Friedman test (cryptology)
In the cryptology is Friedman test is a method for analysis of a text by polyalphabetic substitution (z. B. algorithm of Vigen'ere ) was encrypted. It can be used to determine the length of the key. It was developed by William Frederick Friedman .
- A both are in the same block position,
- B they are in different block positions.
The probability that two letters of type A are the same is : = 0.0762 (corresponds to the coincidence index for longer German texts). Furthermore, the probability that two letters of type B are the same is equal : = 0.0385 (= 1/26 and corresponds to the uniform distribution).
In every block position there are letters and thus pairs. So the number of pairs of type A is the same
Pairs are of the type B . This gives for the coincidence index
If one now solves for n, the result is
The presumed key length of the code word is then an integer that is close to this estimate .
The Kasiski test is used to find out the key length based on repeated groups of characters.
- Albrecht Beutelspacher : Cryptology. An introduction to the science of encryption, concealment, and concealment. Without any secrecy, but not without deceitful rogue, presented for the benefit and delight of the general public. 2nd considerably expanded and hopefully improved edition. Vieweg, Braunschweig 1991, ISBN 3-528-18990-8 .