Frequency Analysis

Statistics for a ciphertext: letter, bigram and trigram frequencies against 12 language references, Shannon entropy and chi-squared, index of coincidence with a key-length estimate, Kasiski spacings of repeated trigrams, and coincidence by lag with per-column IC. Frequency analysis goes back to al-Kindi's ninth-century treatise; F. W. Kasiski published the repeat method in 1863; William Friedman introduced the index of coincidence in 1922.

• Statistics for a ciphertext: letter, bigram and trigram frequencies against 12 language references, Shannon entropy, index of coincidence with a key-length estimate, Kasiski spacings, coincidence by lag, and counts of words, characters and sentences. Frequency analysis goes back to al-Kindi's ninth-century treatise; F. W. Kasiski published the repeat method in 1863; William Friedman introduced the index of coincidence in 1922.
• David Kahn, “Kasiski, Friedrich W.”, Neue Deutsche Biographie 11 (1977), p. 317 — on Die Geheimschriften und die Dechiffrir-Kunst (1863)
• William F. Friedman, The Index of Coincidence and Its Applications in Cryptography, Riverbank Publication No. 22 (Geneva, Illinois: Riverbank Laboratories, 1922)