What is a frequency analysis? When did letter frequency analysis become important? In cryptanalysis, frequency analysis (also known as counting letters) is the study of the frequency of letters or groups of letters in a ciphertext.

The method is used as an aid to breaking classical ciphers. The most ancient description for what we know was made by Al-Kindi, dating back to the IXth century. The letter frequency analysis was made to decrypt ciphers such as monoalphabetical ciphers , for instance Caesar cipher , which means that frequency analysis could have been used before Al-Kindi. Letter frequency is simply the amount of times letters of the alphabet appear on average in written language.
How to use frequency analysis? The most obvious trait that letters have is the frequency with which they appear in a language. Frequency analysis is the study of the distribution of the letters in a text. One way to tell if you have a transposition style of cipher instead of an encrypting method is to perform a letter frequency analysis on the ciphertext. To use this tool, just copy your text into the top box and a chart showing letter frequency will be generated in the bottom.
This frequency analysis tool can analyze unigrams (single letters), bigrams (two-letters-groups, also called digraphs), trigrams (three- letter -groups, also called trigraphs), or longer. (more…)