Cyclometer

The cyclometer was a cryptologic device designed, "probably in 1934 or 1935," by Marian Rejewski of the Polish Cipher Bureau's German section (BS-4) to facilitate decryption of German Enigma ciphertext.

Frode Weierud provides the procedure, secret settings, and results that were used in a 1930 German technical manual.

The first line of the message is not encrypted. The "1035" is the time, "90" is number of characters encrypted under the message key, and "341" is a system indicator that tells the recipient how the message was encrypted (i.e., using Enigma with a certain daily key). The first six letters in the body ("PKPJXI") are the doubled key ("ABLABL") encrypted using the daily key settings and starting the encryption at the ground setting/Grundstellung "FOL". The recipient would decipher the first six letters to recover the message key ("ABL"); he would then set the machine's rotors to "ABL" and decipher the remaining 90 characters. Notice that the Enigma does not have numerals, punctuation, or umlauts. Numbers were spelled out. Most spaces were ignored; an "X" was used for a period. Umlauts used their alternative spelling with a trailing "e". Some abbreviations were used: a "Q" was used for "CH".

Marian Rejewski was a mathematics student at Poznań University. During that time, the Polish Cipher Bureau recruited Rejewski and some other mathematics students including Jerzy Różycki and Henryk Zygalski to take a Bureau-sponsored course on cryptology. The Bureau later hired some of the students to work part-time at a local branch of Bureau. Rejewski left Poznań to study mathematics at University of Göttingen, but after a year he returned to Poznań. In September 1932, Rejewski, Różycki, and Zygalski went to Warsaw and started working for the Polish Cipher Bureau full-time.

During December 1932, Marian Rejewski was tasked by the Cipher Bureau to work on the German Enigma. The Bureau had attempted to break a few years earlier, but had failed. Within a few weeks, Rejewski had discovered how to break the German Enigma cipher machine. The German Enigma message procedures at the time used common but secret daily machine settings, but the procedures also had each code clerk choose a three-letter message key. For example, a clerk might choose "ABL" as the message key. The message key was used to set the initial position of the rotors when encrypting (or decrypting) the body of the message. Choosing a different message key was a security measure: it avoided having all the day's messages sent using the same polyalphabetic key which would have made the messages vulnerable to a polyalphabetic attack. However, the sender needed to communicate the message key to the recipient in order for the recipient to decrypt the message. The message key was first encrypted using the day's Grundstellung (a secret initial position of the Enigma's rotors, e.g., "FOL").

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