Polish disclosures As the likelihood of war increased in 1939, Britain and France pledged support for Poland in the event of action that threatened its independence. In April, Germany withdrew from the
German–Polish Non-Aggression Pact of January 1934. The Polish General Staff, realising what was likely to happen, decided to share their work on Enigma decryption with their western allies. Marian Rejewski later wrote: At a conference near Warsaw on 26 and 27 July 1939, the Poles revealed to the French and British that they had broken Enigma and pledged to give each a
Polish-reconstructed Enigma, along with details of their Enigma-solving techniques and equipment, including Zygalski's perforated sheets and Rejewski's
cryptologic bomb. In return, the British pledged to prepare two full sets of
Zygalski sheets for all 60 possible wheel orders. Dilly Knox was a member of the British delegation. He commented on the fragility of the Polish system's reliance on the repetition in the indicator, because it might "at any moment be cancelled". In August, two Polish Enigma doubles were sent to Paris, whence
Gustave Bertrand took one to London, handing it to
Stewart Menzies of Britain's
Secret Intelligence Service at
Victoria Station. Gordon Welchman, who became head of
Hut 6 at Bletchley Park, wrote:
Peter Calvocoressi, who became head of the Luftwaffe section in Hut 3, wrote of the Polish contribution:
PC Bruno On 5 September 1939 the Cipher Bureau began preparations to evacuate key personnel and equipment from Warsaw. Soon a special evacuation train, the Echelon F, transported them eastward, then south. By the time the Cipher Bureau was ordered to cross the border into allied Romania on 17 September, they had destroyed all sensitive documents and equipment and were down to a single very crowded truck. The vehicle was confiscated at the border by a Romanian officer, who separated the military from the civilian personnel. Taking advantage of the confusion, the three mathematicians ignored the Romanian's instructions. They anticipated that in an internment camp they might be identified by the Romanian security police, in which the German
Abwehr and
SD had informers. The mathematicians went to the nearest railway station, exchanged money, bought tickets, and boarded the first train headed south. After a dozen or so hours, they reached Bucharest, at the other end of Romania, where they went to the British embassy. Told by the British to "come back in a few days", they next tried the French embassy, introducing themselves as "friends of Bolek" (Bertrand's Polish code name) and asking to speak with a French military officer. A French Army colonel telephoned Paris and then issued instructions for the three Poles to be assisted in evacuating to Paris.
PC Bruno and Bletchley Park worked together closely, communicating via a
telegraph line secured by the use of Enigma doubles. In January 1940
Alan Turing spent several days at
PC Bruno conferring with his Polish colleagues. He had brought the Poles a full set of Zygalski sheets that had been punched at Bletchley Park by
John Jeffreys using Polish-supplied information, and on 17 January 1940, the Poles made the first break into wartime Enigma traffic—that from 28 October 1939. From that time, until the
Fall of France in June 1940, 17 per cent of the Enigma keys that were found by the allies were solved at
PC Bruno. Just before opening their 10 May 1940 offensive against the Low Countries and France, the Germans made the feared change in the indicator procedure, discontinuing the duplication of the enciphered message key. This meant that the Zygalski sheet method no longer worked. Instead, the cryptanalysts had to rely on exploiting the
operator weaknesses described below, particularly the cillies and the
Herivel tip. After the June Franco-German armistice, the Polish cryptological team resumed work in France's southern "free zone", although probably not on Enigma. Marian Rejewski and Henryk Zygalski, after many travails, perilous journeys, and Spanish imprisonment, eventually reached Britain, where they were inducted into the Polish Army and put to work breaking German
SS and
SD hand ciphers at a Polish signals facility in
Boxmoor, Hertfordshire. Due to their having been in occupied France, it was thought too risky to invite them to work at Bletchley Park. After the German occupation of
Vichy France, several of those who had worked at
PC Bruno were captured by the Germans. Despite the dire circumstances in which some of them were held, none betrayed the secret of Enigma's decryption.
Operating shortcomings Apart from some less-than-ideal inherent characteristics of the Enigma, in practice the system's weaknesses were the large numbers of messages and some of the ways that Enigma was used. The basic principle of this sort of enciphering machine is that it should deliver a stream of transformations that are difficult for a cryptanalyst to predict. Some of the instructions to operators, and operator sloppiness, had the opposite effect. Without these operating shortcomings, Enigma would almost certainly not have been broken. The shortcomings that Allied cryptanalysts exploited included: • The production of an early Enigma training manual containing an example of plaintext and its genuine ciphertext, together with the relevant message key. When Rejewski was given this in December 1932, it "made [his reconstruction of the Enigma machine] somewhat easier". • The use of easily guessed keys such as
AAA or
BBB, or sequences that reflected the layout of the Enigma keyboard, such as "three [typing] keys that stand next to each other [o]r diagonally [from each other]..." At Bletchley Park such occurrences were called
cillies. Cillies in the operation of the four-rotor
Abwehr Enigma included four-letter names and German obscenities. Sometimes, with multi-part messages, the operator would not enter a key for a subsequent part of a message, merely leaving the rotors as they were at the end of the previous part, to become the message key for the next part. • Having only three different rotors for the three positions in the scrambler. (This continued until December 1938, when it was increased to five and then eight for naval traffic in 1940.) • Using only six plugboard leads, leaving 14 letters
unsteckered. (This continued until January 1939 when the number of leads was increased, leaving only a small number of letters unsteckered.) Other useful shortcomings that were discovered by the British and later the American cryptanalysts included the following, many of which depended on frequent solving of a particular network: • The practice of re-transmitting a message in an identical, or near-identical, form on different cipher networks. If a message was transmitted using both a low-level cipher that Bletchley Park broke by hand, and using Enigma, the decrypt provided an excellent crib for Enigma decipherment. • For machines where there was a choice of more rotors than there were slots for them, a rule on some networks stipulated that no rotor should be in the same slot in the scrambler as it had been for the immediately preceding configuration. This reduced the number of wheel orders that had to be tried. • Not allowing a wheel order to be repeated on a monthly setting sheet. This meant that when the keys were being found on a regular basis, economies in excluding possible wheel orders could be made. • The stipulation, for Air Force operators, that no letter should be connected on the plugboard to its neighbour in the alphabet. This reduced the problem of identifying the plugboard connections and was automated in some Bombes with a Consecutive Stecker Knock-Out (CSKO) device. • The sloppy practice that
John Herivel anticipated soon after his arrival at Bletchley Park in January 1940. He thought about the practical actions that an Enigma operator would have to make, and the short-cuts he might employ. He thought that, after setting the alphabet rings to the prescribed setting, and closing the lid, the operator might not turn the rotors by more than a few places in selecting the first part of the indicator. Initially this did not seem to be the case, but after the changes of May 1940, what became known as the
Herivel tip proved to be most useful. • The practice of re-using some of the columns of wheel orders, ring settings, or plugboard connections from previous months. The resulting analytical short-cut was christened at Bletchley Park
Parkerismus after Reg Parker, who had, through his meticulous record-keeping, spotted this phenomenon. • The re-use of a permutation in the German Air Force METEO code as the Enigma
stecker permutation for the day.
Mavis Lever, a member of
Dilly Knox's team, recalled an occasion when there was an unusual message from the Italian Navy, whose exploitation led to the British victory at the
Battle of Cape Matapan. Postwar debriefings of
German cryptographic specialists, conducted as part of project
TICOM, tend to support the view that the Germans were well aware that the un-steckered Enigma was theoretically solvable, but thought that the steckered Enigma had not been solved.
Crib-based decryption The term
crib was used at Bletchley Park to denote any
known plaintext or
suspected plaintext at some point in an enciphered message. Britain's Government Code and Cipher School (GC&CS), before its move to Bletchley Park, had realised the value of recruiting mathematicians and logicians to work in codebreaking teams.
Alan Turing had started to work for GC&CS on a part-time basis from about the time of the
Munich Crisis in 1938. Gordon Welchman, another Cambridge mathematician, had also received initial training in 1938, and they both reported to Bletchley Park on 4 September 1939, the day after Britain declared war on Germany. Most of the Polish success had relied on the repetition within the indicator. But as soon as Turing moved to Bletchley Park—where he initially joined Dilly Knox in the research section—he set about seeking methods that did not rely on this weakness, as they correctly anticipated that the German Army and Air Force might follow the German Navy in improving their indicator system. The Poles had used an early form of crib-based decryption in the days when only six leads were used on the plugboard. Foremost among the knowledge needed for identifying cribs was the text of previous decrypts. Bletchley Park maintained detailed indexes of message preambles, of every person, of every ship, of every unit, of every weapon, of every technical term, and of repeated phrases such as forms of address and other German military jargon. For each message the
traffic analysis recorded the radio frequency, the date and time of intercept, and the preamble—which contained the network-identifying discriminant, the time of origin of the message, the callsign of the originating and receiving stations, and the indicator setting. This allowed cross referencing of a new message with a previous one. Thus, as
Derek Taunt, another Cambridge mathematician-cryptanalyst wrote, the truism that "nothing succeeds like success" is particularly apposite here.
An die Gruppe ("to the group") and a number that came from weather stations such as
weub null seqs null null ("weather survey 0600"). This was actually rendered as
WEUBYYNULLSEQSNULLNULL. The word
WEUB being short for
Wetterübersicht,
YY was used as a separator, and
SEQS was common abbreviation of
sechs (German for "six"). As another example, Field Marshal
Erwin Rommel's Quartermaster started all of his messages to his commander with the same formal introduction. With a combination of probable plaintext fragment and the fact that no letter could be enciphered as itself, a corresponding ciphertext fragment could often be tested by trying every possible alignment of the crib against the ciphertext, a procedure known as
crib-dragging. This, however, was only one aspect of the processes of solving a key. Derek Taunt has written that the three cardinal personal qualities that were in demand for cryptanalysis were (1) a creative imagination, (2) a well-developed critical faculty, and (3) a habit of meticulousness. Skill at solving crossword puzzles was famously tested in recruiting some cryptanalysts. This was useful in working out plugboard settings when a possible solution was being examined. For example, if the crib was the word
WETTER (German for "weather") and a possible decrypt before the plugboard settings had been discovered, was
TEWWER, it is easy to see that
T with
W are
stecker partners. These examples, although illustrative of the principles, greatly over-simplify the cryptanalysts' tasks. A fruitful source of cribs was re-encipherments of messages that had previously been decrypted either from a lower-level manual cipher or from another Enigma network. This was called a
kiss and happened particularly with German naval messages being sent in the
dockyard cipher and repeated
verbatim in an Enigma cipher. One German agent in Britain,
Nathalie Sergueiew, code-named
Treasure, who had been
'turned' to work for the Allies, was very verbose in her messages back to Germany, which were then re-transmitted on the
Abwehr Enigma network. She was kept going by
MI5 because this provided long cribs, not because of her usefulness as an agent to feed incorrect information to the
Abwehr. Occasionally, when there was a particularly urgent need to solve German naval Enigma keys, such as when an
Arctic convoy was about to depart, mines would be laid by the
RAF in a defined position, whose grid reference in the German naval system did not contain any of the words (such as
sechs or
sieben) for which abbreviations or alternatives were sometimes used. The warning message about the mines and then the "all clear" message, would be transmitted both using the
dockyard cipher and the
U-boat Enigma network. This process of
planting a crib was called
gardening. Although
cillies were not actually cribs, the
chit-chat in clear that Enigma operators indulged in among themselves often gave a clue as to the cillies that they might generate. When captured German Enigma operators revealed that they had been instructed to encipher numbers by spelling them out rather than using the top row of the keyboard, Alan Turing reviewed decrypted messages and determined that the word
eins ("one") appeared in 90% of messages. Turing automated the crib process, creating the
Eins Catalogue, which assumed that
eins was encoded at all positions in the plaintext. The catalogue included every possible rotor position for
EINS with that day's
wheel order and plugboard connections.
British bombe The British bombe was an electromechanical device designed by Alan Turing soon after he arrived at Bletchley Park in September 1939.
Harold "Doc" Keen of the
British Tabulating Machine Company (BTM) in
Letchworth ( from Bletchley) was the engineer who turned Turing's ideas into a working machine—under the codename CANTAB. Turing's specification developed the ideas of the Poles'
bomba kryptologiczna but was designed for the much more general crib-based decryption. The bombe helped to identify the
wheel order, the initial positions of the rotor cores, and the
stecker partner of a specified letter. This was achieved by examining all 17,576 possible scrambler positions for a set of
wheel orders on a comparison between a crib and the ciphertext, so as to eliminate possibilities that
contradicted the Enigma's known characteristics. In the words of Gordon Welchman "the task of the bombe was simply to reduce the assumptions of
wheel order and scrambler positions that required 'further analysis' to a manageable number". This allowed a set of scramblers to be connected
in series by means of 26-way cables. Electrical connections between the rotating drums' wiring and the rear plugboard were by means of metal brushes. When the bombe detected a scrambler position with no contradictions, it stopped and the operator would note the position before restarting it. Although Welchman had been given the task of studying Enigma traffic
call signs and discriminants, he knew from Turing about the bombe design and early in 1940, before the first pre-production bombe was delivered, he showed him an idea to increase its effectiveness. It exploited the reciprocity in plugboard connections, to reduce considerably the number of scrambler settings that needed to be considered further. This became known as the
diagonal board and was subsequently incorporated to great effect in all the bombes. A cryptanalyst would prepare a crib for comparison with the ciphertext. This was a complicated and sophisticated task, which later took the Americans some time to master. As well as the crib, a decision as to which of the many possible
wheel orders could be omitted had to be made. Turing's
Banburismus was used in making this major economy. The cryptanalyst would then compile a
menu which specified the connections of the cables of the patch panels on the back of the machine, and a particular letter whose
stecker partner was sought. The menu reflected the relationships between the letters of the crib and those of the ciphertext. Some of these formed loops (or
closures as Turing called them) in a similar way to the
cycles that the Poles had exploited. The reciprocal nature of the plugboard meant that no letter could be connected to more than one other letter. When there was a contradiction of two different letters apparently being
stecker partners with the letter in the menu, the bombe would detect this, and move on. If, however, this happened with a letter that was not part of the menu, a false stop could occur. In refining down the set of stops for further examination, the cryptanalyst would eliminate stops that contained such a contradiction. The other plugboard connections and the settings of the alphabet rings would then be worked out before the scrambler positions at the possible true stops were tried out on
Typex machines that had been adapted to mimic Enigmas. All the remaining stops would correctly decrypt the crib, but only the true stop would produce the correct plaintext of the whole message. The longer the crib, however, the more likely it was that
turn-over of the middle rotor would have occurred. The production model 3-rotor bombes contained 36 scramblers arranged in three banks of twelve. Each bank was used for a different
wheel order by fitting it with the drums that corresponded to the Enigma rotors being tested. The first bombe was named
Victory and was delivered to Bletchley Park on 18 March 1940. The next one, which included the diagonal board, was delivered on 8 August 1940. It was referred to as a
spider bombe and was named
Agnus Dei which soon became
Agnes and then
Aggie. The production of British bombes was relatively slow at first, with only five bombes being in use in June 1941, 15 by the year end, 30 by September 1942, 49 by January 1943 but eventually 210 at the end of the war. A refinement that was developed for use on messages from those networks that disallowed the plugboard (
Stecker) connection of adjacent letters, was the
Consecutive Stecker Knock Out. This was fitted to 40 bombes and produced a useful reduction in false stops. Initially the bombes were operated by ex-BTM servicemen, but in March 1941 the first detachment of members of the
Women's Royal Naval Service (known as
Wrens) arrived at Bletchley Park to become bombe operators. By 1945 there were some 2,000 Wrens operating the bombes. Because of the risk of bombing, relatively few of the bombes were located at Bletchley Park. The largest two outstations were at
Eastcote (some 110 bombes and 800 Wrens) and Stanmore (some 50 bombes and 500 Wrens). There were also bombe outstations at Wavendon, Adstock, and Gayhurst. Communication with Bletchley Park was by
teleprinter links. When the German Navy started using 4-rotor Enigmas, about sixty 4-rotor bombes were produced at Letchworth, some with the assistance of the
General Post Office. The
NCR-manufactured
US Navy 4-rotor bombes were, however, very fast and the most successful. They were extensively used by Bletchley Park over teleprinter links (using the
Combined Cipher Machine) to
OP-20-G for both 3-rotor and 4-rotor jobs.
Luftwaffe Enigma Although the German army, SS, police, and railway all used Enigma with similar procedures, it was the
Luftwaffe (Air Force) that was the first and most fruitful source of Ultra intelligence during the war. The messages were decrypted in
Hut 6 at Bletchley Park and turned into intelligence reports in
Hut 3. The network code-named 'Red' at Bletchley Park was broken regularly and quickly from 22 May 1940 until the end of hostilities. Indeed, the Air Force section of Hut 3 expected the new day's Enigma settings to have been established in Hut 6 by breakfast time. The relative ease of solving this network's settings was a product of plentiful cribs and frequent German operating mistakes. Luftwaffe chief
Hermann Göring was known to use it for trivial communications, including informing squadron commanders to make sure the pilots he was going to decorate had been properly deloused. Such messages became known as "Göring funnies" to the staff at Bletchley Park.
Abwehr Enigma ''. It had three ordinary rotors and a rotating reflector, multiple notches on the rotor rings, but no plugboard.
Dilly Knox's last great cryptanalytical success, before his untimely death in February 1943, was the solving of the
Abwehr Enigma in 1941. Intercepts of traffic which had an 8-letter indicator sequence before the usual 5-letter groups led to the suspicion that a 4-rotor machine was being used. The assumption was correctly made that the indicator consisted of a 4-letter message key enciphered twice. The machine itself was similar to a
Model G Enigma, with three conventional rotors, though it did not have a plug board. The principal difference to the model G was that it was equipped with a reflector that was advanced by the stepping mechanism once it had been set by hand to its starting position (in all other variants, the reflector was fixed). Collecting a set of enciphered message keys for a particular day allowed
cycles (or
boxes as Knox called them) to be assembled in a similar way to the method used by the Poles in the 1930s. Deriving the indicator setting for that day was achieved using Knox's time-consuming
rodding procedure. It is unclear whether the German Army Enigma operators made deciphering more difficult by making fewer operating mistakes.
German Naval Enigma The German Navy used Enigma in the same way as the German Army and Air Force until 1 May 1937, when they changed to a substantially different system. This used the same sort of setting sheet but, importantly, it included the ground key for a period of two, sometimes three days. The message setting was concealed in the indicator by selecting a trigram from a book (the
Kenngruppenbuch, or K-Book) and performing a bigram substitution on it. This defeated the Poles, although they suspected some sort of bigram substitution. The procedure for the naval sending operator was as follows. First they selected a trigram from the K-Book, say YLA. They then looked in the appropriate columns of the K-Book and selected another trigram, say YVT, and wrote it in the boxes at the top of the message form: They then filled in the "dots" with any letters, giving say: Finally they looked up the vertical pairs of letters in the Bigram Tables and wrote down the resultant pairs, UB, LK, RS, and PW which were transmitted as two four letter groups at the start and end of the enciphered message. The receiving operator performed the converse procedure to obtain the message key for setting his Enigma rotors. As well as these
Kriegsmarine procedures being much more secure than those of the German Army and Air Force, the German Navy Enigma introduced three more rotors (VI, VII, and VIII), early in 1940. The choice of three rotors from eight meant that there were a total of 336 possible permutations of rotors and their positions. Alan Turing decided to take responsibility for German naval Enigma because "no one else was doing anything about it and I could have it to myself". He established
Hut 8 with
Peter Twinn and two "girls". Turing used the indicators and message settings for traffic from 1–8 May 1937 that the Poles had worked out, and some very elegant deductions to diagnose the complete indicator system. After the messages were deciphered they were translated for transmission to the Admiralty in
Hut 4.
German Navy 3-rotor Enigma The first break of wartime traffic was in December 1939, into signals that had been intercepted in November 1938, when only three rotors and six plugboard leads had been in use. It used "Forty Weepy Weepy" cribs. A captured German
Funkmaat ("radio operator") named Meyer had revealed that numerals were now spelt out as words. EINS, the German for "one", was present in about 90% of genuine German Navy messages. An EINS catalogue was compiled consisting of the encipherment of EINS at all 105,456 rotor settings. These were compared with the ciphertext, and when matches were found, about a quarter of them yielded the correct plaintext. Later this process was automated in Mr Freeborn's section using
Hollerith equipment. When the ground key was known, this EINS-ing procedure could yield three bigrams for the tables that were then gradually assembled. Banburismus used large cards printed in Banbury (hence the Banburismus name) to discover correlations and a statistical scoring system to determine likely rotor orders (
Walzenlage) to be tried on the bombes. The practice conserved scarce bombe time and allowed more messages to be attacked. In practice, the 336 possible rotor orders could be reduced to perhaps 18 to be run on the bombes. Knowledge of the bigrams was essential for Banburismus, and building up the tables took a long time. This lack of visible progress led to
Frank Birch, head of the Naval Section, to write on 21 August 1940 to
Edward Travis, Deputy Director of Bletchley Park: Schemes for capturing Enigma material were conceived including, in September 1940,
Operation Ruthless by Lieutenant Commander
Ian Fleming (author of the
James Bond novels). When this was cancelled, Birch told Fleming that "Turing and Twinn came to me like undertakers cheated of a nice corpse..." A major advance came through
Operation Claymore, a
commando raid on the
Lofoten Islands on 4 March 1941. The German
armed trawler Krebs was captured, including the complete Enigma keys for February, but no bigram tables or K-book. However, the material was sufficient to reconstruct the bigram tables by "EINS-ing", and by late March they were almost complete. Banburismus then started to become extremely useful. Hut 8 was expanded and moved to 24-hour working, and a crib room was established. The story of Banburismus for the next two years was one of improving methods, of struggling to get sufficient staff, and of a steady growth in the relative and absolute importance of cribbing as the increasing numbers of bombes made the running of cribs ever faster. Of value in this period were further "pinches" such as those from the
German weather ships München and
Lauenburg and the submarines and . Despite the introduction of the 4-rotor Enigma for Atlantic U-boats, the analysis of traffic enciphered with the 3-rotor Enigma proved of immense value to the Allied navies. Banburismus was used until July 1943, when it became more efficient to use the many more bombes that had become available.
M4 (German Navy 4-rotor Enigma) On 1 February 1942, the Enigma messages to and from Atlantic U-boats, which Bletchley Park called "Shark", became significantly different from the rest of the traffic, which they called "Dolphin". This was because a new Enigma version had been brought into use. It was a development of the
3-rotor Enigma with the reflector replaced by a thin rotor and a thin reflector. Eventually, there were two fourth-position rotors that were called Beta and Gamma and two thin reflectors, Bruno and Caesar, which could be used in any combination. These rotors were not advanced by the rotor to their right, in the way that rotors I through VIII were. The introduction of the
fourth rotor did not catch Bletchley Park by surprise, because captured material dated January 1941 had made reference to its development as an adaptation of the 3-rotor machine, with the fourth rotor wheel to be a reflector wheel. Indeed, because of operator errors, the wiring of the new fourth rotor had already been worked out. This major challenge could not be met by using existing methods and resources for a number of reasons. • The work on the Shark cipher would have to be independent of the continuing work on messages in the Dolphin cipher. • Solving Shark keys on 3-rotor bombes would have taken 50 to 100 times as long as an average Air Force or Army job. • U-boat cribs at this time were extremely poor. It seemed, therefore, that effective, fast, 4-rotor bombes were the only way forward. This was an immense problem and it gave a great deal of trouble. Work on a high speed machine had been started by
Wynn-Williams of the
TRE late in 1941 and some nine months later
Harold Keen of BTM started work independently. Early in 1942, Bletchley Park were a long way from possessing a high speed machine of any sort. Eventually, after a long period of being unable to decipher U-boat messages, a source of cribs was found. This was the
Kurzsignale (short signals), a code which the German navy used to minimise the duration of transmissions, thereby reducing the risk of being located by
high-frequency direction finding techniques. The messages were only 22 characters long and were used to report sightings of possible Allied targets. A copy of the code book had been captured from on 9 May 1941. A similar coding system was used for weather reports from U-boats, the
Wetterkurzschlüssel, (Weather Short Code Book). A copy of this had been captured from on 29 or 30 October 1942. These short signals had been used for deciphering 3-rotor Enigma messages and it was discovered that the new rotor had a neutral position at which it, and its matching reflector, behaved just like a 3-rotor Enigma reflector. This allowed messages enciphered at this neutral position to be deciphered by a 3-rotor machine, and hence deciphered by a standard bombe. Deciphered Short Signals provided good material for bombe menus for Shark. Regular deciphering of U-boat traffic restarted in December 1942.
Italian naval Enigma In 1940 Dilly Knox wanted to establish whether the Italian Navy were still using the same system that he had cracked during the Spanish Civil War; he instructed his assistants to use rodding to see whether the crib
PERX (
per being Italian for "for" and
X being used to indicate a space between words) worked for the first part of the message. After three months there was no success, but
Mavis Lever, a 19-year-old student, found that rodding produced
PERS for the first four letters of one message. She then (against orders) tried beyond this and obtained
PERSONALE (Italian for "personal"). This confirmed that the Italians were indeed using the same machines and procedures.
American bombes Unlike the situation at Bletchley Park, the United States armed services did not share a combined cryptanalytical service. Before the US joined the war, there was collaboration with Britain, albeit with a considerable amount of caution on Britain's side because of the extreme importance of Germany and her allies not learning that its codes were being broken. Despite some worthwhile collaboration among the cryptanalysts, their superiors took some time to achieve a trusting relationship in which both British and American bombes were used to mutual benefit. In February 1941, Captain
Abraham Sinkov and Lieutenant
Leo Rosen of the US Army, and Lieutenants Robert Weeks and
Prescott Currier of the US Navy, arrived at Bletchley Park, bringing, among other things, a replica of the
"Purple" cipher machine for Bletchley Park's Japanese section in
Hut 7. The four returned to America after ten weeks, with a naval radio direction-finding unit and many documents, including a "paper Enigma". The main American response to the 4-rotor Enigma was the US Navy bombe, which was manufactured in much less constrained facilities than were available in wartime Britain. Colonel
John Tiltman, who later became Deputy Director at Bletchley Park, visited the US Navy cryptanalysis office (OP-20-G) in April 1942 and recognised America's vital interest in deciphering U-boat traffic. The urgent need, doubts about the British engineering workload, and slow progress prompted the US to start investigating designs for a Navy bombe, based on the full
blueprints and wiring diagrams received by US Navy Lieutenants Robert Ely and Joseph Eachus at Bletchley Park in July 1942. Funding for a full, $2 million, Navy development effort was requested on 3 September 1942 and approved the following day. Commander Edward Travis, Deputy Director and
Frank Birch, Head of the German Naval Section travelled from Bletchley Park to Washington in September 1942. With
Carl Frederick Holden, US
Director of Naval Communications they established, on 2 October 1942, a UK:US accord which may have "a stronger claim than
BRUSA to being the forerunner of the
UKUSA Agreement", being the first agreement "to establish the special
Sigint relationship between the two countries", and "it set the pattern for UKUSA, in that the United States was very much the senior partner in the alliance". It established a relationship of "full collaboration" between Bletchley Park and OP-20-G. An all electronic solution to the problem of a fast bombe was considered, but rejected for pragmatic reasons, and a contract was let with the
National Cash Register Corporation (NCR) in
Dayton, Ohio. This established the
United States Naval Computing Machine Laboratory. Engineering development was led by NCR's
Joseph Desch, a brilliant inventor and engineer. He had already been working on electronic counting devices. Alan Turing, who had written a memorandum to OP-20-G (probably in 1941), was seconded to the British Joint Staff Mission in Washington in December 1942, because of his exceptionally wide knowledge about the bombes and the methods of their use. He was asked to look at the bombes that were being built by NCR and at the security of certain speech cipher equipment under development at Bell Labs. He visited OP-20-G, and went to NCR in Dayton on 21 December. He was able to show that it was not necessary to build 336 Bombes, one for each possible rotor order, by utilising techniques such as
Banburismus. The initial order was scaled down to 96 machines. The US Navy bombes used drums for the Enigma rotors in much the same way as the British bombes, but were very much faster. The first machine was completed and tested on 3 May 1943. Soon these bombes were more available than the British bombes at Bletchley Park and its outstations, and as a consequence they were put to use for Hut 6 as well as Hut 8 work. A total of 121 Navy bombes were produced. The machine was designed to analyse 3-rotor, not 4-rotor traffic. It did not use drums to represent the Enigma rotors, using instead telephone-type relays. It could, however, handle one problem that the bombes with drums could not. A 3-rotor run took about 10 minutes.
German suspicions The German navy was concerned that Enigma could be compromised. They printed key schedules in water-soluble inks so that they could not be salvaged. They policed their operators and disciplined them when they made errors that could compromise the cipher. The navy minimised its exposure. For example, ships that might be captured or run aground did not carry Enigma machines. When ships were lost in circumstances where the enemy might salvage them, the Germans investigated. After investigating some losses in 1940, Germany changed some message indicators. In April 1940, at the
Battles of Narvik the British sank eight German destroyers in Norway. The Germans concluded that it was unlikely that the British were reading Enigma. The Germans investigated, but concluded Enigma had not been breached by either seizures or brute-force cryptanalysis. The Germans took steps to make Enigma more secure. Grid locations (an encoded latitude and longitude) were further disguised using digraph tables and a numeric offset. The U-boats were given their own network,
Triton, to minimise the chance of a cryptanalytic attack. In August 1941, the British captured . The Germans concluded the crew would have destroyed the important documents, so the cipher was safe. Even if the British had captured the materials intact and could read Enigma, the British would lose that ability when the keys changed on 1 November. Although Germany realised that convoys were avoiding its
wolfpacks, it did not attribute that ability to reading Enigma traffic. Instead,
Karl Dönitz thought that Britain was using radar and direction finding. The
Kriegsmarine also improved the Enigma. On 1 February 1942, it started using a four-rotor version. The improved security meant that convoys no longer had as much information about the whereabouts of wolfpacks, and were therefore less able to avoid areas where they would be attacked. The increased success of wolfpack attacks following the strengthening of the encryption might have given the Germans a clue that the previous Enigma codes had been broken. This was not noticed because other things changed at the same time: the United States had entered the war, and Dönitz had sent U-boats to raid the US East Coast, where there were many easy targets. In early 1943, Dönitz was suspicious that the Allies were reading Enigma. German cryptanalysis of Allied communications showed surprising accuracy in its estimates of wolfpack sizes. It was concluded that Allied direction finding was the source. The Germans also recovered a
cavity magnetron, used to generate radar waves, from a British bomber. The conclusion was that the Enigma was secure. The Germans were still suspicious and each submarine got a key net in June 1944. By 1945, almost all German Enigma traffic (Wehrmacht military; comprising the
Heer, Kriegsmarine, and Luftwaffe; and German intelligence and security services like the Abwehr, SD, etc.) could be decrypted within a day or two, yet the Germans remained confident of its security. They openly discussed their plans and movements, handing the Allies huge amounts of information, not all of which was used effectively. The
Battle of Kasserine Pass was foreshadowed in decrypted Enigma traffic, but the Americans did not properly appreciate the information. After the war, Allied
TICOM project teams found and detained a considerable number of German cryptographic personnel. Among the things learned was that German cryptographers, at least, understood very well that Enigma messages might be read; they knew Enigma was not unbreakable. They found it impossible to imagine anyone going to the immense effort required. When Abwehr personnel who had worked on
Fish cryptography and Russian traffic were interned at
Rosenheim around May 1945, they were not at all surprised that Enigma had been broken, only that someone had mustered all the resources in time to do it. Dönitz had been advised that a cryptanalytic attack was the least likely of all security problems. ==After World War II==