Vigenère Cipher Encoder / Decoder
Encode and decode the Vigenère cipher with a keyword, plus Beaufort and autokey variants, and break an unknown key with the index of coincidence and frequency analysis.
The Vigenère cipher encrypts text with a repeating keyword, shifting each letter by a different amount. For centuries it was called "le chiffre indéchiffrable" — the unbreakable cipher — until frequency analysis cracked it. This tool encodes and decodes it, supports the Beaufort and autokey variants, and can break an unknown key automatically.
How to use this tool
- Type your text and a keyword (letters only).
- Choose encode, decode, or break to recover an unknown key.
- Pick the variant if you are not using plain Vigenère.
- Copy the result; non-letters are left in place.
Formula
Encoding adds the key letter to the text letter modulo 26: Cᵢ = (Pᵢ + Kᵢ) mod 26. Decoding subtracts it. The key repeats over the message, advancing only on letters.
Breaking first finds the key length with the index of coincidence, then solves each column as a Caesar shift using letter frequencies.
Example
"ATTACKATDAWN" with the key "LEMON" encrypts to LXFOPVEFRNHR, the textbook example of the cipher.
Frequently asked questions
Is Vigenère secure today?
No. It is easily broken by the frequency methods built into this tool when the message is longer than the key. Use it for puzzles and learning, never for real secrets.
What is the Beaufort variant?
Beaufort computes (key − text) mod 26 instead of adding, which makes it its own inverse — the same operation both encrypts and decrypts.
How long must text be to break it?
Roughly 20 letters or more, and the longer the better. Short messages do not give the statistics enough signal, so the recovered key may need manual tweaking.
Last updated:
Page address with these inputs: https://kakao.mobile/tools/vigenere-cipher-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). Vigenère Cipher Encoder / Decoder. https://kakao.mobile/tools/vigenere-cipher-tool/
“Vigenère Cipher Encoder / Decoder.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/vigenere-cipher-tool/.
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