Editing Vigenère cipher (section)

===Frequency analysis===
{{Main|Frequency analysis}}
Once the length of the key is known, the ciphertext can be rewritten into that many columns, with each column corresponding to a single letter of the key. Each column consists of plaintext that has been encrypted by a single [[Caesar cipher]]. The Caesar key (shift) is just the letter of the Vigenère key that was used for that column. Using methods similar to those used to break the Caesar cipher, the letters in the ciphertext can be discovered.

An improvement to the Kasiski examination, known as [[Auguste Kerckhoffs|Kerckhoffs]]' method, matches each column's letter frequencies to shifted plaintext frequencies to discover the key letter (Caesar shift) for that column. Once every letter in the key is known, all the cryptanalyst has to do is to decrypt the ciphertext and reveal the plaintext.<ref>{{cite web |url=http://courses.umass.edu/cs415/labs/lab1/415-lab1-crypto.pdf |title=Lab exercise: Vigenere, RSA, DES, and Authentication Protocols |access-date=2006-11-10 |work=CS 415: Computer and Network Security |archive-url=https://web.archive.org/web/20110723140549/http://courses.umass.edu/cs415/labs/lab1/415-lab1-crypto.pdf |archive-date=2011-07-23 |url-status=dead }}</ref> Kerckhoffs' method is not applicable if the Vigenère table has been scrambled, rather than using normal alphabetic sequences, but Kasiski examination and coincidence tests can still be used to determine key length.