public String encrypt(String klartext) {
        String cipherText = "";
        for (int i = 0; i < klartext.length(); i++) {
            int charPosition = -1;
            char replaceVal;
            int keyVal = -1;
            char val = klartext.charAt(i);

            if (Character.isUpperCase(val)) {
                charPosition = ALPHABET_UPPER.indexOf(val);
                if (charPosition != -1) {
                    keyVal = (offset + charPosition) % 26;
                    replaceVal = ALPHABET_UPPER.charAt(keyVal);
                } else {
                    replaceVal = klartext.charAt(i);
                }
            } else {
                charPosition = ALPHABET_LOWER.indexOf(val);
                if (charPosition != -1) {
                    replaceVal = ALPHABET_LOWER.charAt(charPosition);
                } else {
                    replaceVal = klartext.charAt(i);
                }
            }
            cipherText += replaceVal;
        }
        return cipherText;
    }