Courier (procedure)
The Kurier short-signal method was a method for wireless communication that was introduced by the German navy in the final phase of World War II . In order to avoid the risk of Allied warships being able to find bearings , the messages to be sent were highly compressed and the length of the radio transmissions was reduced to less than half a second.
background
The aim of the German naval war strategy in World War II was to "starve" this enemy by preventing the Allied supplies to Great Britain, so forcing them to give up and eliminate them as an enemy. Since the United Kingdom could not be self-sufficient due to its island location , but was dependent on the supply of essential goods of all kinds, such as food, raw materials and military goods, by sea, it maintained a large merchant fleet . At the beginning of the Second World War this comprised around three thousand ships with a total tonnage of around 17.7 million tons. These merchant ships were attacked by the German submarines and sunk in large numbers (see also submarine war ).
As a countermeasure, which led the Royal Navy , the convoy system, which means they let the merchant ships in the Association as convoys drive and escort by warships. For its part, the German opposing side introduced the " pack tactic " in which, after seeing a convoy by a single submarine, it did not attack immediately, but only "kept in touch" and via radio in cooperation with the commander of the submarines (BdU) as a command center brought further submarines to the convoy, before they all attacked the convoy's ships together like a "wolf pack".
The prerequisite for this coordinated action by several submarines was tap-proof, secret communication with one another and with the command post of the BdU. This was achieved in the first years of the war and especially in 1942 after the submarines prevented the Allies from reading along with the help of the ENIGMA-M4, which was introduced on February 1, 1942, using improved encryption methods . In the first eight months of 1942, for example, more than 4,000,000 tons of Allied shipping space was sunk, that is between 60 and 108 ships per month and an average of more than 500,000 tons.
In March of the following year this quota even increased to 627,000 tons, but then dropped two months later to 264,000 tons for May 1943. The reason was that the Allied countermeasures became effective, such as the alignment of the submarines with " Huff-Duff " ( high-frequency direction finders on the Allied warships) and the successful deciphering of the German M4 radio messages.
While it remained hidden from the German side during the entire war (and for almost thirty years afterwards) that the Allies had broken the ENIGMA encryption (under the code name Ultra ) , the danger that the German submarines would be located by Allied radio measurement technology was definitely known. As a countermeasure, the courier system was designed, the development of which Professor Karl Küpfmüller at Telefunken was responsible for from December 1943 .
technology
A central component of a courier transmission system was an impulse generator, called KZG 44/2. The abbreviation KZG means token generator and the two numbers indicate the year 1944 and the model number 2. The required short impulses were generated with the help of a drum with a diameter of about 30 cm, on the edge of which, evenly distributed over almost three quarters of the circumference, there were 85 elements made of magnetizable soft iron (see web links: "The Courier Short Signal Procedure" ). The soft iron cores were arranged so they could be moved in the radial direction and could be locked in one of two positions by hand. In the inner position, they influenced the magnetic flux of a horseshoe-shaped permanent magnet that quickly passed by the inner edge of the drum during the transmission process , which acted like the read head of a tape recorder or a modern hard drive . In the outer position, however, the soft iron elements had no effect on the magnetic head.
In this way, depending on the position of the 85 iron cores, up to 85 impulses could be generated, some of which were missing, depending on the message content. It was therefore an early form of pulse code modulation , which is still of great importance in today's communications technology.
To initialize and synchronize the receiver, a so-called distant signal was sent before the actual message signal. For this purpose, 25 start pulses were generated in the same way as described for the message signal. In the lower quarter of the drum there were 25 permanently arranged soft iron cores that the reading head passed at the beginning of a shipment. This resulted in 25 pulses, each 1 ms long, at 4 ms intervals. The pulse repetition frequency was therefore 250 Hz and the total duration of the start signal was 97 ms. This distant signal was followed by a pause of 20 ms before the first signal pulse appeared. For technical reasons, the actual signal transmission always began with an impulse. The first of the 85 iron elements was always set. This was followed by up to a maximum of 84 further impulses, which - depending on how the corresponding soft iron core was set - could be present or not present and thus transmitted the actual message content. (In modern language it was possible to transmit a message with up to 84 bits of information .) As with the distant signal, each pulse with the message signal was 1 ms long and the possible pulses also occurred in a time grid of 4 ms. The last usable pulse (number 85) thus appeared 84 · 4 ms = 336 ms after the first pulse (number 1). The message signal therefore had a maximum length of 337 ms.
In total, the maximum duration of the overall transmission was thus
97 ms + 20 ms + 337 ms = 454 ms
Depending on the message content, the signal could end a few milliseconds before the maximum duration specified above.
The signal pulses that can be used for message transmission were summarized as follows for the transmission of Morse code . A point was coded by a single pulse, a line by two consecutive pulses. The time gap between the individual elements ( dots and / or lines ) of a single Morse code was represented by a missing pulse. The somewhat longer pause between the different Morse symbols (= individual letters) was implemented by two consecutive pulses, while three consecutive missing pulses represented the time separator between groups of letters.
If one symbolizes the occurrence of one of the 1 ms long signal pulses with a “1” and the alternatively possible absence of such a pulse with a “0” and omits the 3 ms long pauses between these events, which are irrelevant for information technology, in the following binary representation , then this results, for example for the letter A, whose Morse code is .- ( dot dash ), the pulse sequence "1011" for the courier method. The word ABBA as a further example, now a four-letter message (Morse code of B is -...), would result in the following pulse sequence: "101100110101010011010101001011". This signal uses 30 of the 85 soft iron cores and its transmission (including distant signal) would have taken 97 ms + 20 ms + 30 · 4 ms + 1 ms = 238 ms, i.e. less than a quarter of a second.
As a further protective measure against the alignment of the German submarines, the courier signals - beyond the signal shortening described - were not sent on a fixed frequency, but this was based on a "conversion table", which was secret at the time, by a value between about 60 kHz and 400 kHz changed. Since the Allies initially did not have a broadband panorama receiver sufficient to monitor the entire possible frequency band, the Germans succeeded in "escaping" the English direction finding stations. Only after developing new broadband receivers did the British manage to overcome this difficulty as well.
Procedure
The short weather signal consisted of seven letters which were symbolically arranged as follows: "PDFWBBU". This message was - as described above - sent as a pulse train, with the seven letters being used individually according to their individual meaning for the purpose of data compression according to the following procedure.
The first letter indicated the air pressure . Instead of the symbolic “P”, a letter from Table 1 (see below) was used, which assigned an air pressure in millibars to the total available alphabet letters . Thereby, 25 letters of the Latin alphabet were used twice, leaving out the letter "x", which was used as a special character, namely for the range from 948 to 996 millibars and additionally for 998 to 1046 millibars. The assignment to the relevant of the two air pressure ranges was the responsibility of the recipient of the message, who, knowing the approximate weather situation, concluded the correct of the two possible values. It was thus possible to cover an air pressure range from 948 mbar (and less) to 1046 mbar (and more) with just one letter and at the same time to achieve a sufficiently fine discretization in steps of 2 mbar. So this is a polyphonic encryption . The left out letter "x" was reserved for the case of a defective barometer and signaled the failure of this measured value.
Tafel 1
P = Luftdruck in Zweiermillibaren
1046 mb = a 1020 mb = n 996 mb = a 970 mb = n 1044 = b 1018 = o 994 = b 968 = o 1042 = c 1016 = p 992 = c 966 = p 1040 = d 1014 = q 990 = d 964 = q 1038 = e 1012 = r 988 = e 962 = r 1036 = f 1010 = s 986 = f 960 = s 1034 = g 1008 = t 984 = g 958 = t 1032 = h 1006 = u 982 = h 956 = u 1030 = i 1004 = v 980 = i 954 = v 1028 = j 1002 = w 978 = j 952 = w 1026 = k 1000 = y 976 = k 950 = y 1024 = l 998 = z 974 = l 948 = z 1022 = m 972 = m
Luftdruck wegen Schadens am Messgerät nicht messbar = x
The second letter “D” related to the wind direction and its possible change according to Table 2. The reference for the change in wind direction was the wind flow determined at the time three hours before the current measurement. A distinction was made between three cases. First, the wind has since turned to the left, this is the case I if the wind was blowing from the north (N) at 1:00 p.m. and blowing from the northwest (NW) at 4:00 p.m. at the time of observation. Case II does not concern a change in wind direction. If, on the other hand, the wind was blowing from the southwest (SW) at 13:00 and is now blowing from the northwest, then it has turned to the right. This is the case III. A special case was considered to be calm or weak circulating wind, and a distinction was made between whether this had occurred in the last three hours or whether this had been the case for more than three hours.
Tafel 2
D = Windrichtung und Änderung der Windrichtung
Fall I Fall II Fall III
N = a N = b N = c NO = d NO = e NO = f O = g O = h O = i SO = j SO = k SO = l S = m S = n S = o SW = p SW = q SW = r W = s W = t W = u NW = v NW = w NW = x
Windstille oder schwacher umlaufender Wind in den letzten drei Stunden eingetreten = y
Windstille oder schwacher umlaufender Wind
seit mehr als drei Stunden = z
The wind strength and its change were indicated by the third letter “F” according to Table 3, whereby here, too, reference was made to the wind strength at the last observation three hours ago.
Tafel 3
F = Windstärke und Änderung der Windstärke
Windstärke nach I II III Beaufort abgeflaut Stärke nicht geändert aufgefrischt
1 – 2 a b c
3 d e f
4 g h i
5 j k l
6 m n o
7 p q r
8 – 9 s t u
10 – 12 v w x
Windstille (0) y z
The sky cover and the weather phenomena were given by the fourth letter "W" via the assignments in Table 4.
Tafel 4
W = Himmelsbedeckung und Wettererscheinungen
wolkenlos = a
bis 1/4 bedeckt = b
1/4 bis 3/4 bedeckt, Haufenwolken = c
3/4 bis fast bedeckt, Schichtwolken = d
3/4 bis fast bedeckt, Haufenwolken = e
3/4 bis fast bedeckt, tiefe oder mittelhohe
Schichtwolken = f
3/4 bis fast bedeckt, nur hohe Schichtwolken = g
ganz bedeckt, tiefe oder mittelhohe Wolken = h
ganz bedeckt, hohe Wolken = i
stark wechselnd bewölkt = j
dünner Nebel = k
dichter Nebel = l
Nebelschwaden = m
Sprühregen = n
leichter Regen = o
mäßiger Regen = p
starker Regen = q
Schnee oder Schnee mit Regen gemischt = r
Regenschauer = s
Schneeschauer = t
Durchzug ausgeprägter Regenböen = u
Durchzug von ausgeprägten Schnee- oder Hagelböen = v
Gewitter = w
Nach Durchgang einer Warmfront in den
letzten drei Stunden = x
Nach Durchgang einer Kaltfront in den letzten
drei Stunden = y
Nach Durchgang einer Okklusion in den letzten
drei Stunden = z
Finally, the observation location, i.e. the position of the boat, was to be reported as "BB" using the weather square map of the weather short code (WKS), while the last character "U" of the weather short signal indicated an identifier for the sea area according to the weather square map and at the same time the "signature" of the submarine, i.e. an identifier of the sender.
The sequence of letters encoded in this way, for example for a weather report with the following content: "Air pressure 1000 millibars, wind turning counterclockwise from the southeast, refreshed, wind force 6, rain shower" would include an assumed position (IU) and a submarine signature (W) as follows appearance:
YJOS IUW
Like all messages sent by the German submarines, weather short signals generated using the courier system according to the above scheme were not sent directly, i.e. in " plain text ", but were encrypted in advance. The ENIGMA-M4 key machine was used for this . This would have the above letter sequence in
DQEH ZYJ
transformed. Represented as Morse code, these seven encoded letters correspond to the following string:
-.. --.- . .... --.. -.-- .---
The soft iron elements on the courier token generator (KZG 44/2) would have to be set according to the following scheme:
110101001101101011001001010101000110110101001101011011001011011011
These are 66 characters (bit) and including the distant signal would have resulted in a courier signal with a duration of 97 ms + 20 ms + 66 · 4 ms + 1 ms = 382 ms.
In addition to the transmission of short weather signals - as described - the courier procedure was also intended for tactical short signals, such as location reports, fuel and torpedo levels, course information and enemy situation.
effect
In contrast to the usual ENIGMA radio messages, which lasted from a few tens of seconds to a few minutes, short courier signals were difficult to perceive for the British listening stations due to their short length of less than half a second. They sounded like one of the many typical atmospheric disturbances (caused for example by distant thunderstorm lightning) that appeared as a brief crack in the headphones and were subjectively faded out by the listening posts. Aligning these brief signals was hard to imagine. Thus, the two most important skills of the Allies in the fight against the German submarines, which were of outstanding importance for the Allied victory in the Battle of the Atlantic , namely the bearing of the positions of the submarines via high-frequency location and the deciphering of the German Radio messages, into the void. If the courier procedure had been introduced for submarines earlier than 1944, this would have had a massive impact on the course of the Atlantic battle and possibly even influenced its outcome.
In fact, this innovative countermeasure came too late to take effect. From autumn 1944 until the end of the war, 30 courier units were completed and the procedure was tested in at least 19 submarines (U 285, U 325, U 416, U 482, U 680, U 864 , U 866 , U 878, U 880 , U 925, U 977 (?), U 978, U 979, U 1055, U 1221 (?), U 1223 and three not exactly known boats). However, it has probably only been used on a single boat in the front line.
On the basis of deciphered ENIGMA radio messages, in which the courier procedure was often mentioned from mid-1944, the Allies knew it early on and gave it the code name “ Squash ” (German: “Matsch”). Even in the last months of the war, the British succeeded in essentially reconstructing the German receiving device and recording courier shipments.
The proven cryptology expert and historian Ralph Erskine assesses the possible effect of the courier procedure as follows: “If couriers had been used on type XXI and XXIII submarines , the Allies would have found it difficult to counter this massive threat. [...] That could have led to the fact that intelligence findings from radio messages from submarines would then only have been obtained to a very limited extent. [...] Even in mid-1950, the US Navy's direction finding network was still having difficulties dealing with short signals of the courier type, since these broadcasts could not be identified, recorded and localized at the same time if the frequencies were not known beforehand. Kurier was an almost perfect match that had only one weakness - the restriction of the signal to a maximum of seven letters. "
literature
- Arthur O. Bauer: Radio direction finding as an Allied weapon against German submarines 1939-1945. Self-published, Diemen Netherlands 1997, ISBN 3-00-002142-6 , pp. 205-225.
Web links
- Courier - "The short signal procedure"
- The short signal procedure courier . Excerpt from the book by Arthur O. Bauer. PDF; 7.9 MB .
- The different types of submarine radio messages . Excerpt from the book by Arthur O. Bauer. PDF; 0.4 MB .
- Testing, countermeasures, evaluation . Posted by Ralph Erskine. Excerpt from the book by Arthur O. Bauer. PDF; 0.4 MB .
- The Kurier system by Dirk Rijmenants (English)
supporting documents
- ^ Hugh Sebag-Montefiore : ENIGMA - The battle for the code . Cassell Military, London 2004, ISBN 0-304-36662-5 , p. 70 ( Cassell Military Paperbacks ).
- ^ Hugh Sebag-Montefiore: ENIGMA - The battle for the code. Cassell Military, London 2004, ISBN 0-304-36662-5 , p. 225 ( Cassell Military Paperbacks ).
- ^ Hugh Sebag-Montefiore: ENIGMA - The battle for the code. Cassell Military, London 2004 ISBN 0-304-36662-5 , p. 248 ( Cassell Military Paperbacks ).
- ^ Hugh Sebag-Montefiore: ENIGMA - The battle for the code. Cassell Military, London 2004 ISBN 0-304-36662-5 , p. 274 ( Cassell Military Paperbacks ).
- ↑ Arthur O. Bauer: Radio direction finding as an allied weapon against German submarines 1939-1945. Self-published, Diemen Netherlands 1997, ISBN 0-19-280132-5 , p. 206.
- ↑ Arthur O. Bauer: Radio direction finding as an allied weapon against German submarines 1939-1945. Self-published, Diemen Netherlands 1997, ISBN 0-19-280132-5 , p. 208.
- ↑ Dirk Rijmenants: Kurzsignale . (English, accessed June 6, 2011).
- ↑ Arthur O. Bauer: Radio direction finding as an allied weapon against German submarines 1939-1945. Self-published, Diemen Netherlands 1997, ISBN 0-19-280132-5 , p. 219.
- ↑ Arthur O. Bauer: Radio direction finding as an allied weapon against German submarines 1939-1945. Self-published, Diemen Netherlands 1997, ISBN 0-19-280132-5 , p. 225.