Kurier was a burst transmission system for U-boat communications that was first sea trialed by the Kriegsmarine in 1943 and subsequently fitted to the Type XXI submarine.: 103–104 Having learned of the success of the UK's "huff-duff" systems in rapidly locating radio transmissions, Kurier was developed to dramatically reduce message transmission times from a typical 20 seconds to about 250 ms, and never longer than 450 ms (just under ½ a second). Due to the deteriorating position of Germany by that time, Kurier never became operationally effective before the war ended.
Kurier was a simple system largely consisting of the KZG 44/2, a metal drum with 85 small metal bars that could be pushed into two positions. A message was prepared using the Navy's kurzsignale encoding method that reduced messages to a short series of four-letter codes, and then encrypted using the Naval Enigma machine. Conventional transmission of the kurzsignale at this stage would take about 20 seconds, fast enough to defeat conventional radio direction finding, but not huff-duff. The resulting short message was converted into Morse code and entered onto the KZG 44/2, with dots as a single bar and dashes as two. A section of the drum was always pre-set as a series of dots to provide a timing signal. Once the message was encoded onto the drum, it was connected to the radio and activated. The drum rapidly rotated past a magnetic pickup; each of the bars created a pulse that was fed into the transmitter.
The pulsed signal was sent at 250 Hz with a duty cycle of 25%. So each pulse was 1 ms long followed by a 3 ms pause. Dots consisted of one pulse and dashes of two consecutive pulses. They were always followed by a 4 ms pause. The burst started with a timing signal consisting of twenty-five 1-ms-long pulses with a pause of 3 ms in between, taking 97 ms to send. This was followed by a 20 ms pause. After this the message was sent with a 4 ms pause added after each letter. The entire message could be encoded into not more than 85 pulses—depending upon the dots and dashes in the message—for a total of not more than 337 ms. The longest possible message was thus 97 + 20 + 337 = 454 ms, less than half a second.
- Matthews, Peter (2013). SIGINT: The Secret History of Signals Intelligence in the World Wars. The History Press. ISBN 9780752493015.
- Llewellyn-Jones, Malcolm (2006). The Royal Navy and Anti-Submarine Warfare, 1917-49 (Kindle ed.). Abingdon: Routledge. ISBN 0 415 38532 6.
- "Kurier". TICOM Archive.
Each pulse was 1 millisecond long and there was a 3 milliseconds gap between each pulse. One bar pushed in represented a dot, two bars pushed in adjacent to each other represented a dash, and an unset bar represented a pause between pulses.
- "Kurzsignale on German U-boats". CIPHER MACHINES AND CRYPTOLOGY.
Each pulse was 1 millisecond long and there was a 3 milliseconds gap between each pulse. [...] Each dot was set on the Kurier device as one pulse, a dash was two pulses. Between dots and dashes there was a pause of one pulse length, and between letters two pauses.