跳转请求安全验证

从万全云电商跳转到应用的请求，在参数中都包含hmac，应用可以验证这个字段，来确认请求是否来源自万全云电商。
以下示例如果计算hmac，在不同场景的跳转中，所带参数会有区别，除了hmac以外的参数，都需参与计算。
例如万全云电商返回授权码的请求参数如下：

{
  "shop=a94a110d86d2452eb3e2af4cfb8a3828&code=a84a110d86d2452eb3e2af4cfb8a3828&merchant_id=1&time_stamp=2013-08-27T13:58:35Z&hmac=a2a3e2dcd8a82fd9070707d4d921ac4cdc842935bf57bc38c488300ef3960726"
}

将请求中的所有参数，转换成如下的字典：

{
  "shop": "a94a110d86d2452eb3e2af4cfb8a3828",  
  "code": "a84a110d86d2452eb3e2af4cfb8a3828",  
  "merchant_id": "1",
  "time_stamp": "2013-08-27T13:58:35Z",    
  "hmac": "a2a3e2dcd8a82fd9070707d4d921ac4cdc842935bf57bc38c488300ef3960726"
}

{
  "shop": "a94a110d86d2452eb3e2af4cfb8a3828",  
  "code": "a84a110d86d2452eb3e2af4cfb8a3828",  
  "merchant_id": "1",
  "time_stamp": "2013-08-27T13:58:35Z"
}

{
  "code=a84a110d86d2452eb3e2af4cfb8a3828&merchant_id=1&shop=a94a110d86d2452eb3e2af4cfb8a3828&time_stamp=2013-08-27T13:58:35Z"
}

using System;
using System.Linq;
using System.Text;

namespace RestSharp.Authenticators
{
    public class HttpBasicAuthenticator : IAuthenticator
    {
        private readonly string authHeader;

        public HttpBasicAuthenticator(string username, string password)
        {
            var token = Convert.ToBase64String(Encoding.UTF8.GetBytes(string.Format("{0}:{1}", username, password)));

            authHeader = string.Format("Basic {0}", token);
        }

        public void Authenticate(IRestClient client, IRestRequest request)
        {
            // only add the Authorization parameter if it hasn't been added by a previous Execute
            if (!request.Parameters.Any(p => "Authorization".Equals(p.Name, StringComparison.OrdinalIgnoreCase)))
                request.AddParameter("Authorization", authHeader, ParameterType.HttpHeader);
        }
    }
}