27 September, 2005

KL-7 rotor machine on the HMS Belfast

I was recently in London and had chance to spend a few hours on board HMS Belfast, a retired British warship now serving as a floating museum on the Thames. The ship was pretty fascinating (that is, it had big guns), but my main motivation for visiting was to have a look at their KL-7 cipher machine. The KL-7 was an advanced NSA rotor machine design (think "souped-up Enigma") introduced in the 1950s and used within NATO.

Although some details are still classified, quite a lot of information has emerged about how the machine worked. We (that is, the plebs with no security clearance) know that it used a scrambler consisting of 8 rotors, each with 36 contacts. It appears that some of the contacts were used in a "loopback" arrangement; that is, 10 of the contacts at the end of the scrambler were wired to 10 of the contacts at the start of the scrambler. This meant that the machine operated on a 26-letter alphabet, but that some outputs represented current which had passed several times through the scrambler. Here's a diagram of how this sort of thing works:

This feature would make cryptanalysis much more complex, as an attacker would have to take into account that an output could represent one, two, or more passes through the rotors.

One detail that's not yet known is precisely how the rotors stepped. We do know that one of the middle rotors was stationary during encipherment, and that each rotor had a detachable ring of plastic around its circumference with a series of bumpy bits. These operated microswitches to control the stepping.

While it was great to see a KL-7 exhibited on board the Belfast, I was a little miffed to find that it was poorly illuminated and, more damningly, that it was not even labelled.


Blogger Dirk Rijmenants said...

Hi Matt,

Some details are still classified??? Regretfull, for such a historical nice machine. Maybe one of my future projects, if the spooks allow me. Currently researching on then CX-52 for my next sim, but technical - read mechanical - details are few. Any tip welcome!

8:12 pm  
Anonymous Anonymous said...

Matt - v. interesting about the KL-7. I often used to visit HMS Belfast when I lived in London but before I became interested in cryptography and the associated machinery. I must go again when I'm next up in town.

I have long been curious about two historical aspects of cryptography: firstly the cryptanalysis techniques and machines used by the Germans to crack Allied codes in WW2 (Goering's Forschungsamt and the Foreign Ministry's Pers Z department had significant successes) and secondly the cryptographic techniques and kit of the Allies (eg, the UK's TypeX). But just as history is written by the victors, so they also determine what remains secret. At least the USofA are relatively more enlightened than us Brits - if only possibly in this respect!

I have looked for publications and web info on both these areas and there are precious few sources of information. Perhaps you or another reader of your blog have had more luck .. I would be interested to know.


9:39 am  
Blogger Matt Crypto said...


At about the same time I visited the HMS Belfast, I spent a couple of days in the National Archives (PRO) at Kew reading up on Typex. One document was an operating manual for the Typex Mk VI, accompained by a superb set of photographs and diagrams. I also read a little about German PoW reports that Typex was being read in North Africa for a brief period. The British investigation was inconclusive, but it seems the Germans were indeed reading a little Typex, but whether this was due to cryptanalysis or to capture of material is unknown (they definitely some captured Typex equipment; one PoW gave an accurate description of a Typex Mk II).

I don't know how much info you already have on Typex, but I can send you the references I have, if you like.

6:14 pm  
Anonymous Anonymous said...

Matt - Thanks for your reply to my comments. Yes, the references would be useful if you have time to send them to me. I am still hankering after more info on Axis cryptanalysis (I am saving up for the 'German Naval Codebreakers' book!). I am dipping into a beat-up copy of David Kahn's 'Hitler's Spies' and am constantly frustrated by the absence of an index and a total lack of references! Yet there are tantalising references in this and 'The Codebreakers' to the codes the Germans broke and the aids they had to do so .. v. frustrating ..

My email is patrick at kinkajouknowhow dot co dot uk


3:37 pm  
Anonymous David C. said...

I can't believe I'm reading all of this information about the KL7 on the internet. I was trained to maintain this unique machine back in 1968. In the Air Force, this item was maintained by AFSC 363X0 personnel, (Communications Relay Center Equipment Repairmen, Electrro/Mechanical/Crypto). My training took place at Sheppard AFB, Texas. Security into and out of the "Vault" was stringent, and security was emphasised and strickly enforced. We were instructed not to discuss anything about the KL7, not even periferally. Certification on the KL7 was an annual requirement, and could only be performed at certain location in CONUS. The last time I saw one of these was at Andrews AFB for my certification back in 1972. I would love to put my hands on one of these again. I would even consider buying one as a momento.

10:49 am  
Blogger Anchor Face said...

I have to agree with David C.
I served in the RN Comms Branch in the 60,s. I used a KL7 every day when at sea. All Comms staff had to sign the official secrets act.We were not allowed to talk about our job, outside of the work place. I'm amazed that a piece of kit like the KL7, is on view on the internet. Too much military confidential information is now gifted to our enemies.Unforgivable !!! Steve H

9:40 pm  
Blogger David Perry said...

I too was a RN Radio Operator in the late sixties early seventies. Our KL7s looked a bit more....?? Modern than that one on the Belfast.
~The job I found most tedious was setting the machine up for the next day. Tedious and one mistake and you'd end up having to take the whole lot apart to find out which rota you'd set wrong.

As for it being on show. Does it matter? It isn't used now and what can you possibly do if you know how it works anyway?

7:11 pm  
Blogger mjmuk said...

Came across this in relation to a Bletchley Park story search. I also worked on these devices in the RN in late 60's. They have now been superseded by much more sophisticated devices, can't say too much as I would have to terminate you!!!! However, they were pretty hush hush at the time.

4:06 pm  
Blogger mjmuk said...

Another bit of background and resource if you have not seen it already.

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12:35 pm  
Anonymous Anonymous said...

As a student at Scott AFB learning
the KL 7 we had to wear a nametag
with a red circle so nobody would talk to us.

5:21 pm  
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