如何写出安全的API接口？接口参数加密签名设计思…

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace test3
{
    public class SampleCode
    {
        public static string test()
        {
            int _timestamp = 12345678;
            var param = new SortedDictionary<string, string>(new AsciiComparer());
            param.Add("z", "ZZZ");
            param.Add("a", "AAA");
            param.Add("Z", "zzz");
            param.Add("A", "aaa");
            param.Add("2", "贰");
            param.Add("1", "壹");
            param.Add("_appid", "club");
            param.Add("_timestamp", _timestamp.ToString());
            string _sign = GetSign(param);
            string urlParam = string.Join("&", param.Select(i => i.Key + "=" + i.Value));
            string url = "http://api.demo.com/dog/add?" + urlParam + "&_sign=" + _sign;
            return url;
        }
        public static string GetSign(SortedDictionary<string, string> paramList, string appKey = "test")
        {
            paramList.Remove("_sign");
            StringBuilder sb = new StringBuilder(appKey);
            foreach (var p in paramList)
                sb.Append(p.Key).Append(p.Value);
            sb.Append(appKey);
            return GetMD5(sb.ToString());
        }
        public static string GetMD5(string str)
        {
            if (string.IsNullOrEmpty(str))
                return str;
            var sb = new StringBuilder(32);
            var md5 = System.Security.Cryptography.MD5.Create();
            var output = md5.ComputeHash(Encoding.UTF8.GetBytes(str));
            for (int i = 0; i < output.Length; i++)
                sb.Append(output[i].ToString("X").PadLeft(2, '0'));
            return sb.ToString();
        }
    }
    /// <summary>
    /// 基于ASCII码排序规则的String比较器
    /// Author：HeDaHong
    /// </summary>
    public class AsciiComparer : System.Collections.Generic.IComparer<string>
    {
        public int Compare(string a, string b)
        {
            if (a == b)
                return 0;
            else if (string.IsNullOrEmpty(a))
                return -1;
            else if (string.IsNullOrEmpty(b))
                return 1;
            if (a.Length <= b.Length)
            {
                for (int i = 0; i < a.Length; i++)
                {
                    if (a[i] < b[i])
                        return -1;
                    else if (a[i] > b[i])
                        return 1;
                    else
                        continue;
                }
                return a.Length == b.Length ? 0 : -1;
            }
            else
            {
                for (int i = 0; i < b.Length; i++)
                {
                    if (a[i] < b[i])
                        return -1;
                    else if (a[i] > b[i])
                        return 1;
                    else
                        continue;
                }
                return 1;
            }
        }
    }
}