Encryption technology of the Cold War is a very interesting, yet also difficult topic. Much of the relevant information is still classified. This is also the case for a simple German encryption device named ACP 212. Does a reader know anything about it?

The biennial Symposium on Cryptologic History is just around the corner. Organized by the NSA this symposium is the most important crypto history event in the world. This time I will be featured with even two presentations. As mentioned in my last post, one of my talks will be about steganography in the First World War. The other one will cover Western German encryption technology in the early Cold War.


Random numbers in the Cold War

The Cold War is a very interesting topic for crypto historians. Among other things, the early years of the Cold War are considered as the time when, after several millenia, codemakers gained the upper hand over codebreakers. Many of the ciphers developed in the 1950s and later are so secure that it might never be possible to break them – even with the best computers.

However, the Cold War is also a difficult topic for crypto historians. Much of the information is still classified. Nevertheless, I found more than enough information about Western German ciphers in the early Cold War (covering the late Cold War would have been much more difficult). One of my main sources of information was the former director of the Western German cipher authority, Dr. Otto Leiberich (1927-2015), whom I had the pleasure to interview several times. To my regret, Dr. Leiberich died two years ago.

According to Otto Leiberich, the Western German cryptologists in the 1950s chose to use the One Time Pad wherever possible. The One Time Pad is a cipher that uses a key that is as long as the cleartext with the encryption simply being an addition of key and cleartext. The One Time Pad is unbreakable, provided that the key cannot be guessed. Using the One Time Pad requires a lot of random numbers (or random letters, which is the same for a cryptologist) to be generated, which are used as keys.

Here’s an early random number generator the Western Germans used in the early Cold War:


The Reihenschieber, which I covered in another blog post, is a Western German random number generator, too:


The following device named Hazardo is another Western German random number generator from the 1950s:


ACP 212

For using the OTP the Western German cryptologists developed a number of devices and forms. About 20 years ago a pretty simple One Time Pad encryption device used in Western Germany was on display at an exhibition held at the BSI (the German IT security authority) in Bonn. It is named “ACP 212”.

The only photograph of the ACP 212 I have ever seen is shown on page 430 of my book Codeknacker gegen Codemacher (second edition). For copyright reasons I omitted this picture in the third (and latest) edition. For the same reason I can’t show this photograph on my blog. Click on this link to get to the book (including the ACP photograph) on Google Books.

The following drawing shows how the ACP 212 looks like:


The only online resource I found about the ACP 212 is a page by the Imperial War Museum in London. This page contains a short description (but no picture):

Square aluminium frame with hinged lid for holding material to be enciphered, fitted with sliding metal bar with letters of the alphabet printed on it in red.On the left hand side of the bar is an aperture labelled “Key”. Three further apertures on the frame are labelled “B” (top and bottom left) and “A” (top right)

1 / 2 / Auf einer Seite lesen

Kommentare (9)

  1. #1 David Wilson
    5. Oktober 2017

    Apparently it’s in the Imperial War Museum, but it’s not pictured on their web site.:


    I’d contact them if you want more information.

  2. #2 Thomas
    5. Oktober 2017

    Was it used in practice? Yes, for the NATO General Cipher since 1952: https://archives.nato.int/provision-of-secure-means-for-second-level-communications-within-nato-forces-4

  3. #3 Thomas
    6. Oktober 2017

    The answers you can find in this NATO document: https://archives.nato.int/cryprographic-systems-for-merchant-ships-in-time-of-war

  4. #4 Thomas
    6. Oktober 2017

    The device was named after the ACP (Allied Communication Publication) 212 and was used for the NATO General Cipher and as a cryptographic System for merchant ships in time of war. The instructions are given in ACP 213 (starting on page 1 -1): https://archives.nato.int/uploads/r/null/1/1/114422/SG_171_1_ENG_PDP.pdf (unfortunately I can´t copy and paste the PDF).

  5. #5 Thomas
    6. Oktober 2017

    Was it really a German device? The NATO document:
    “the Military Representatives Committee approved the adoptions for NATO use of the Merchant Ships Crypto Systems (MERSEX) as set forth in ACP 213, Merchant Ship Cipher Instructions, copy attached , MERSEX has been made available for this
    use by the United Kingdom and the United States.
    2. The United Kingdom and the United States are requested to supply the other Member Nations now with ACPs 212 and 213”

  6. #6 Gerd
    6. Oktober 2017

    @Thomas: Oje, wenn damals SOLCHE Schreibmaschinen zur Verfügung standen, ist klar, dass ein man da kein aufwändigeres Chiffriergerät als das ACP 212 einführen konnte…

  7. #7 Klaus Schmeh
    6. Oktober 2017

    @Thomas: Thank you very much! This information is very helpful. Apparently the ACP 212 was a NATO encryption device, which was also used by the Germans.

  8. #8 Breaker
    12. Oktober 2017

    A strange thing happened again Klaus, you posted another kryptos key.decoding tool that perhaps holds a variance to the puzzle’s second layer……

    The rearrangement of the vigenere tableau forms an extra segment of 2 lines and a side panel section.

    I believe that the slide rule methods are the intended theme but that the foundations of this type of crypto tool that you showed is based on the original ideas based on the rules of cryptography.

    Below is the segment that I feel is related to these tools that is found from operating a ‘t square’ shape of lettering across a vigenere tableau for an additional layer of encryption.


  9. #9 Christopher Howell
    11. September 2021

    When I was in the British Royal Navy, I used this frame in anger. The pictures given above are not quite correct but does give a good representation. Used by the NATO, UK and Merchant Services it was a fairly good but slow method of cryptography. Would have been limited to approximately 250 five letter groups.