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Books by John Barratt

Armada 1588

The Battle of Marston Moor

The Civil War in South-West England 1642-1646

The First Battle of Newbury: 1643

Cavalier Generals

The Great Siege of Chester

The Battle for York: Marston Moor 1644

Cavalier Generals

Enigma and Ultra - the Cypher War
Enigma and Ultra - the Cypher War
by John Barratt

It would not be until some thirty years after the end of World War II and the Battle of the Atlantic that details of the vital role played by Allied code-breakers began to be revealed.

Whilst many German military and naval communications were transmitted by normal media such as wireless and telephone, many of the messages of most vital importance were exchanged in code by means of the so-called "Enigma " machines.

The German Enigma machine was initially developed after World War I as a commercial encrypting device, but the military were quick to recognise its value, and developed it for their own uses. In basic terms, the Enigma resembled a typewriter, which scrambled the text typed into it by means of notched wheels or rotors. The messages could be unscrambled by a similar machine with its rotors adjusted to the same settings as the sender. German cypher experts refined the basic machine by adding plugs with variable electronic circuits, whose settings operators changed approximately every 24 hours, according to code books, listing the daily variations, with which they were issued. 

With millions of possible code variations, the German High Command remained convinced until the end of the war that Enigma was unbreakable, and indeed, with the limited technology then available to Allied code-breakers, this confidence might have have been well-placed, had it not been for a series of mistakes committed by the Germans themselves.

The first "leak" came in 1931, when a German Defence Ministry official, Hans Thilo Schmidt, sold some manuals to French Intelligence. Neither Britain nor France recognised the significance of the material which Schmidt continued to sell them, and such progress as there was in the inter-war years in breaking Enigma was largely the work of the Polish Intelligence services, which had obtained an Enigma machine in 1929. They developed a type of primitive computer, known as a "Bomby", which had some limited success in deciphering Enigma , although German refinements, such as the addition of extra rotors, prevented any major breakthrough.

In July 1939, as war approached, the Poles revealed their successes to British and French intelligence, and gave them replica Enigma machines.

Britain and France largely worked independently in attempting to pierce the secret of Enigma, and with France's defeat, the main burden fell on Britain.

The Battle of the Cyphers

During World War I Britain's code-breakers had known as ID 25 or more popularly, "Room 40". In 1920 they became part of the Secret Intelligence Service, and a few days before the outbreak of World War II changed their title from the Code and Cypher School to Government Communications Head Quarters. They were based at a large country house, Bletchley Park, whose extensive grounds provided space for the vast collection of huts erected to house a workforce which would eventually number several thousand.

A concerted drive was made to enlist the services of leading mathematicians from British universities, and, thanks largely to the pre-war work of the Poles, the Enigma codes used by the Luftwaffe were fairly quickly and comprehensively broken, as were some of those employed by the Wehrmacht. Those used by the Kriegsmarine, however, proved a much tougher proposition.

Thanks to the refined "bombes" developed by the brilliant mathematician Alan Turing, the first complete "Enigma" message was deciphered in January 1940, and by April some messages were being read within 24 hours of despatch. The intelligence data provided by these and other means were given the codename "Ultra". The first significant breakthrough in reading the Naval Enigma came in February 1940, when, after U-33 was sunk off the Scottish coast, three rotor wheels found in the possession of survivors gave Bletchley a partial insight into the Naval Code. More captured enemy material was needed to progress any further, and this was provided in April 1940, when some Enigma documents were found on board the German armed trawler, Polaris , taken off Norway. This enabled Bletchley to make its first, brief, reading of the Naval code, and provided Turing with material to work towards a more comprehensive breakthrough. Unfortunately, by the time that he made any real progress, new codes had been introduced, rendering messages once more unreadable.

The introduction by Donitz, from late 1940 onwards, of "wolf-pack" tactics, gave the Enigma codes still greater importance. In order to rendezvous U-boats had to signal their positions to Donitz's operations room. If these messages could be deciphered, it would be possible to divert convoys away from known ambushes. But it was becoming clear that little progress could be made without further captures of enemy material, itself a hazardous procedure, for if the Germans became aware that Enigma material had fallen into enemy hands, their whole cypher system might be changed.

On March 4th 1941, during a Commando raid on the Lofoten Islands off Norway, the Royal Navy captured the German trawler Krebs, along with two Enigma machines and the current settings for use in home waters. This allowed another partial breakthrough, allowing some messages to be read. Donitz, whilst concerned by increased British naval successes, was assured by his cypher experts that Enigma was unbreakable, and tended to suspect that the problem was due to increasingly effective tracking by means of HF/DF signals.

It was in the spring of 1941 that Britain made an important breakthrough in the battle for Enigma. Harry Hinsley, one of the Bletchley codebreakers, realised that the network of German weather and supply ships currently operating in the Atlantic, would carry code information. The problem lay in capturing some of these without betraying to the enemy exactly what was going on.On May 7th, in a highly secret operation, Royal Navy ships intercepted and captured the weather ship Munchen, seizing the code books to be used in June. Two days later, in one of the most dramatic episodes of the war at sea, depth charges fired by British destroyers forced to the surface U-110, whose commander, Lemp, had sunk the liner Athenia on the opening day of the war. Believing his vessel to be sinking, Lemp failed to destroy either his Enigma machine or its codes. Whilst sailors opened up on the U-boat crew with rifles and machine guns to panic them, and prevent any returning below deck, HMS Bulldog closed in and boarded U-110. Both machine and codes were seized. Lemp was not among the survivors of the U-boat crew, and once again the extent of their success remained a carefully guarded British secret.

The capture of U-110 was not in fact as decisive as sometimes claimed, but it provided useful additional information which would eventually be of considerable help in the breaking of Enigma. More significant, in fact, was the capture next month of the German weather ship Lauenberg , with the keys for June and July. This would enable Bletchley to break the German Home Waters code virtually until the end of the war, normally within 50 hours of transmission. 

Once again the problem lay in how to make use of the information provided by the code breakers without arousing enemy suspicion. This almost happened during the comprehensive elimination of the German surface supply and weather ship network, which was intended to be a gradual process, but went rather too quickly when the Royal Navy stumbled across two enemy vessels by accident.

From the second half of 1941 onwards, information from Enigma was one of the key factors enabling the Royal Navy to divert convoys away from waiting wolf packs. Decoded messages went initially to the Royal Navy section at Bletchley Park, then, if relevant, were passed on to Submarine Tracking Room in the Admiralty and later to the HQ Western Approaches, in Liverpool. The German practice of changing their rotor settings every day or two meant that messages were often at least several days old when deciphered. Before sending them on, analysts would add notes on any significant content, such as the identities of persons mentioned. The gist of the information contained in the signals, carefully edited to conceal its source, was passed on to operational commanders, only a very few of the most senior of whom were let even partially into the secret of Enigma .

The Enigma material, known as Ultra , was, of course, combined with intelligence from a wide variety of other sources, including HF/DF and wireless intercepts and reconnaissance reports, into a body of information known collectively as "SIGINT".

The effect of the improved flow of intelligence information was apparent during the second half of 1941. Increasing numbers of convoys were being diverted away from waiting U-boats. In July, for example, not a single convoy was sighted by the Germans over a period of three weeks, and during July and August monthly sinkings went below 100,000 tons, the lowest for over a year.

Not all of this improvement could be put down to Ultra and SIGINT. Among other factors involved were the diversion of U-boats to the Mediterranean and Arctic, and increasingly effective Allied air patrols. It was also fortunate in the long term, if the Germans were to remain ignorant of Allied success in breaking Enigma , that inability fully to understand a newly introduced code meant that not all merchant shipping could avoid U-boat ambush.

Even as it was, Donitz had recurring suspicions about the security of Enigma, as, for example, when the U-570 was captured, and it seemed likely that some codes might have been taken. However German naval analysts eventually decided that only one codebook had been captured, providing insufficient material with which to penetrate Enigma. His cypher experts assured Donitz that the Naval Enigma was "one of the most secure systems for enciphering messages in the world." Even so, the Royal Navy faced the constant dilemma of how much advantage to take of their knowledge without the risk of revealing their source to the enemy. Though every effort was made both to limit the circulation of information, and to disguise its origins, there were times that the breaking of Enigma came dangerously close to discovery.

Throughout the war there would be occasional breaks in the flow of information, when the Germans changed some of the cyphers, but these were usually solved either by the increasingly sophisticated "bombes", by the growing experience of the cryptologists, or by further captures of enemy material. Even so, such breaks could cause serious problems; one such temporary inability to decipher enemy signals played an important part in the heavy losses suffered by Arctic convoy PQ17.

A major crisis began on February 1st 1942, when a new rotor was added to the machines used on the Atlantic U-boat network. Known to the Germans as "Triton", and to the Allies, with sinister aptness, as "Shark", this additional refinement allowed 26 times as many different code combinations. The introduction of the new rotor coincided with greatly increased shipping losses due to the German "Happy Time" following the entry into the war of the United States. Though dire in other respects, the slaughter which the U-boats were making along the eastern seaboard of the USA at least prevented German Naval Command from linking their increased success to the refinement of Enigma .

The British Admiralty had always been reluctant to share Enigm a derived information with the USA, mainly, it appears, because of fears of security breaches. But in the current crisis, it was recognised that potentially much greater US resources, in for example, the construction of "bombes", would be invaluable. However no US "bombes" would be online before May 1943, and in the meantime the level of sinkings threatened to become unbearable.

Once again the situation was saved by captures from the enemy. In October 1942, the British destroyer HMS Petard commanded by the slightly crazy Commander Mark Thornton, depth-charged to the surface U-559, and, although two British seamen were lost when the U-boat sank, captured the latest code books. These provided invaluable aid in penetrating "Shark", aided as on previous occasions, by German carelessness and lapses in security. 

A further break in deciphering occurred in the spring of 1943, but in March, again in the nick of time, more captured codes enabled "Shark" to be broken again. The crisis of the Battle of the Atlantic came in May. By then, although reading Enigma messages remained important, the greatly increased numbers of sea and air convoy escorts, with better detection equipment, were playing an increasingly predominant role in the defeat of the U-boats.

Though there would be other short breaks in the ability to decipher the Naval Enigma, by now the worst of the U-boat threat was over. None of the interruptions lasted for long. One such difficulty was resolved on June 4th, 1944, when the USS Pittsburg captured and boarded the U-505, taking her code books. Unlike some previous occasions, the U-boat crew were aware of the failure to destroy Enigma material, and as a result they were denied access to the International Red cross, or any contact with their families. Kept in isolation in the US, they were not released until 1947. Ironically the commander of the Us naval task group involved was almost court-martialled by Admiral Ernest King, who feared, groundlessly as it proved, that the failure to sink the U-505 might have alerted the Germans to the breaking of Enigma on the eve of D-Day.

By the time of the D-Day landings, the Naval Enigma was being broken almost instantly by the improved knowledge of the codes and the greater number of "bombes" available in the UK and USA. Only in the very last days of the war did the Germans introduce another code variation which threatened Allied supremacy in this field, and by then it no longer mattered.
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David Kahn, Seizing the Enigma. London, 1996.

Lewin, Ronald. "Ultra" Goes to War. London. 1978.

Sebag-Montefiore, Hugh. Enigma- the Battle for the Code . London, 2000,

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Copyright © 2002 John Barratt.

Written by John Barratt. If you have questions or comments on this article, please contact John Barratt at:

About the author:
John Barratt has authored many books to include: Armada 1588,  The Battle of Marston Moor, The Civil War in South-West England 1642-1646, and Cavalier Generals.

Published online: 12/15/2002.
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