Autokey.java

packagecom.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
 */
publicclassAutokey {

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

StringBuilderextendedKey = newStringBuilder(keyword);
extendedKey.append(plaintext); // Extend key with plaintext

StringBuilderciphertext = newStringBuilder();

for (inti = 0; i < plaintext.length(); i++) {
charplainChar = plaintext.charAt(i);
charkeyChar = extendedKey.charAt(i);

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

returnciphertext.toString();
 }

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

StringBuilderplaintext = newStringBuilder();
StringBuilderextendedKey = newStringBuilder(keyword);

for (inti = 0; i < ciphertext.length(); i++) {
charcipherChar = ciphertext.charAt(i);
charkeyChar = extendedKey.charAt(i);

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

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

returnplaintext.toString();
 }
}