Felo

Vigenere Cipher Encoder Decoder Online

Encrypt or decrypt text with the classical Vigenère cipher using your own secret key. Unlike tools that flatten your text to bare uppercase letters, this agent preserves spacing, punctuation and case by default and shows the key alignment so you can verify every shift. Try it online.

What is Vigenere Cipher Encoder Decoder Online?

This agent is a conversational tool for the classical Vigenère cipher, the repeating-key Caesar shift that has been used for centuries to obscure written messages. You give it two things: the text you want to transform and a secret key word. It then shifts each letter of your message by the corresponding letter of the key, cycling the key as many times as needed. Ask it to encrypt and you get cipher text; ask it to decrypt and you get the original message back.

The important difference from many free text tools is what happens to everything that is not a letter. Some encoders strip spaces, punctuation and digits and force everything to uppercase, which makes the output hard to read and impossible to paste back into a document. This agent keeps your formatting intact by default, shifting only the letters and leaving every other character exactly where it was.

Key Features

Two-way Vigenère transform. Encrypt plaintext into cipher text or decrypt cipher text back into plaintext with the same secret key.
Formatting preserved by default. Spaces, punctuation, digits and letter case stay in place; only alphabetic characters are shifted. A strict mode is available if you want the classic strip-and-uppercase output.
Key alignment display. See how the key lines up against your message and which shift value applies to each letter, so the result can be checked by hand.
Clear handling of problem keys. An empty key or a key with no letters is reported with an explanation rather than producing silent garbage.
Round-trip check. Re-encrypt a decrypted result to confirm that the key was correct.

How to Use

State what you want: for example, "Encrypt 'Meet me at the bridge at noon' with key LEMON" or "Decrypt this cipher text with key SECRET".
Paste your text and key in the same message. If you only give one of them, the agent will ask for the missing piece before starting.
Read the result in a copyable block. Ask for the key alignment table, strict mode, or a round-trip verification if you want more detail.

Example Use Cases

Classroom cryptography exercise. A teacher wants students to encrypt a short message with the Vigenère cipher and then check their work, so the agent returns both the cipher text and the per-letter shift table.
Puzzle and escape-room design. A designer needs a Vigenère cipher encoder to hide a clue inside a paragraph while keeping the punctuation readable, and uses the formatting-preserving mode.
Decoding a friend's cipher text. Someone receives a block of letters with a key word and asks the agent to decrypt it, then runs a round-trip check to confirm the key was right.

Tips for Best Results

Give the key as a single word or short phrase with letters only; digits and symbols in the key are ignored, and the agent will tell you when that happens.
If you are decrypting, paste the cipher text exactly as received. Extra letters will change the alignment of the key.
Ask for strict mode when you need output that matches the classic all-uppercase, letters-only convention.

FAQ

What input format does it accept?

Plain text of any length plus a secret key. Letters are shifted; spaces, punctuation, digits and other characters are preserved by default. Accented and non-Latin letters are passed through unchanged rather than deleted.

How accurate is the result?

The transform follows the standard Vigenère rule exactly: each letter is shifted by the key letter's alphabet position, with the key repeating. Because the arithmetic is deterministic, the same text and key always produce the same output, and a round-trip check confirms correctness.

How does this compare with the original source page?

Some free text tools apply the same repeating-key shift but strip everything except A–Z and force uppercase, so spacing and punctuation disappear. This agent performs the same cipher by default while keeping your formatting, and it adds a key alignment view and explicit handling of empty or non-letter keys. It does not crack unknown keys or provide historical analysis.

Can it break a cipher without the key?

No. This is an encoder and decoder, not a cryptanalysis tool. You need the key to decrypt.