Class Autokey

src/main/java/com/thealgorithms/ciphers/Autokey.java:12–55

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.wikipe

 * @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();
    }
}