怎么在.net4.0中实现IdentityServer4客户端JWT解密

public static IDictionary<string, object> DecodeJWT(string jwttoken,string key)
    {

      //从/.well-known/openid-configuration路径获取jwks_uri
      var webClient = new WebClient();

      var endpoint = "http://localhost:5000/.well-known/openid-configuration";

      var json = webClient.DownloadString(endpoint);

      JObject metadata = JsonConvert.DeserializeObject<JObject>(json);

      var jwksUri = metadata["jwks_uri"].ToString();

      //从jwks_uri获取keys
      json = webClient.DownloadString(jwksUri);

      var keys = JsonConvert.DeserializeObject<CustomJWKs>(json);


      //从jwt获取头部kid,并从keys中找到匹配kid的key
      string[] tokenParts = jwttoken.Split('.');
      byte[] bytes = FromBase64Url(tokenParts[0]);
      string head= Encoding.UTF8.GetString(bytes);
      string kid = JsonConvert.DeserializeObject<JObject>(head)["kid"].ToString();

      var defaultkey=keys.keys.Where(t => t.kid == kid).FirstOrDefault();

      if(defaultkey==null)
      {
        throw new Exception("未找到匹配的kid");
      }

      //jwt解密
      return RS256Decode(jwttoken, key, defaultkey.e, defaultkey.n);
    }


     public static IDictionary<string, object> RS256Decode(string token, string secret, string exponent,string modulus)
    {
      try
      {
        IJsonSerializer serializer = new JsonNetSerializer();
        IDateTimeProvider provider = new UtcDateTimeProvider();
        IJwtValidator validator = new JwtValidator(serializer, provider);
        IBase64UrlEncoder urlEncoder = new JwtBase64UrlEncoder();
        RSAlgorithmFactory rS256Algorithm = new RSAlgorithmFactory(() =>
        {
          RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
          rsa.ImportParameters(
           new RSAParameters()
           {
             Modulus = FromBase64Url(modulus),
             Exponent = FromBase64Url(exponent)
           });


          byte[] rsaBytes = rsa.ExportCspBlob(true);

          X509Certificate2 cert = new X509Certificate2(rsaBytes);
          return cert;
        });

        IJwtDecoder decoder = new JwtDecoder(serializer, validator, urlEncoder, rS256Algorithm);
        var json = decoder.DecodeToObject(token, secret, verify: false);
        return json;
      }
      catch (TokenExpiredException)
      {
        throw new Exception("token已过期");
        //Console.WriteLine("Token has expired");
        //return null;
      }
      catch (SignatureVerificationException)
      {
        throw new Exception("token验证失败");
        //Console.WriteLine("Token has invalid signature");
        //return null;
      }
    }


    public static byte[] FromBase64Url(string base64Url)
    {
      string padded = base64Url.Length % 4 == 0
        ? base64Url : base64Url + "====".Substring(base64Url.Length % 4);
      string base64 = padded.Replace("_", "/")
                 .Replace("-", "+");
      return Convert.FromBase64String(base64);
}