Tuesday, April 21, 2015

The Kurier System

In the same vein as my previous postings on the Enigma cipher, I have been doing some reading about some of the challenges that faced the Bletchley Park code breakers during WWII, particularly with regard to Kriegsmarine M4 U-Boat message traffic.

A technique employed by the Gernman U-boats involved coordination between multiple U-boats.  Wolfepacks or Rudeltaktik as it was known to the Germans were created as a means to defeat allied ship convoys.  First tested in 1940, the concept is simple.  Gather U-boats in patrole lines to scout for convoys.  Once spotted by the first boat, it was designated as "shadower" and would follow the convoy and report its heading and speed to the U-boat tactical command.  Coordination between U-boats allowed them to form up around the convoy and attack with as many U-boats as possible during night to overwhelm the escorts.  So successful were wolfepacks that in 1942 convoy operations in the North Atlantic were paused for a time.

To combat the wolfepacks the biggest impact was obtained through HF/DF technologies (High Frequency/Direction Finding, known as "Huff Duff" to sailors).  Once a U-boat sighted a convoy, coordination between boats involved Enigma encoded messages being exchanged.  HF/DF was not new having been used for navigation purposes for years.  The Royal Navy designed an apparatus that could take bearings on the transmitters employed by German U-boats.

Shore-based direction finding installations were constructed on both shores of the Atlantic as well as Iceland, Greenland and Bermuda.  Anytime the Germans transmitted , the HF/DF equipment could get bearings on the approximate location of the U-boat.  Note that it was not important to understand the content of the U-boat messages.  It was sufficient to know the general area from which the transmissions were emanating and thus avoid the area.  Ship-borne HF/DF equipment allowed U-boats to be located and sank or at least forced to dive and remain submerged for hours disrupting coordination of wolfepacks.  Thus the HF/DF efforts struck at the very core of wolfepack tactics.  The German Navy needed to minimize exposure to HF/DF by reducing significantly the time required to transmit messages.

The Kurier system was an experimental burst transmission system that was introduced late in 1944.  The goal of the system was to dramatically reduce transmission times required to send Kurzsignalen encoded message.  Unfortunately for the German efforts, Kurier never became operational due to deteriorating late-ware conditions. Entire standard messages were reduced to a series of four-letter codes that were then encrypted using the Naval Enigma machine. The entire message could then be transmitted in a maximum of 454 milliseconds.

Kurzsignalen involved the use of several different codebooks.  The Kurzsignalheft codebook was used for all sorts of operational messages.  The Wetterkurzschlussel was used for weather reports.  The Kurzsignalheft contained tables that converted entire sentences into four letter groups.  A large number of expressions and topics were listed including logistic matters such as refueling, rendezvous with supply ships were able to be accommodated along with tables of dates and times, etc. all reduced to four letter code groups which were then encrypted by Enigma.  Another codebook contained Kenngruppen and Spruchschlussel key identification and message keys (the starting position of the Enigma rotors).  Codebooks were printed on a special paper with red, water soluble ink to allow for easy disposal/destruction by tossing into water.

Once encoded and encrypted a Kurzsignalen message could be sent by an experienced radio operator in Morse code in about 20 seconds.  Still too long...

The Kurier system encodes Morse code dots as a 1 millisecond (ms) pulse followed by a 3 ms pause.  A Morse code dash is encoded as a 1 ms pulse, 1 ms pause, another 1 ms pulse and a 3 ms pause.

The entire signal starts as a timing signal of twenty-five 1 ms long dot signals taking 97 ms to send followed by a 20 ms pause.  After this, the encoded and enciphered message is sent.  The entire message could encode 85 pulses for a total of 337 ms.  Thus the longest message is 454 ms.

The transmission was accomplished by a rotary mechanical device attached to a transmitter.  A receiver produced an optical output that was captured by photosensitive paper.

So, with that bit of history in place, I thought I would  attempt to encode and decode Kurier signals.

More to come...

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