Period (cryptology)
In cryptology , especially with the polyalphabetic substitution method, the period is the number of characters after which the alphabet used for encryption is repeated.
Examples
Vigenère encryption
With the classic encryption method of Vigenère encryption , a total of 26 different alphabets are available according to the number of letters of the usual Latin alphabet , which can be arranged in the form of a classic tabula recta , for example, and some of which are selected by the keyword . The length of the keyword determines the number of alphabets used and thus the encryption period. Long passwords result in long periods, which benefits the cryptographic security of the method against unauthorized decryption . The Friedman test can be used to break (“crack”) the encryption. It tries to determine the length of the period using the coincidence index as the first step in decoding.
Enigma machine
The period length of the German key machine ENIGMA I is 26 · 25 · 26 = 16,900 characters. This results from the number of rollers used - usually three were used - and the number of letters on each roller, the factor 25 for the middle roller being caused by an (unimportant) anomaly in the indexing mechanism . The ENIGMA was well protected against cryptanalytic attacks with the help of the Friedman test due to its relatively large period compared to the prescribed maximum length of the radio messages of 250 letters.
M-209
In contrast to the Enigma, the American M-209 contained six key rotors (and not just three or at most four like the Enigma-M4 ). Also different from the German machine, these were divided differently (26, 25, 23, 21, 19 and 17). These numbers were chosen deliberately coprime , which resulted in the product 26 · 25 · 23 · 21 · 19 · 17 = 101,405,850.
literature
- Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, ISBN 3-540-67931-6 .