What is frequency analysis in cryptanalysis?

Frequency analysis is the study of the frequency of letters or letter groups in a ciphertext and serves as a method to aid in breaking classical ciphers. It relies on the principle that written languages have characteristic, roughly uniform letter distributions, such as the high frequency of E, T, A, and O in English. When ciphers preserve these natural language patterns, analysts can exploit them through ciphertext-only attacks.

What we know

• Frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext to help break classical ciphers. [1, 2]
• The method is based on the fact that written languages exhibit characteristic letter distributions that remain roughly the same across samples. [2]
• In English text, letters such as E, T, A, and O are most common, while Z, Q, X, and J are rare. [2]
• Common bigrams in English include TH, ER, ON, and AN, while frequent repeats include SS, EE, TT, and FF. [2]

Where to go next

• a case in pointHow did Bletchley Park break the Enigma machine?
• how it workedWhat role did Colossus play in decoding the Lorenz cipher?
• the bigger pictureHow have cryptanalytic methods evolved over time?
• compared withWhat are side-channel attacks in modern security?
• whoWho was Alan Turing and how did he contribute to decryption?
• an unexpected connectionHow did teleprinter stream ciphers connect to early programmable computers?The complex challenges of decrypting high-level German teleprinter traffic led directly to the creation of Colossus, the world's first electronic, programmable digital computer.

Sources

• [1]frequency analysis (Wikidata Q565636) · CC0 1.0
• [2]Frequency analysis · CC BY-SA 4.0
• [3]Cryptanalysis · CC BY-SA 4.0
• [4]Cryptanalysis of the Enigma · CC BY-SA 4.0
• [5]Cryptanalysis of the Lorenz cipher · CC BY-SA 4.0