan, 1940 for messages on the previous 23rd October, 1939. The following is from 'Ultra goes to war'.

"The Germans made many mistakes, and though the breakthrough was the result of immense effort in several fields it was one particular error that led directly to the conquest of Enigma.  When the operator had set up his machine according to the instructions for the day, and was about to encipher a signal, he would begin by tapping at random a small group of letters.  The machine gave him an encipherment of this group, which he now incorporated at the start of the signal.  A recipient of the message would then know, from these few letters, how to set the rotors of his own machine for deciphering that particular text.  It was, one might say, a key built into the message itself.  In their meticulous way, however, the Germans repeated the group at the beginning of each message.  To Hut 6, (in BP) once the significance of the letters was realised, this duplication offered great possibilities."

The enormous number of combinations on the Enigma ed to the development of electro-mechanical devices for trying to discover the key settings, these were known as bombes, and were a development on the Polish Bomba, which they had also made the British aware of in 1939.  Before the bombe however, there were a large number of people recruited into Bletchley Park from Oxford and Cambridge, many were mathematicians or chess players.  All would work long hours trying to break the codes.  There were some thousands of people who worked there, many not knowing what others were doing, and not asking, most working in large huts.  Two of the main huts were Hut 8 and Hut 6.  Army and Air Force signal traffic went to Hut 6,  Navy traffic to Hut 8.  After deciphering the messages would go to Hut 3 for translation and analysis, and re-writing so as to hide the fact that the information had come from an Enigma decrypt.
Churchill came to rely very heavily on his information from Ultra, one reason being that it had been shown many times to be accurate, it gave him prior warning.  The time taken to break the codes meant that it was not useful for information on daily movements, or things that were out of date by the time the message was decoded.  Its value lay in the decipherment of the enemy's intentions and strategies, so allowing the Allies to be informed ahead of time.  An early example was that of Knickebein, or 'crooked leg'.  Ultra allowed the British to discover that the Germans had developed a system of radio beams to allow their bomber crews to bomb British targets accurately at night.  By sending beams with 'dots' and 'dashes' the crews would fly these beams, and the dots or dashes would indicate if they were to the right or left of the intended track.  A constant sound would indicate that the crew were on track in the centre of the beam.  A second beam, from another transmitting station would send out an intersecting beam, the intersection of the two beams would be the point for bomb release.  By decoding the signals associated with this bombing aid, discovering the stations and the directions of the beams and planned raid timings the British were able to fly out themselves and prove the existence of the beams.  This showed the accuracy of the Ultra information to Churchill and his senior staff.  They knew that they could rely on it, but they also knew they had to protect it.  There have been stories that the need to protect Ultra was such that no action was taken when Coventry was bombed, to protect the source, or that Freyberg did not take sufficient action on Crete to protect the source of his information prior to the invasion.  These cases do not stand up to scrutiny.
On the 25th April Hitler issued his Hitler Directive 28, for the invasion of Crete.  By the 1st of May General Freyberg was given a detailed picture of General Student's intentions and capabilities for that invasion.  Even with that information, denying the Germans the element of surprise, Crete would fall.  Ultra could not make up for the inadequacies of the island's defences, or the interpretation that Freyberg would put on some of that information.  He convinced himself that the main threat was the seaborne invasion, not the airborne one.  Because of this he allowed the Germans to take Maleme airfield, and so Crete.
The following is the text of an Ultra transcript sent to Freyberg, 13 May, 1941.  It outlines in some detail the German intentions, but as with other Ultra output, it includes analysis by Hut 3.  This signal OL (Orange Leonard) 2/302, led Freyberg to believe that the 22nd Airlanding Division would be coming, as well as 7th Parachute Division.  It also convinced him that the seaborne invasion was the bigger threat to the island.
The following summarises intentions against Crete from operation orders issued.  
Para 1. The island of Crete will be captured by the 11th Air Corps and the 7th Air Division and the operation will be under the control of the 11th Air Corps.
Para 2. All preparations, including the assembly of transport aircraft, fighter aircraft, and dive bombing aircraft, as well as of troops to be carried both by air and sea transport, will be completed on 17th May.
Para 3. Transport of seaborne troops will be in cooperation with admiral south-east, who will ensure the protection of German and Italian transport vessels (about twelve ships) by Italian light naval forces.  These troops will come under the orders of the 11th Air Corps immediately on their landing in Crete.
Para 4. A sharp attack by bomber and heavy fighter units to deal with the allied air forces on the ground as well as with their anti-aircraft defences and military camps, will precede the operation.
Para 5. The following operations will be carried out as from day one.  The 7th Air Division will make a parachute landing and seize Maleme, Candia and Retimo.  Secondly.  Dive bombers and fighters (about 100 aircraft of each type) will move by air to Maleme and Candia.  Thirdly.  Air landing of 11th Air Corps, including corps headquarters and elements of the Army placed under its command probably including the 22nd Division.  Fourthly.  Arrival of the seaborne contingent consisting of anti-aircraft batteries as well as of more troops and supplies.
Para 6. In addition the 12th Army will allot three Mountain Regiments as instructed.  Further elements consisting of motor-cyclists, armoured units, anti-tank units, anti-aircraft units will also be allotted.
Para 7. Depending on the intelligence which is now awaited, also as the result of air reconnaissance, the aerodrome at Kastelli (Pediados) south east of Candia and the district west and south west of Canea will be specially dealt with, in which case separate instructions will be included in detailed operation orders.
Para 8. Transport aircraft, of which a sufficient number - about 600 - will be allotted for this operation, will be assembled on aerodromes in the Athens area.  The first sortie will probably carry parachute troops only.  Further sorties will be concerned with the transport of the air landing contingent, equipment and supplies, and will probably include aircraft towing gliders.
Para 9. With a view to providing fighter protection for the operations, the possibility of establishing a fighter base on Skarpanto will be examined.
Para 10. The Quartermaster General's branch will ensure that adequate fuel supplies for the whole operation are available in the Athens area in good time, and an Italian tanker will be arriving at the Piraeus before May 17th.  This tanker will probably also be available to transport fuel supplies to Crete.  In assembling supplies and equipment for invading force it will be borne in mind that it will consist of some 30 to 35,000 men, of which some 12,000 will be the parachute landing contingent, and 10,000 will be transported by sea.  The strength of the long range bomber and heavy fighter force which will prepare the invasion by attacking before day one will be of approximately 150 long range bombers and 100 heavy fighters.
Para 11. Orders have been issued that Suda Bay is not to be mined, nor will Cretan aerodromes be destroyed, so as not to interfere with the operations intended.
Para 12. Plottings prepared from air photographs of Crete on one over ten thousand scale will be issued to units participating in this operation.
In his thesis "The Fall of Crete 1941: Was Freyburg (sic) culpable?" Major Bliss makes a number of good points on Freyberg's preparation for the attack and his use of the intelligence provided by Ultra.  Bliss makes the point that Freyberg could not ignore the possibility of the seaborne invasion when preparing his plans, but he was over cautious in his use of Ultra.  To protect the source, and Churchill was adamant on this, Freyberg felt that he could not over defend Maleme lest it should give the impression that he was basing his decision on intelligence.  However, protecting the airfields would have been natural since their loss would be such an obvious disaster.  Also, failing to defend the area west of the Tavronitis would have serious consequences.  As Bliss points out 'hindsight is cruel to Freyberg' because it gives historians the opportunity to criticise his dispositions and his use of the intelligence.  Bliss points out on a couple of occasions that Student did not know himself at the outset of the invasion where the main point of the attack would be.  We now know that it was maleme, but as Bliss points out, Heraklion had the better developed runway for an aerial invasion, that on Maleme was temporary and was still being developed by the British, but it was closer to Suda Bay which would be important in a seaborne invasion.  Without going into all the detail in Bliss's account, it is worth reading by anyone seriously interested in the events of May 1941.  It is not in stock at Amazon but available at www.mlrsbooks.co.uk 
Ralph Bennett, from his obituary in the Daily Telegraph, August 2002.  A Cambridge historian he spent four years with the Bletchley Park codebreaking operation.
"In 1941, Bennett was put to work in Hut 3, the intelligence section, producing reports based on decrypts of German Army and Air Force messages which used the Enigma cipher.  These helped to turn the tide for the Allies in North Africa, and provided invaluable intelligence during the allied advances through Italy and France.
The fastidious young Cambridge don was not overly impressed by the ramshackle wooden building, with its nauseating fumes from leaky coke-burning stoves during the blackout.  Messages, he rcalled, arrived from Hut 6 next door in a cardboard box which was pushed through a small tunnel with a broom handle "either in a trickle or a flood", depending on how well the codebreakers were doing.  These were sorted into different degrees of urgency, then handed out to officers sitting around a horseshoe-shaped table.
Their first task was to fill in any gaps left because of radio interference or garbled letters.  Then, to avoid any hint that the British had broken the Enigma cipher, Bennett and his fellow intelligence reporters rewrote the messages for distribution by MI6.  They removed anything which might suggest that a deciphered radio signal had not come from Source Boniface, a supposed British spy with a network of agents inside Germany."