Frequency Analysis

Introduction

A ninth-century scholar, al-Kindi, wrote the first surviving account of breaking a cypher by counting letters. It remains the foundation of classical cryptanalysis.

Tally the cypher, line the tallies up against English, and the tallest bars point at E, T and A. Bigrams and trigrams do the rest.

Key ideaCompare the counts in the cypher with the counts of ordinary English and the mapping falls out.