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.
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 the supremacy of the Kriegsmarine 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.
To apply the Kurzsignale, the Kriegsmarine used several different codebooks. The two most important codebooks were the Kurzsignalheft for all kinds of operational messages, and the Wetterkurzschlussel for weather reports. The Kurzsignalheft contained tables that converted sentences into four-letter groups. All kinds of expressions in many different topics were listed. Logistic matters such as refueling and rendez-vous with supply ships, positions and grid lists, names of harbors, countries, weapons, weather conditions, enemy positions and ships, date and time tables. All possible situations and topics were listed. Another codebook contained the Kenngruppen and Spruchschlussel, resp key identification and message key, that is the start position to the rotors of the Enigma. The Kurzsignalheft codebooks were printed on special paper with red, water soluble, ink. If the codebooks could be captured, they were destroyed by throwing them into water.
In our example we will explain a Short Message, encoded with the 1944 edition of the Kurzsignalheft. This edition was more complex than the previous. The Kurzsignalheft 44 consisted of two parts, Heft I and II. Heft I contained the Satzbuch or sentence book, to convert sentences into four-figure groups, and the Schlusselzahltafel or key number table. Heft II, called Buchgruppenheft, was used to convert four-number groups into four-letter words. In addition, to sign messages or identify other U-boats in a message, the Kriegsmarine used a codebook called Marinefunknamenliste or Naval Callsign List. This was a list with all existing U-boats and trigrams, three letter words, assigned to each U-boat. Unfortunately for the Kriegsmarine, the more complex 1944 edition didn't came into service on time to change the odds for the decimated U-boat fleet in the Atlantic.
The message that had to be sent:
Translated: Convoy of 16 to 20 steamships at Grid CA 9133, signed U-999
First, the three sentences are converted into figures, using the Satzbuch:
Next, a key number, retained from the Schlusselzahlentafel or key number table is added, without carry:
Finally, the resulting figures are translated into four-letter words with the Buchgruppenheft, and the message is signed with the trigram, representing our U-boat in the Marinefunknamenliste:
Not only the Kriegsmarine transmission procedures and message format were different from the Wehrmacht and Luftwaffe. The key sheets for the Enigma settings were also different. The Wehrmacht used one table with rotors, ring settings, plugs for each day of a given month. The Kriegsmarine used various code sheets. The Kriegsmarine TRITON code sheets consisted of two parts.
The first sheet, called Schlusseltafel M Algemein - Innere Einstellung, contained the three rotors and their ring settings, the thin beta or gamma rotor and the reflector, and this for all odd days of a whole month. The second sheet, called Schlusseltafel M Algemein - Aussere Einstellung, contained the plugs and Grundstellung or start position for each day of the month.
The Kriegsmarine Sonderschlussel M, used for private conversations between the Commander of the U-Boat Fleet and a particular U-boat, had a special key sheet with only three inner settings, and three plugboard settings, each for a period of ten days, and a list of Spruchschlussel or message keys, designated by a code word. The Sonderschlussel M was similar to the Schlusseltafel M Offizier from TRITON. Examples of the Kriegsmarine keys on the procedure page.
In order to prepare the message for transmission, the operator had to encipher the message with the Enigma cipher machine. The internal settings and plugboard of the Enigma would already be set. The operator selected a Kenngruppe and Spruchschlussel from his Kenngruppenheft codebook. A Spruchschlussel or message key was the initial start position of the Enigma rotors prior to enciphering. The Kenngruppe was a trigram to identify the Spruchschlussel. The kenngruppenheft was a fixed codebook, which was not regularly replaced. It had three parts: The first part is the Zuteilungsliste, a table where you could find sets with ranges of numbers, according to the day an radio net. Within a range that was derrived from the table, the operator selected a figure. The second part was Teil A, where you could look-up the figure and find the Kenngruppe and Spruchschlussel behind it. The Spruchschlussel was set as startposition on the Enigma and the Kenngruppe (in our example RDF) was added (not encrypted!) at the beginning of the message to identify the Spruchschlussel. The receiving operator would look-up the Kengruppe in the third part of his codebook, Teil B, and find the figure that was related to that Kenngruppe. With this figure, he could now find the appropriate Spruchschlussel (message key) to decipher his message.
The groups, enciphered with Enigma:
The completed message contained the following information:
a. The introduction signal ßß (beta
The complete Kurzsignal message, ready for transmission:
An experienced radio operator could easily transmit this little message in morse in about 20 seconds.
At the end, the operator used 7 tables or key sheets to encipher his message! The Kurzsignalheft Heft I with its Satzbuch to convert sentences into four-figure groups and the Schlusselzahltafel to add the key number, Heft II with the Buchgruppenheft to convert the four-figure groups into four-letter groups, the Marinefunknamenliste to identify the U-boats, the two key sheets Schlusseltafel M for inner an outer settings of the Enigma machine and finally the Kenngruppenheft to select the message key. No wonder they were confident their communications were secure. Nonetheless, Allied codebreakers succeeded in breaking into the U-boats communications as you can read in Enigma and the U-boat War.
Some pages from the Kurzsignalheft (click to enlarge).
Some pages of the Kenngruppenheft. The Zuteilungsliste to select a figure, part A to find the trigram and message key, and part B to return a trigram into a figure (click to enlarge).
Weather reports were vital tactical information for the Kriegsmarine. Every few hours, the U-boats had to send detailed weather reports by radio. However, each broadcast from a submarine increased the risk of detection by Allied direction finding systems. Therefore, it was important to put as much as possible information into a message that was as small as possible. The Wetterkurzschlussel or WKS did just that. Each Wetterkurzsignal consisted of 23 or 24 letters, representing a certain weather condition. Thirteen tables determined which letter or letter combination was used for a particular weather condition. These included barometric pressure, clouds, winds, visibility, rain fall and so on. Thanks to the Wetterkurzschlussel a large amount of weather variables was compressed in only a few letters.
Format of a Wetterkurzsignal
Some pages of the Wetterkurzschlussell book (click to enlarge).
In August 1944 the Kriegsmarine began testing an experimental system called "Kurier", designed as a counter-measure against the High Frequency Direction Finding. It was a system, based on a principle now known as burst-encoding. The Kurier device was connected to a radio transmitter. The main component of Kurier was the pulsgenerator KZG 44/2, a drum with 85 small adjustable bars. Each bar represented a signal pulse. When started, an arm with a magnetic pickup element made one single rotation, passing the 85 pre-set bars. Each pulse was 1 millisecond long and there was a 3 milliseconds gap between each pulse. Together with start pulses and pauses, the complete transmission of the short message took never more than 460 milliseconds! The Kurzsignal that had to be sent was converted into Morse code. Each dot was set on the Kurier device as one puls, a dash was two pulses. Between dots and dashes there was a pause of one pulse length, and between letters two pauses. The Kurier receiver KGR-1 converted the pulses into a lightbeam that was projected on a rotating drum with photosensitive paper (more technical details on the website for German communications).
The Kurier system was to be used to transmit Kurzsignalen and Wetterkurzsignalen, and combined with a complex schedule of frequency changes with frequency shifts of plus or minus 200 KHz. Each Kurier Wetterkurzsignalen was seven letters long. Each letter of the Kurzsignal stands for a value, obtained from a table in the Kurier book. For instance: if the first letter of the Wetterkurzsignal is G this means 1034 millibar. As always with kurzsignalen, each message was to be enciphered with Enigma prior to transmission with the Kurier device.
Composition of the Kurier Wetterkurzsignal:
By the end of 1944 Berlin made the Kurier tests a top priority but the program was interrupted before the Kurier system was operational on the U-boat fleet. Events would finally catch up with the program and the end of the war stopped further experiments. Had the Kurier system been operational at an earlier stage of the U-boat war, it could have resulted in serious consequences. Allied intelligence would have been deprived of direction finding and monitoring kurzsignal messages. This would not only mean loss of U-boat positions but would also deprive the codebreakers in Bletchley Park from the essential cribs to break the Enigma keys, used to encrypt the Kriegsmarine message traffic. This could have changed the outcome of the war in the Atlantic.
Some pages of the Kurier Wetterkurzsignal procedures. This book contained the guidelines, frequency shift tables and the tables to convert weather values in letters (click to enlarge).