情景：公司項目基於.net4.0,web客戶端實現單點登錄需要自己解密id_token，對於jwt解密，.net提供了IdentityModel類庫，但是4.0中該類庫不可用，所以自己實現了解密方法..

使用了類庫：https://github.com/jwt-dotnet/jwt

下面直接貼代碼，直接調用DecodeJWT方法就行，參數為id_token，key默認為空字符串"",

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

            
 
//從/.well-known/openid-configuration路徑獲取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();

            //從jwks_uri獲取keys
            json = webClient.DownloadString(jwksUri);

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


            //從jwt獲取頭部kid,並從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已過期");
                //Console.WriteLine("Token has expired");
                //return null;
            }
            catch (SignatureVerificationException)
            {
                throw new Exception("token驗證失敗");
                //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);
        }

IdentityServer4客戶端JWT解密實現（基於.net4.0）