Энигма шифрын бұзудағы "Сағат" әдісі

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. For the Poles, learning the rightmost rotor reduced the rotor order search space by a factor of 3 (the number of rotors). The British improved the method, and it allowed them to use their limited number of bombes more effectively (the British confronted 5 to 8 rotors).

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. The clock method was developed by Jerzy Różycki during 1933–1935. Marian Rejewski's grill method could determine the right hand rotor, but that involved trying each possible rotor permutation (there were three rotors at the time) at each of its 26 possible starting rotations. The grill method tests were also complicated by the plugboard settings. In contrast, the clock method involved simple tests that were unaffected by the plugboard. In the early 1930s, determining the rotor order was not a significant burden because the Germans used the same rotor order for three months at a time. The rotor order could be determined once, and then that order could be used for the next three months. On 1 February 1936, the Germans changed the rotor order every month. On 1 November 1936, the Germans changed the rotor order every day. 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 Cipher Bureau received German radio intercepts enciphered by the Enigma machine. With about 60 messages, the Bureau could determine Marian Rejewski's characteristic structure for the message key encoding. By exploiting poor message keys, the Bureau could determine the message key encoding. At that point, the cryptanalysts may know only the message keys and their ciphertext. They may not know the other secrets of the daily key such as the plugboard setting, the ring settings, the rotor order, or the initial setting. With such little information and some luck, the Poles could still determine which rotor was the rightmost. In the daily traffic, there might be about a dozen message pairs whose message key starts with the same two letters. That means the left and middle rotors are in the same position. There are two ways to align the ciphertexts of the message pair. Both alignments are tried; one of the alignments will use an identical polyalphabetic substitution. From that, the cryptanalyst can determine the rotor turnover happened within a particular range of letters. The rotors had different turnover positions. The British used the mnemonic "Royal Flags Wave Kings Above", which meant Rotor I turned over at R, Rotor II turned over at F, Rotor III turned over at W, Rotor IV turned over at K, and all other rotors turned over at A. 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. Only Rotor I satisfies both message pairs, so Rotor I is the right hand rotor.

Early on, the clock method was not very important. In 1932, the Germans kept the same rotor order for three months at a time. On 1 February 1936, the Germans changed the rotor order every month. Daily wheel order changes started 1 November 1936. In October 1936, the Germans increased the number of plugs from six to eight, and that complicated the grill method. The Poles developed the cyclometer and card catalog. Although the new method was not ready for a year, it identified the entire rotor order (not just the right rotor) with little work. Unfortunately, the catalog was rendered useless on 2 November 1937 when the Germans changed the reflector; a new catalog needed to be made. On 15 September 1938, the Germans changed their procedures so that the messages on a network did not use the same Grundstellung. The change would complicate the clock method because the message key was no longer easily known. The British codebreakers extended the clock method; see Banburismus. German naval Enigma messages used the same Grundstellung, and the British codebreakers could determine the encrypted message keys. If all but the final letter of the encrypted keys matched, then they would have the same rotor positions except for the right rotor. The problem was the British were not matching plaintext message keys (as the Poles) but rather encrypted message keys, so the last letter of the encrypted message key did not have a natural "ABCDE WXYZ" ordering but rather an arbitrary order. Rather than looking at just two offset, the British had to look at all the possible offsets and infer enough of the third wheel order before they could determine the right rotor. Correctly guessing the last rotor could save the British a lot of valuable Bombe time.