Class ADFGVXCipher

src/main/java/com/thealgorithms/ciphers/ADFGVXCipher.java:27–167

The ADFGVX cipher is a fractionating transposition cipher that was used by the German Army during World War I. It combines a Polybius square substitution with a columnar transposition to enhance encryption strength. The name "ADFGVX" refers to the six letters (A, D, F, G, V, X) used as row

 * @author bennybebo
 */
public class ADFGVXCipher {

    // Constants used in the Polybius square
    private static final char[] POLYBIUS_LETTERS = {'A', 'D', 'F', 'G', 'V', 'X'};
    private static final char[][] POLYBIUS_SQUARE = {{'N', 'A', '1', 'C', '3', 'H'}, {'8', 'T', 'B', '2', 'O', 'M'}, {'E', '5', 'W', 'R', 'P', 'D'}, {'4', 'F', '6', 'G', '7', 'I'}, {'9', 'J', '0', 'K', 'L', 'Q'}, {'S', 'U', 'V', 'X', 'Y', 'Z'}};

    // Maps for fast substitution lookups
    private static final Map<String, Character> POLYBIUS_MAP = new HashMap<>();
    private static final Map<Character, String> REVERSE_POLYBIUS_MAP = new HashMap<>();

    // Static block to initialize the lookup tables from the Polybius square
    static {
        for (int i = 0; i < POLYBIUS_SQUARE.length; i++) {
            for (int j = 0; j < POLYBIUS_SQUARE[i].length; j++) {
                String key = "" + POLYBIUS_LETTERS[i] + POLYBIUS_LETTERS[j];
                POLYBIUS_MAP.put(key, POLYBIUS_SQUARE[i][j]);
                REVERSE_POLYBIUS_MAP.put(POLYBIUS_SQUARE[i][j], key);
            }
        }
    }

    /**
     * Encrypts a given plaintext using the ADFGVX cipher with the provided keyword.
     * Steps:
     * 1. Substitute each letter in the plaintext with a pair of ADFGVX letters.
     * 2. Perform a columnar transposition on the fractionated text using the keyword.
     *
     * @param plaintext The message to be encrypted (can contain letters and digits).
     * @param key       The keyword for columnar transposition.
     * @return The encrypted message as ciphertext.
     */
    public String encrypt(String plaintext, String key) {
        plaintext = plaintext.toUpperCase().replaceAll("[^A-Z0-9]", ""); // Sanitize input
        StringBuilder fractionatedText = new StringBuilder();

        for (char c : plaintext.toCharArray()) {
            fractionatedText.append(REVERSE_POLYBIUS_MAP.get(c));
        }

        return columnarTransposition(fractionatedText.toString(), key);
    }

    /**
     * Decrypts a given ciphertext using the ADFGVX cipher with the provided keyword.
     * Steps:
     * 1. Reverse the columnar transposition performed during encryption.
     * 2. Substitute each pair of ADFGVX letters with the corresponding plaintext letter.
     * The resulting text is the decrypted message.
     *
     * @param ciphertext The encrypted message.
     * @param key        The keyword used during encryption.
     * @return The decrypted plaintext message.
     */
    public String decrypt(String ciphertext, String key) {
        String fractionatedText = reverseColumnarTransposition(ciphertext, key);

        StringBuilder plaintext = new StringBuilder();
        for (int i = 0; i < fractionatedText.length(); i += 2) {