JavaAlgorithms/ciphers/Vigenere.java

package ciphers;

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

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

    String result = "";

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

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

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

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

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

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

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

  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));
  }
}