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));

}