Java/src/main/java/com/thealgorithms/ciphers/Autokey.java

package com.thealgorithms.ciphers;

/**
 * The Autokey Cipher is an interesting and historically significant encryption method,
 * as it improves upon the classic Vigenère Cipher by using the plaintext itself to
 * extend the key. This makes it harder to break using frequency analysis, as it
 * doesn’t rely solely on a repeated key.
 * https://en.wikipedia.org/wiki/Autokey_cipher
 *
 * @author bennybebo
 */
public class Autokey {

    // Encrypts the plaintext using the Autokey cipher
    public String encrypt(String plaintext, String keyword) {
        plaintext = plaintext.toUpperCase().replaceAll("[^A-Z]", ""); // Sanitize input
        keyword = keyword.toUpperCase();

        StringBuilder extendedKey = new StringBuilder(keyword);
        extendedKey.append(plaintext); // Extend key with plaintext

        StringBuilder ciphertext = new StringBuilder();

        for (int i = 0; i < plaintext.length(); i++) {
            char plainChar = plaintext.charAt(i);
            char keyChar = extendedKey.charAt(i);

            int encryptedChar = (plainChar - 'A' + keyChar - 'A') % 26 + 'A';
            ciphertext.append((char) encryptedChar);
        }

        return ciphertext.toString();
    }

    // Decrypts the ciphertext using the Autokey cipher
    public String decrypt(String ciphertext, String keyword) {
        ciphertext = ciphertext.toUpperCase().replaceAll("[^A-Z]", ""); // Sanitize input
        keyword = keyword.toUpperCase();

        StringBuilder plaintext = new StringBuilder();
        StringBuilder extendedKey = new StringBuilder(keyword);

        for (int i = 0; i < ciphertext.length(); i++) {
            char cipherChar = ciphertext.charAt(i);
            char keyChar = extendedKey.charAt(i);

            int decryptedChar = (cipherChar - 'A' - (keyChar - 'A') + 26) % 26 + 'A';
            plaintext.append((char) decryptedChar);

            extendedKey.append((char) decryptedChar); // Extend key with each decrypted char
        }

        return plaintext.toString();
    }
}