protected String hashToString(String serializedModel, byte[] key) {

        String result = null;

        Mac sha512_HMAC;

        try {

            sha512_HMAC = Mac.getInstance("HmacSHA512");

            SecretKeySpec secretkey = new SecretKeySpec(key, "HmacSHA512");

            sha512_HMAC.init(secretkey);

            byte[] mac_data = sha512_HMAC.doFinal(serializedModel.getBytes("UTF-8"));

            // if this next line not compiled, test the second line.
            result = Base64.encodeBase64String(mac_data);
            //result = Base64.encodeToString(mac_data, Base64.DEFAULT);

        }catch(Exception e){
        }

        return result;
    }