Vigenère Cipher
Encode or decode with a keyword — classic Vigenère, Beaufort and autokey variants — with the text, repeated key and result lined up letter by letter.
The ciphertext or plaintext with case and punctuation kept, the cleaned key, the letter count, and the three-line alignment of text, key stream and output.
Example: ATTACKATDAWN with key LEMON → LXFOPVEFRNHR (key stream LEMONLEMONLE); Beaufort gives LLTOLBETLNPR and is its own inverse; autokey with QUEENLY → QNXEPVYTWTWP.
One Caesar shift
per key letter.
How the keyword drives the shifts, how Beaufort and autokey differ, and what the alignment shows.
Vigenère
The key is cleaned to letters A–Z and repeated under the text; each text letter is shifted by the position of the key letter beneath it (A = 0 … Z = 25): c = (p + k) mod 26. Decoding subtracts instead. Only letters take part — spaces, digits and punctuation pass through and do not use up a key letter — and the case of each letter is kept.
Beaufort and autokey
Beaufort reverses the arithmetic, c = (k − p) mod 26, which makes it its own inverse: applying it twice with the same key returns the text. Autokey uses the keyword once and then continues the key stream with the plaintext itself, so the key never repeats; when decoding, each recovered letter feeds the key stream in turn.
Reading the alignment
The three lines show the text, the key stream actually used at each position and the result, so any single letter can be checked on a tabula recta. Vigenère is a classical cipher: the key length shows through repeated patterns (Kasiski examination) and each column is then a Caesar shift. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Vigenère: c = (p + k) mod 26; Beaufort: c = (k − p) mod 26; autokey: the plaintext continues the key stream after the keyword; non-letters pass through without consuming a key letter
Last reviewed 22 September 2026. How results are checked: How we verify.