Autokey Cipher

Extend a keyword with plaintext letters for a stronger polyalphabetic stream.

Key and method

Keyword

Only letters in the key are used. Spaces, numbers, and punctuation are ignored.

Autokey starts with a keyword, then extends the keystream using plaintext letters. Encryption uses C = P + K mod 26. Decryption must rebuild the key from recovered plaintext.

Type: Polyalphabetic substitution Key: Keyword + plaintext Direction: Different each way Normalised key: QUEENLY Key length: 7

Keystream preview: -

First mapping: A with Q becomes Q

Options

Preserve case

Keep punctuation & numbers

Remove whitespace

Group letters

Autokey is not reciprocal: decrypt mode rebuilds the keystream from recovered plaintext.

Output stats

Mode: Encrypt Key letters: 0 Input letters: 0 Input chars: 0 Output chars: 0 Blocks: 0

Example: with key QUEENLY, ATTACKATDAWN becomes QNXEPVYTWTWP.

Input

Output

Using This Tool: Guide & Notes Show guide

Autokey starts with a keyword, then extends the key stream with message text instead of simply repeating the keyword forever.

How to use it

Enter the starting keyword.
Choose Encrypt for plaintext or Decrypt for ciphertext.
Paste the message and watch the key stream details to see how the key extends.
Keep the same starting keyword for decryption.

Options and settings

Keyword is only the beginning of the key stream, so even a small typo changes everything after it.
Preserve case keeps letter styling without changing the mathematical result.
Keep punctuation and numbers controls whether unsupported characters stay in the visible output.
Group letters is useful when you want traditional grouped ciphertext.

Notes

Autokey reduces the repeated-key pattern of Vigenere, but it is still a classical cipher.
Decryption is sensitive because recovered plaintext becomes part of the following key stream.

Autokey Cipher: When the Message Becomes the Key

How a Vigenère-style cipher tried to escape repetition by feeding the plaintext back into the keystream.

Read the article

Signal acquired

A note from the team

We build these tools because we love cryptography, encoding, and making difficult ideas easier to explore. If they've helped you, even a small coffee means the world to the project. Thank you!

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