Cryptanalyst’s Index of Coincidence
Index of coincidence measures how likely it is to draw two matching letters by choosing two random letters from a text.
This article is a work in progress.
Given a text of length using an alphabet consisting of letters and given the number of appearances for each letter , the Index of coincidence (IC) is defined by the sum :
The normalized Index of coincidence can be obtained by multiplying the above by .
- Friedman, William F. (1922). "The index of coincidence and its applications in cryptography." Department of Ciphers. Publ 22. Geneva, Illinois, USA: Riverbank Laboratories. Download