Rasterschluessel 44 (RS44)

RS44 was a German field cipher that gave the Allies a major headache during the second world war. It took so long to break that the information was out of date and of no operational use.

RS 44 was a transposition cipher with a twist. Random gaps were left between letters that broke-up the continuity of the ciphertext and prevented attack by anagramming. The basis of the cipher was a ‘stencil’ that contained white cells for writing-in the message and black cells, distributed randomly, that were left empty.

Every row and column was given a different bigram identifier and in addition every column was numbered in a random order to form the transposition key.

The idea of the cipher was to pick a cell at random to start and then write-in the message horizontally, row by row. Then to take-out the ciphertext vertically, column by column, in the order of the key.

A different stencil was used each day so that the location of the black cells, the column identifiers and the transposition key were changing daily. Other camouflaging tactics were used which are explained in detail in the later section on enciphering.

The stencils were printed by machine in pads of 31 -- one for each day of the month. The Germans made only 36 different printing patterns of black and white cells , from which 24 were chosen to print each stencil. This limitation was enforced by an extreme shortage of materials due to the increasing encirclement by the Allies.

A 20-page Instruction Manual was issued by General Fellgiebel of the German High Command (OKW) in March 1944. The intention was that the Army, Airforce and regular Police would begin using the cipher in August. Material shortages hindered progress and the Army was not at all keen to adopt the cipher. They finally took it up in February 1945.

The cipher was used by forward troops who needed a simple and highly portable cipher. They had previously been using the Double Playfair cipher but this was not sufficiently secure. In fact the Allies broke it every day. With the prospect of invasion in the West, the OKW wanted something more secure and RS 44 certainly provided that as I will describe in the next section.

Most of the messages enciphered by RS 44 were fairly trivial, dealing with troop movements, deliveries of petrol and other logistical matters. However even such mundane matters would have been of interest to the Allies to aid tactical decisions.