@Override
public String digestParams(RestInvocation restInvocation) {
final Params params = restInvocation.getParamsMap().get(FormParam.class);
final Map<String, String> nameValueMap = params.asHttpHeaders();
nameValueMap.remove("sign");
nameValueMap.put("secret_key", secretKey);
final List<Map.Entry<String, String>> nameValueList = new ArrayList<Map.Entry<String, String>>(nameValueMap.entrySet());
Collections.sort(nameValueList, comparator);