public void setCipherKey(String cipherKey) {
	if (cipherKey == null || cipherKey.length() == 0) throw new IllegalArgumentException();
	for (int i = 0; i < cipherKey.length(); i++) {
	    if(cipherKey.charAt(i)<97 || cipherKey.charAt(i)>122)
		throw new IllegalArgumentException();
	}
	this.cipherKey = cipherKey.toLowerCase();
    }