Vigenèrova šifra

/**
 * Vigenerova sifra - demonstrace
 * @author Pavel Micka
 */
public class VigenereCipher {
    /**
     * Zasifruje text Vigenerovou sifrou
     * @param s otevreny text
     * @param key klic (jen velka pismena)
     * @return zasifrovany text (jen velka pismena)
     */
    public static String encipher(String s, String key){
        StringBuilder builder = new StringBuilder();
        for(int i = 0; i < s.length(); i ++){
            if(s.charAt(i) < 65 || s.charAt(i) > 90){ //znak v ASCII
                throw new IllegalArgumentException("" +
                        "Sifrovany retezec neobsahuje jen velka pismena");
            }
            //modularne pricteme shift
            char encyphered = s.charAt(i) + getShift(key, i) > 90 ? (char)((s.charAt(i) + getShift(key, i)) - 26) : (char)(s.charAt(i) + getShift(key, i));
            builder.append(encyphered);
        }
        return builder.toString();
    }
    /**
     * Desifruje dany zasiforvany text Vigenerovou sifrou
     * @param s zasifrovany text
     * @param key klic
     * @return otevreny text
     */
    public static String decipher(String s, String key){
        StringBuilder builder = new StringBuilder();
        for(int i = 0; i < s.length(); i ++){
            if(s.charAt(i) < 65 || s.charAt(i) > 90){ //znak v ASCII
                throw new IllegalArgumentException("" +
                        "Desifrovany retezec neobsahuje jen velka pismena");
            }
            //modularne odecteme shift
            char decyphered = s.charAt(i) - getShift(key, i) < 65 ? (char)((s.charAt(i) - getShift(key, i)) + 26) : (char)(s.charAt(i) - getShift(key, i));
            builder.append(decyphered);
        }
        return builder.toString();
    }
    /**
     * Zjisti shift
     * @param key klic
     * @param i pozice v textu
     * @return shift
     */
    private static int getShift(String key, int i) {
            if(key.charAt(i % key.length()) < 65 || key.charAt(i % key.length()) > 90){ //znak v ASCII
                throw new IllegalArgumentException("" +
                        "Klic retezec neobsahuje jen velka pismena");
            }
        return ((int)key.charAt(i % key.length())) - 65;
    }
    
    public static void main(String[] args){
        String text = "KULTURNIATASEJESPION";
        String key = "PES";
        String enciphered = encipher(text, key);
        System.out.println(enciphered);
        System.out.println(decipher(enciphered, key));
    }
}