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.

The method determined the rightmost rotor in the German Enigma by exploiting the different turnover positions.

In contrast, the clock method involved simple tests that were unaffected by the plugboard.

[4] 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.

"[5] The Cipher Bureau received German radio intercepts enciphered by the Enigma machine.

If the message pairs cooperated, the Poles could narrow the window where the turnover happens to include only one rotor.

One message pair might say the turnover happened in the window B to U; that meant rotors I (R), II (F), and IV (K) were viable.

A second message pair might produce a window of M to C; that meant rotors I (R), III (W), V+ (A) were viable.

The Enigma cipher machine relied on the users having some shared secrets.

In a grave procedural mistake, the Germans encrypted the message key twice.

The Poles had to use other techniques to get those remaining secrets; the clock method helped determine the rotor order.

The clock method exploited the three rotors (I, II, III) having different turnover positions.

[13] With sufficient traffic, the Poles would find message keys that started with the same two characters.

To emphasize the index of coincidence to an absurd level, the two example messages above consist entirely of the letter "A", so the coincidences occur at every position that shares the same rotor positions (something that would not happen for normal messages).

In practice, long messages are needed to get a good statistical indication.

The Poles searched the daily traffic to find a pair of messages whose keys started the same two letters.

The Poles wanted the first two letters to match because that meant the left and middle rotors were at identical rotations and would produce the same permutation.

[14] In October 1936, the Germans increased the number of plugs from six to eight, and that complicated the grill method.

[16] The change would complicate the clock method because the message key was no longer easily known.

Correctly guessing the last rotor could save the British a lot of valuable Bombe time.