Vigenere Cipher Encoder / Decoder

How to use

1. 1Pick Encode to convert plaintext into cipher text, or Decode to recover plaintext from cipher text. Use Swap direction to flip quickly.
2. 2Choose a variant: Standard Vigenere is the classical algorithm, Beaufort is self-reciprocal (encode equals decode), and Autokey extends the key with the plaintext.
3. 3Enter a keyword (letters only; case does not matter). LEMON, KEY, and FORTIFICATION are common textbook examples.
4. 4Paste your text. Optionally toggle Strip non-letters to clean the input, or Group output in blocks of 5 for the classic ciphertext layout.
5. 5Read the result with character and key-length stats, check the key alignment preview, and click Copy result. Use Use result as input to round-trip back through the cipher and verify the encode and decode pair.

About this tool

Vigenere Cipher Encoder / Decoder is a complete, browser-only toolkit for the classical polyalphabetic cipher that resisted cryptanalysis for three centuries. Pick a keyword of any length, type your message, and the tool shifts each plaintext letter forward by the value of the corresponding key letter (A is shift 0, B is shift 1, all the way to Z is shift 25). The key is repeated under the plaintext as long as needed, so a five-letter key recycles every five positions of the input. Decoding runs the same key in reverse to recover the original message. Three cipher variants are built in. Standard Vigenere is the historical algorithm from the 16th century, the one Blaise de Vigenere is named after and the one CTF organisers, escape rooms, and cryptography classes still reach for. The Beaufort variant subtracts the plaintext letter from the key letter, which makes it self-reciprocal: the same operation encodes and decodes, no mode toggle required. Autokey Vigenere extends the key with the plaintext itself after the keyword runs out, so the key stream never repeats and Kasiski analysis no longer cracks it the way it cracks the standard variant. Case is preserved automatically so Hello with key KEY becomes Rijvs rather than RIJVS. Non-letter characters (digits, spaces, punctuation) pass through untouched by default, which keeps line breaks, paragraph structure, and number tokens intact so the cipher text is readable. Optional toggles cover the classical conventions: a Strip non-letters option removes spaces and punctuation before encrypting (the textbook convention), and a Group output in blocks of five inserts a space every five letters so the ciphertext looks the way cryptanalysts expect to see it. A live key alignment preview under the result shows the first 40 positions of input with the key letter applied at each position, so you can visually confirm the cipher is wired correctly before you trust the output. Sample buttons load known plaintext and known ciphertext for one-click verification, including the famous LEMON keyword example and an autokey demonstration with FORTIFICATION. Useful for school cryptography assignments, capture-the-flag puzzles, escape-room and ARG clue construction, geocaching, programming interview practice, history reenactment, novel writing where a character needs to send a secret note, and any time you encounter a string of jumbled letters that look like a Caesar cipher but is not breakable by a single shift. Everything from the modular arithmetic to the key alignment preview runs locally in your browser, so the messages you encrypt or decrypt here never leave your device.

Free to use. Works in your browser. No signup, no login.