IdentityServer4客户端JWT解密实现（基于.net4.0）

情景：公司项目基于.net4.0,web客户端实现单点登录需要自己解密id_token，对于jwt解密，.net提供了IdentityModel类库，但是4.0中该类库不可用，所以自己实现了解密方法..

使用了类库：https://github.com/jwt-dotnet/jwt

下面直接贴代码，直接调用DecodeJWT方法就行，参数为id_token，key默认为空字符串"",

         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[]);
            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 %  ==
                ? base64Url : base64Url + "====".Substring(base64Url.Length % );
            string base64 = padded.Replace("_", "/")
                                  .Replace("-", "+");
            return Convert.FromBase64String(base64);
        }