Index of coincidence
Índice de coincidencia
- Date
- 1920-1922 · confidence high
- Place
- Geneva, Illinois, United States
- Credited to
- William Frederick Friedman
- Field
- Knowledge & symbols
What it is
A statistical cryptanalysis technique that compares how often identical letters coincide at the same position between two texts, allowing estimation of a key's periodicity or distinguishing ciphertext from random text; developed by William F. Friedman at the Riverbank Laboratories.
What it led to
- Fundamental tool of classical cryptanalysis against polyalphabetic ciphers
- Used to break historical ciphers
- Influenced modern statistical cryptanalysis methods