# Autokey Cipher

Encrypts or decrypts a message using the Autokey cipher.

Source library: `secretpy`

## Historical Context

The Autokey cipher is a polyalphabetic substitution cipher that was invented by Girolamo Cardano in 1550. It is similar to the Vigenère cipher, but it uses the plaintext itself to extend the key.

## How it Works

The Autokey cipher uses a keyword to start the encryption process. The rest of the key is generated by taking the plaintext of the message. The encryption is then performed using a tabula recta, similar to the Vigenère cipher.

## Parameters

- **text**: The message to encrypt or decrypt.
- **alphabet**: The alphabet to use for the cipher. Can be a string of characters or a predefined alphabet from `secretpy.alphabets`.
- **key**: The key for the cipher.
- **mode**: Whether to encrypt or decrypt the message.
- **keep_formatting**: Whether to keep the original formatting of the message.