Productivity and Utilities

Vigenere Cipher Tool

Encrypt or decrypt text with a keyword Vigenere cipher, a historical puzzle cipher.

The Vigenere cipher is a historical cipher from the 1500s that was broken long ago. Use it for puzzles, teaching and games, never to protect real information. For modern encryption see the AES text tool.

Direction

Key word

Letters A to Z only. Other characters in the key are ignored.

Text

Up to 500,000 characters. Nothing is uploaded.

Uppercase letters only, in groups of five

Result

Each letter is shifted forward (or back, to decrypt) by the position of the matching key letter, A = 0 to Z = 25, and the key repeats. Capital and small letters keep their case. Spaces, digits, punctuation and letters outside A to Z, such as accented letters, pass through unchanged and do not use up a key letter. The result is 12 letters for the classic example ATTACKATDAWN with the key LEMON: LXFOPVEFRNHR.

Next step

Caesar CipherTry the simpler cipher that shifts every letter by the same amount.
Encrypt and Decrypt Text (AES)Encrypt text with modern encryption instead of a puzzle cipher.
NATO Phonetic Alphabet ConverterSpell words out with the phonetic alphabet.

How to use the vigenere cipher tool

Choose Encrypt or Decrypt.
Type a key word made of letters. Only letters A to Z are used.
Type or paste your text. Capital and small letters keep their case.
Copy the result. Tick the option for uppercase groups of five if you want the classic cipher look.

ATTACK AT DAWN with the key LEMON

The key repeats along the letters: LEMONLEMONLE. A + L = L, T + E = X, T + M = F, A + O = O, C + N = P, K + L = V, and so on, where each letter is shifted by the key letter's position, A = 0 to Z = 25.

The text ATTACKATDAWN becomes LXFOPVEFRNHR. With spaces kept, ATTACK AT DAWN becomes LXFOPV EF RNHR, because spaces do not use up a key letter. Decrypting with the same key turns it back.

A historical cipher, not security

The Vigenere cipher was described in the 1500s and was long called unbreakable, but methods to break it were published in the 1800s, and a computer breaks it in a moment today. Use it for puzzles, escape rooms, classroom lessons and games. Never use it to protect passwords, messages or anything private. For real encryption of text, use the AES text tool.

Compared with the Caesar cipher, which shifts every letter by the same amount, the Vigenere cipher shifts each letter by a different amount from a repeating key. That hides the usual letter frequencies better, but a long message and a short key still leave patterns.

Rules used here

Only the 26 letters of the English alphabet are changed. Digits, spaces, punctuation, accented letters and emoji pass through unchanged and do not use up a key letter. The key is turned to capital letters and anything not A to Z is ignored. An empty key is an error. A key of A shifts by 0, so it leaves the text as it was.

Text of up to 500,000 characters is handled in your browser and nothing is uploaded. For other fun encodings see the NATO phonetic alphabet and the binary translator.

Assumptions and limits

The alphabet is the 26 letters A to Z. Other characters pass through unchanged.
Non-letters do not advance the key.
The key uses only its letters, in capitals.
The cipher is shown for learning and puzzles and gives no real protection.

Formulas reviewed October 10, 2026. See the calculation methodology for how SumPanda rounds, tests and sources its formulas.

Frequently asked questions

How does the Vigenere cipher work?

Each letter is shifted forward by the number matching the key letter at that position, A = 0 to Z = 25, and the key repeats. To decrypt, shift back.

What is ATTACKATDAWN with the key LEMON?

LXFOPVEFRNHR. It is the classic example for this cipher.

Is the Vigenere cipher secure?

No. It was broken long ago and a computer can break it quickly. Use it only for games, puzzles and learning.

Do spaces use up a key letter?

Not here. Spaces and punctuation pass through and the key moves on only when a letter is shifted.