Contact analysis
Contact analysis is a cryptanalysis method that looks at how often certain symbols appear next to each other. It helps crack old ciphers by using patterns in language: in any language, some symbols tend to neighbor specific others, and these patterns stay fairly consistent across texts even if overall symbol frequencies vary. This can hold true whether you’re using letters or whole words.
Some ciphers keep these natural language patterns in the ciphertext, so they can be exploited even when you only have the ciphertext. Unlike basic frequency analysis, which counts how often a symbol appears, contact analysis studies conditional frequencies—how often a symbol follows a given symbol, or how often a sequence occurs given the preceding symbols. For example, what is the chance the next letter is B after an A, or the chance of C after the pair A, B? It can also describe probabilities like the next symbol belonging to one set given that the previous symbol belonged to another. In short, contact analysis uses second- or third-order statistics rather than just first-order frequencies.
This page was last edited on 2 February 2026, at 16:15 (CET).