private static String hexToString(byte[] bytes) {
		StringBuffer buff = new StringBuffer();
		for (int i=0; i<bytes.length; i++) {
			int val = bytes[i];
			val = val & 0xff;  // remove higher bits, sign
			if (val<16) buff.append('0'); // leading 0
			buff.append(Integer.toString(val, 16));
		}
		return buff.toString();
	}