Triton Strikes Back
The dropping results in Uboat success in 1941, made Admiral Dönitz suspicious. Although reassured by the Abwehr, German Foreign Intelligence, that Enigma was unbreakable, he insisted on improving the security of Enigma. On 1 February 1942 the famous Enigma M4 model with four rotors and new code books were introduced. The new and more complicated Triton code, designated Shark by Bletchley Park, caused a complete black-out. They could no longer break the U-boat radio traffic and no longer had any idea of the U-boat positions. On top of that, new Wetterkurzschlussel code books were introduced one month earlier. This deprived Bletchley Park completely of any cribs. The Kriegsmarine called the springtime of 1942 Die Glücklichen Zeiten or Happy Times, because of the enormous shipload they sunk. Note that the propaganda term Happy Times is relative, taken in account the dangerous and extreme circumstances the U-boat crews were operating in. The problem was not in the machines being used. betchely park already had afour rotor machine. the problem was in the new codes and new procedures introduced by the KM in february 1942.
After ten nerve wrecking months of heavy losses, Bletchley Park succeeded in breaking into Shark. This was due to several different reasons (but dieppe was not one of them). An important event was the attack on Kapitänleutnant Hans Heidtmann's U-559 by the British destroyer HMS Petard on 30 October 1942. After taking heavy fire from HMS Petard, the sinking U-boat was boarded by three British sailors. They managed to get the Enigma code books and the new edition of the Wetterkurzschlussel. Two of them returned once more to recover the Enigma M4, but went down with the U-boat. They payed their bravery with their lives, but not without result. This mission proved to be a turning point in breaking Shark. On 13 December 1942, more than ten months after the start of the black-out, Bletchley Park could finally inform the Admiralty again about U-boats positions.
After breaking a substantial number of messages, the codebreakers realized that the U-boat weather reports, encoded with the WetterKurzschlussel, were sent with the four-rotor Enigma in the less complicated M3 mode (the fourth rotor in A position with ring setting A). This was done to be compatible with the three-rotor Enigma M3, used on weather ships. The Bombes in Bletchley Park, developed to crack the three-rotor Enigma, took more than 20 days to crack the four-rotor Enigma key settings. However, a three-rotor key setting could be retrieved in less than 24 hours. This discovery was an enormous time profit. With the broken meteorological reports from Hut 10 and the recovered Wetterkurzschlussel they finally broke Triton continuously. When a new edition of the Wetterkurzschlussel came into service in March 1943, the seized U-559 Wetterkurzschlussel became useless, resulting in a new black-out. Fortunately, the Kurzsignalheft code book, also recovered from U-559, provided new ways to find cribs in U-boat short-signals and enabled the codebreakers to re-enter shark after nine days. Except for some brief periods, the codebreakers never lost Shark again.
Breaking Kurzsignale
The U-boats used the Kurzsignalheft (short-signal book) to encipher contact messages. The Kriegsmarine converted default tactical expressions with a code table, called Kurzsignalheft, before enciphering them with Enigma. A contact with a convoy could for example be converted into UGKU, an enemy airplane into HKJL, or a meeting point for refueling into KLUG.The use of Kurzsignale was a clever approach. It was harder for Allied Signal Intelligence to trace these short messages with HDFD (High Frequency Direction Finding or Huffduff). Moreover, attempts to decipher these short messages didn't give any readable sentences, and approaching the correct key did not reveal pieces of normal sentences, helping to find the key settings. Also, the conversion of text into four letter codes shortened the cipher text. Less cipher text also provides less statistics to the code breakers. Nothing but advantages...they believed.
Unfortunately for the Germans, the use of Kurzsignale resulted into recognizable patterns in the Enigma messages. A convoy, nearing a U-boat, would probably evoke a contact message. An airplane, spotting a U-boat, would result in a airplane contact message. In Bletchley Park, tactical information was linked to positions, obtained by HFDF and reconnaissance reports, to find out what type of message was sent by that particular U-boat. In combination with the recovered Kurzsignalheft code book, Bletchley Park was able to predict the content of the enciphered messages, thus providing them again with crucial cribs to feed into their Bombes. Meanwhile, new Bombes were developed to deal with the four-rotor Enigma. By June 1943, the first four-rotor Bombes came into action, and by the end of 1943 another fifty four-rotor Bombes went operational at OP-20-G, the American Naval codebreakers. In the fall of 1943, Shark messages were generally broken within 24 hours.
U-boats down
The 'Ultra' information was extremely effective in the strategically very important North Atlantic Ocean. After the initial hard times, Bletchley Park broke the Enigma messages on a daily base. The tide of U-boat war was turned. Except for some brief periods, the entire German communication system was intercepted by a large number of listening stations called Y-stations, and the codes broken in Bletchley Park, with over 7000 employees at its peak. With the positions of the U-boats unveiled, Allied ships were simply re-routed to avoid fatal confrontations with the U-boats, and an active hunt for the U-boats begun. The elite weapon of the Kriegsmarine became decimated, resulting in heavy losses among the U-boat crews. An estimated 700 U-boats and 30,000 crewman were lost at sea. The German command related these losses to new detection techniques like the ASDIC sonar system, U-boat detection planes, and destroyers escorting convoys. This was partially a correct assumption, but they never suspected cryptanalysis of their Enigma encrypted radio traffic. The Enigma machine provided without a doubt a for those days unbreakable encryption. However, unsafe procedures and tactical mistakes turned the Enigma machine into the Achilles heel of the German war machine. Germany kept on using Enigma without any suspicion in all parts of their forces, which resulted in catastrophic consequences for Nazi Germany.
Origins of Kurzsignale
During the Second World War the German U-boats used Kurzsignale or Short Signals to send their messages. The Kurzsignale were an important part of the complex Kriegsmarine communications system. In general, the Kurzsignale were four letter groups representing all kinds of sentences regarding tactical information such as course, enemy reports, position grids or weather reports.
An important reason for the Kriegsmarine to apply these Kurzsignale was the Allied use of High Frequency Direction Finding, also called HFDF or Huff Duff. This system enabled Allied Forces to accurately determine the position of German broadcastings. This was an important tactical advantage in the Atlantic, revealing the positions of German ships and U-boats. The use of Kurzsignale decreased the length of the morse messages enormously, often reducing broadcasting time to less then one minute. This way, the German Navy made it harder to fix positions with Huff Duff.
Kurzsignale on U-Boats
The Kriegsmarine procedures on sending messages with the Enigma cipher machine were far more complex and elaborate than the Wehrmacht and Luftwaffe procedures. Of course, secure communications were a most vital part of Kriegsmarine ooperations in the Atlantic. The U-boats relied completely on secure communications to receive their orders, coordinate patrolling on sea, and their Wolfpack tactics. If the communications were compromised, this would reveal Germans naval positions and result in Allied tactical countermeasures or active hunt on the U-boats.
During the War, several different Kurzsignale methods were used on U-boats. Until 1942, Alpha signals were used. An Alpha signal was a small message, usually containing a single four-letter groups. From 1942 on, U-boats commonly used the Beta signals. Various editions of Kurzsignalhefte, the Short Signal Codebooks, were applied during the war. Each Kurzsignal message, or Beta signal, had a strict format, containing an introduction, an identification to the key, and the message, encrypted with the Enigma cipher machine.
This knowledge was not known until after October 1942….