Vigenère Cipher Encoder / Decoder

Encrypt or decrypt text with a keyword using the Vigenère cipher.

Keyword · stays on your device, never in the shared link

Input

Ciphertext

LXFOPVEFRNHR

About the Vigenère cipher

The Vigenère cipher is a polyalphabetic substitution cipher: instead of a single fixed shift, it repeats a keyword across the message and shifts each letter by the position of the matching key letter. With the key LEMON, ATTACKATDAWN encrypts to LXFOPVEFRNHR. For centuries it resisted casual code-breaking, earning the nickname le chiffre indéchiffrable. Only letters are enciphered — spaces, digits, and punctuation pass through, and the key only advances on letters.

For the simpler single-shift version, see the Caesar cipher or ROT13. All enciphering happens locally in your browser.

Built and maintained by Meet Shah · Last updated 25 July 2026

What this tool is used for

Working through a classical cipher exercise where a repeating keyword is the mechanism under study.
Solving a puzzle whose ciphertext resists single-letter frequency analysis but has a short period.
Demonstrating why a repeating key is the weakness rather than the strength of the scheme.
Reproducing a historical message against a published keyword.
Showing the contrast between a repeating key and a genuine one-time pad.

Frequently Asked Questions

How does the cipher work?: Each letter is shifted by the corresponding letter of a repeating keyword: with key CAT, the shifts are 2, 0, 19, 2, 0, 19… So the same plaintext letter encrypts differently depending on position, which is what defeats simple letter-frequency analysis.
Why was it called "le chiffre indéchiffrable"?: Because for roughly three centuries nobody published a general break. Described by Bellaso in 1553 and misattributed to Vigenère, it held until Babbage broke it privately in the 1850s and Kasiski published a method in 1863.
How is it broken?: First find the key length — repeated ciphertext sequences tend to be a multiple of it apart (Kasiski), or the index of coincidence reveals it. Then the message splits into that many simple Caesar ciphers, each of which falls to frequency analysis immediately.
What makes a one-time pad different?: The key is as long as the message, random, and never reused. That removes the repetition Vigenère depends on, and it is the only cipher with a proof of perfect secrecy. Reuse a one-time pad even once and it degrades straight back into this.
Is it usable today?: Only as a puzzle or a teaching example — a few hundred characters of ciphertext is enough to break it with software in seconds. It is genuinely valuable for understanding *why* modern ciphers avoid key repetition.
What is a running-key variant?: One where the key is a long text — a book passage — rather than a repeated word, removing the period that Kasiski examination exploits. It is stronger, but still breakable, because natural-language keys are themselves predictable.

Common errors and gotchas

Reusing a short keyword on a long message, which is exactly what Kasiski examination and the index of coincidence exploit.
Expecting non-letters to be enciphered. Punctuation and spaces usually pass through, and they leak the word boundaries.
Treating it as unbreakable because of its historical nickname. It has been broken for well over a century.
Losing the key. Without it the ciphertext is recoverable only by analysis, and that needs a decent quantity of text.
Confusing it with a one-time pad, whose security depends on a key as long as the message and never reused.