I've just returned from a two-semester exchange at the Charles University[1] of Prague. There I had the chance to take a course about classical cryptography, for which I wrote an essay that might be interesting to others:

Everybody knows about the Enigma machine[2]. It was a cryptographic device developed by the Germans, used extensively by Hitler's troops, and famously broken by the allies at Bletchley Park. It was an "offline" device in that the plaintext was keyed in letter by letter, indicating the corresponding ciphertext character with each keystroke. Now, what many are not aware of is that the Germans also used a series of online crypto devices during WWII. One of these was the Siemens T52[3], a.k.a. the "Geheimschreiber". This machine was an actual teleprinter with built-in automatic encryption and decryption, i.e. it encrypted and transmitted the characters as they were typed (or fed via punched tape), and vice-versa. It's cryptographic algorithm is considered significantly more sophisticated than that of the Enigma.

When Germany leased several telegraph lines from Sweden they were immediately tapped but the T52 was soon employed on them. My essay[4] describes how the Swedish cryptanalyst Arne Beurling broke the encryption without ever having seen one of the machines. It's mainly a condensed form of the description from "Codebreakers"[5], with less history and more mathematics.

Enjoy.

References:

Update: Typesetting a reference list in Advogato

For those of you who, like me, are not completely fluent in CSS (and maybe a little stuck with HTML wisdom of the 90's), here's how you produce hanging indents as in the reference list above:

1. Somehow determine the width of your left column. For the reference list, I took the maximum number of characters in it plus one, times the width of the 'x' character (ex): "[n]:" are four characters, so I use 5ex.
2. Put the left column text into a <div> and give it the attribute 'style="float: left"', so that the following text will flow down to the right of it.
3. Put the right column text into another <div> and give it the attribute 'style="margin-left: 5ex"' (replacing 5ex with whatever left-column width you want). This results in the hanging indent.

<div style="float: left">[1]:</div>
<div style="margin-left: 5ex">
  "Univerzita Karlova v Praze"<br>
  <a href="http://www.cuni.cz">http://www.cuni.cz/</a>
</div>

For a reference list, it also looks good to indent the whole thing a little by putting it in another <div style="padding: 1ex">.

Enjoy