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.

Tags: KL-7 , Rotor machine , Cryptography

Heilbronn Institute

The ominously-named Heilbronn Institute is due to open next month. It's being established as a partnership between Bristol University and a obscure little British intelligence agency known as GCHQ. Apparently, the institute will be pursuing a research programme "into key areas of mathematics of interest to GCHQ".

Each researcher will get to spend half their time on their own personal stuff, and the other half working on GCHQ projects. The director is Elmer Rees (one of the world's leading mathematicians working in the field geometry, according to Wikipedia), and the deputy director is Richard Pinch, a "civil servant living in Cheltenham", albeit a civil servant with research interests in "computational number theory, the arithmetic of elliptic curves, algebraic combinatorics and public-key cryptography".

Tags: Cryptography , GCHQ

An illustrated guide to IPsec

"An Illustrated Guide to IPsec" is an excellent introduction to IPsec, written by software/network consultant Steve Friedl. I've read a number of overviews of IPsec recently, and this is by far the clearest explanation I've found of the basic AH/ESP, Tunnel/Transport Mode stuff. A little while back Steve wrote "An Illustrated Guide to Cryptographic Hashes", which was similarly lucid and helpful. Hopefully, he'll be writing some more crypto tech-tips soon.

While the article is a good explanation of the basics of what IPsec does, sadly, the article doesn't really cover why IPsec is the way it is. I'd like to read an "apology" for IPsec, explaining the somewhat complex, convoluted and perplexing architecture, and what the expected uses are for the various modes.

Tags: Cryptography , IPsec