Clock (cryptography)

In cryptography, the clock was a method devised by Polish mathematician-cryptologist Jerzy Różycki, at the Polish General Staff's Cipher Bureau, to facilitate decrypting German Enigma ciphers.

This method sometimes made it possible to determine which of the Enigma machine's rotors was at the far right, that is, in the position where the rotor always revolved at every depression of a key.

Różycki's "clock" method was later elaborated by the British cryptologist Alan Turing at Bletchley Park in the development of a cryptological technique called "Banburismus."

The Enigma cipher machine relied on the users having some shared secrets. Here are the secret daily settings from a 1930 Enigma manual:

The daily settings told the code clerks how to configure the machine so message could be exchanged. Initially, the machine had three rotors that could be arranged in any order (the wheel order or rotor order). Each rotor had a ring with numbers or letters on it, and that ring could be in any of 26 positions. A plugboard interchanged additional characters.

For each message, the operator would choose a three-letter message key to encrypt the body of the message. The intention was for this key to be random, and using a random key for each message was a good security practice. The message key needed to be communicated to the recipient so the recipient could decrypt the message.

Instead of sending the message keys in the clear, the message keys would be encrypted with the Grundstellung (ground setting). In a grave procedural mistake, the Germans encrypted the message key twice. If the message key were "ABL", then the Germans would encrypt the doubled key "ABLABL" and send the result ("PKPJXI"). Sending the message key twice allowed keys garbled in transmission to be recovered, but the cryptographic mistake was encrypting the doubled key rather than sending the encrypted key twice (e.g., "PKPPKP"). The doubled key gave the Poles an attack. If there were sufficient message traffic using the same daily key (about 70 messages) and the code clerks used weak keys (such as "CCC" or "WER"), then the Poles could use Rejewski's method of characteristics to determine all the day's message keys. Surprisingly, the Poles cracked the message keys without learning the substantial secrets of the daily machine settings: the plugboard settings, the rotor order, the rotor positions, or the ring settings.

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