JavaAlgorithms/Ciphers/Vigenere.java

package Ciphers;

/**
 * A Java implementation of Vigenere Cipher.
 *
 * @author straiffix
 * @author beingmartinbmc
 */
public class Vigenere {

    public static String encrypt(final String message, final String key) {

        StringBuilder result = new StringBuilder();

        for (int i = 0, j = 0; i < message.length(); i++) {
            char c = message.charAt(i);
            if (Character.isLetter(c)) {
                if (Character.isUpperCase(c)) {
                    result.append((char) ((c + key.toUpperCase().charAt(j) - 2 * 'A') % 26 + 'A'));

                } else {
                    result.append((char) ((c + key.toLowerCase().charAt(j) - 2 * 'a') % 26 + 'a'));
                }
            } else {
                result.append(c);
            }
            j = ++j % key.length();
        }
        return result.toString();
    }

    public static String decrypt(final String message, final String key) {
        StringBuilder result = new StringBuilder();

        for (int i = 0, j = 0; i < message.length(); i++) {

            char c = message.charAt(i);
            if (Character.isLetter(c)) {
                if (Character.isUpperCase(c)) {
                    result.append((char) ('Z' - (25 - (c - key.toUpperCase().charAt(j))) % 26));

                } else {
                    result.append((char) ('z' - (25 - (c - key.toLowerCase().charAt(j))) % 26));
                }
            } else {
                result.append(c);
            }

            j = ++j % key.length();
        }
        return result.toString();
    }

    public static void main(String[] args) {
        String text = "Hello World!";
        String key = "itsakey";
        System.out.println(text);
        String ciphertext = encrypt(text, key);
        System.out.println(ciphertext);
        System.out.println(decrypt(ciphertext, key));
    }
}