Rasterschluessel 44

Introduction

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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 It took so long to break that the information was out of date and of no operational use.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.

A tough challenge

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A tough challenge.

The number of different grids possible was, in principle, enormous. For each row there were 25x24x...x16 = 12 thousand billion different ways of arranging the white and black squares. Taking into account that there were 24 rows, the number of different grids increases to the astronomical figure of 7 followed by 36 zeros, a considerably greater number of possibilities than generated by an Enigma machine. It was consequently completely out of the question for a cryptanalyst to try every possibility!

The Swedish mathematician Arne Beurling (who has been ranked alongside Rejewski and Turing both as a mathematician and a cryptanalyst) gave his conclusion on RS44 after the war: “The possibilities for successful cryptanalysis are nil, even under the best of circumstances”. Maj. Hugh Skillen, an officer in the British Y Service, wrote that ‘it defeated our cryptographers’. Sir Harry Hinsley, referring to a ‘new’ medium-grade cipher by which he undoubtedly meant RS44, said that it was practically unbreakable.

Bletchley Park tried hard to find a solving technique, with over 100 people employed on the task, but after a few months gave up. An exception was made for Police messages that gave a unique picture of the last days of the Nazi regime and of its individual leaders. Although the intelligence gained suffered from long delays and had a disproportionately high cost, it nevertheless provided posterity with a unique picture and for this reason wartime messages continued to be decrypted after VE-day.

Encryption details

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Details of Encryption process.

The cipher clerk was handed the message to be enciphered. He began by writing the time and the number of letters in the message on a transmission sheet. He laid a transparent ‘flimsy’ over the stencil and selected a cell to start the message. This cell was identified by its row and column coordinates, let us say ea ac, which were then encoded with a bigram table such as:

a b c d e
l c g q j
o y e v z
n b s k x
i a p r u
d w m f h

So coordinates 'eaac' could be encoded to a choice of letters, including 'zios' or 'jodg' and so on. The transmission sheet would then look something like this:

1307 – 54 – zios

Next he looks at the message text. Following his Instruction Manual, he replaces numerals with text making abbreviations where appropriate – for example ‘aqt’ for ‘acht’ – and he enciphers place names with a given substitution cipher.

Now he writes the message onto the flimsy, from the starting cell from left to right, row by row, using the white cells only. If the end of the stencil is reached, continuation proceeds in the top row. He puts an arrow to mark the starting column and rules of the top of the message (see below).

Finally he identifies the offset for the take-out column by adding together the digits of the minutes and the number of letters. In our example this is 0+7+5+4=16. He counts 16 columns to the right of the start column and puts a cross there. And he rules off the bottom of the text.

Now he is ready to take out the ciphertext. He starts in the column marked 19, then follows the transposition key 20,21...25,1,2...18. The ciphertext is thus

NITOT SNILM AISMT OOISA HENEE AIRTP SDTPP HMSSS YATHO AWIOO TSOS

Messages may be enciphered and deciphered using the Windows program written by Geoff Sullivan to accompany my article in Cryptologia "RS44 - the epitome of hand field ciphers". The next section gives details on how to use this program.

Implementing the RS 44 Program

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1. Download the Windows exec file RS44.exe by clicking here and put it into a directory called RS44.

2. In the same directory make a text file named plain.txt and type the plaintext into the file. As an example:

A PESSIMIST IS A PERSON WHO HAS HAD TO LISTEN TO TOO MANY OPTIMISTS

3. Now run the program RS44.exe by double-clicking the icon.

4. In the title bar at the top of the stencil click Options/Enter Start Cell . A dialogue box appears in which you enter the start cell, the start column and the offset (which is the number of columns to the right where taking out will begin). In the example we enter 'ea' as the start row, 'ac' as the start column and '16' as the offset.

5. Now click File/Open PT file and select the file plain.txt The plaintext will appear in the stencil starting at the position specified as below:

6. Finally click File/Save CT file and enter the name for your ciphertext file, and click save.

If you now look in the directory at the file cipher.txt it will contain the ciphertext NITOT SNILM AISMT OOISA HENEE AIRTP SDTPP HMSSS YATHO AWIOO TSOS

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