追本溯源，从使用开始　　

　　首先看一下我们通常是如何使用微软自带的认证，一般在Startup里面配置我们所需的依赖认证服务，这里通过JWT的认证方式讲解

public void ConfigureServices(IServiceCollection services)
{
    services.AddAuthentication(authOpt =>
    {
        authOpt.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
        authOpt.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
    })
    .AddJwtBearer(o =>
    {
        o.TokenValidationParameters = new TokenValidationParameters
        {
            //配置自己所要验证的参数
            
        };
    });
}

　　我们来看一下源码AddAuthentication主要做了什么

public static class AuthenticationServiceCollectionExtensions
  {
    public static AuthenticationBuilder AddAuthentication( this IServiceCollection services, Action<AuthenticationOptions> configureOptions)
    {
      if (services == null)
        throw new ArgumentNullException(nameof (services));
      if (configureOptions == null)
        throw new ArgumentNullException(nameof (configureOptions));
      AuthenticationBuilder authenticationBuilder = services.AddAuthentication();
      services.Configure<AuthenticationOptions>(configureOptions);
      return authenticationBuilder;
    }

    public static AuthenticationBuilder AddAuthentication( this IServiceCollection services)
    {
      if (services == null)
        throw new ArgumentNullException(nameof (services));
      services.AddAuthenticationCore();
      services.AddDataProtection();
      services.AddWebEncoders();
      services.TryAddSingleton<ISystemClock, SystemClock>();
      return new AuthenticationBuilder(services);
    }

    public static AuthenticationBuilder AddAuthentication(
      this IServiceCollection services,
      string defaultScheme)
    {
      return services.AddAuthentication((Action<AuthenticationOptions>) (o => o.DefaultScheme = defaultScheme));
    } 

  .....
}

　　ConfigureServices方法基本都是服务的注册，基于微软的风格，这里的AddAuthenticationCore肯定是我们的认证服务注册方法，来看一下

public static class AuthenticationCoreServiceCollectionExtensions
  {
    /// <summary>
    /// Add core authentication services needed for <see cref="T:Microsoft.AspNetCore.Authentication.IAuthenticationService" />.
    /// </summary>    
    public static IServiceCollection AddAuthenticationCore(
      this IServiceCollection services)
    {
      if (services == null)
        throw new ArgumentNullException(nameof (services));
      services.TryAddScoped<IAuthenticationService, AuthenticationService>();
      services.TryAddSingleton<IClaimsTransformation, NoopClaimsTransformation>();
      services.TryAddScoped<IAuthenticationHandlerProvider, AuthenticationHandlerProvider>();
      services.TryAddSingleton<IAuthenticationSchemeProvider, AuthenticationSchemeProvider>();
      return services;
    }

    /// <summary>
    /// Add core authentication services needed for <see cref="T:Microsoft.AspNetCore.Authentication.IAuthenticationService" />.
    /// </summary>   
    public static IServiceCollection AddAuthenticationCore(
      this IServiceCollection services,
      Action<AuthenticationOptions> configureOptions)
    {
      if (services == null)
        throw new ArgumentNullException(nameof (services));
      if (configureOptions == null)
        throw new ArgumentNullException(nameof (configureOptions));
      services.AddAuthenticationCore();
      services.Configure<AuthenticationOptions>(configureOptions);
      return services;
    }
  }

　　我们看到这里主要注册了AuthenticationService, AuthenticationHandlerProvider, AuthenticationSchemeProvider这三个对象，如文章开头所说，追本溯源，从使用开始，我们先看一下这三个对象是如何在认证体系中使用的，且是如何发挥作用的。

　　从使用开始

　　看一下我们的认证管道构建

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
   {
       ...
       app.UseAuthentication();
       ...
   }


　public static class AuthAppBuilderExtensions
  {
    public static IApplicationBuilder UseAuthentication( this IApplicationBuilder app)
    {
      if (app == null)
        throw new ArgumentNullException(nameof (app));
      return app.UseMiddleware<AuthenticationMiddleware>();
    }
  }

　　这里使用了约定的注册方式UseMiddleware，并且指定使用中间件AuthenticationMiddleware

public class AuthenticationMiddleware
  {
    private readonly RequestDelegate _next;

    public AuthenticationMiddleware(RequestDelegate next, IAuthenticationSchemeProvider schemes)
    {
      if (next == null)
        throw new ArgumentNullException(nameof (next));
      if (schemes == null)
        throw new ArgumentNullException(nameof (schemes));
      this._next = next;
      this.Schemes = schemes;
    }

    public IAuthenticationSchemeProvider Schemes { get; set; }

    public async Task Invoke(HttpContext context)
    {
      context.Features.Set<IAuthenticationFeature>((IAuthenticationFeature) new AuthenticationFeature()
      {
        OriginalPath = context.Request.Path,
        OriginalPathBase = context.Request.PathBase
      });
      IAuthenticationHandlerProvider handlers = context.RequestServices.GetRequiredService<IAuthenticationHandlerProvider>();
      foreach (AuthenticationScheme authenticationScheme in await this.Schemes.GetRequestHandlerSchemesAsync())
      {
        IAuthenticationRequestHandler handlerAsync = await handlers.GetHandlerAsync(context, authenticationScheme.Name) as IAuthenticationRequestHandler;
        bool flag = handlerAsync != null;
        if (flag)
          flag = await handlerAsync.HandleRequestAsync();
        if (flag)
          return;
      }
      AuthenticationScheme authenticateSchemeAsync = await this.Schemes.GetDefaultAuthenticateSchemeAsync();
      if (authenticateSchemeAsync != null)
      {
        AuthenticateResult authenticateResult = await context.AuthenticateAsync(authenticateSchemeAsync.Name);　　//实际的认证业务
        if (authenticateResult?.Principal != null)
          context.User = authenticateResult.Principal;
      }
      await this._next(context);
    }
  }

　　在继续往下之前，我们先看一下这个认证中间件的作用结果，当认证通过时，在HttpContext的User属性（ClaimPrincipal）赋予身份标识，所以在后续的请求管道中都是基于认证结果中的身份标识做鉴权，这个我们会在后面的实际操作中会提到。

　　言归正传，在这里引出了我们的两个对象AuthenticationHandlerProvider, AuthenticationSchemeProvider。

重要对象讲解

IAuthenticationSchemeProvider

　　从名字来看，IAuthenticationSchemeProvider的作用应该是提供Scheme的，这也是Provider在微软的风格里面起的作用（类似于工厂模式）。

　　这个Scheme是什么呢？很明显，在Framework时代，也是有基于不同Scheme验证的，比如Bearer，Cookie，在Aspnet Core中定义不同的Scheme代表着不同的认证处理方式，具体体现是在每个Scheme中包含对应的IAuthenticationHandler类型的Handler，由它来完成跟自身Scheme相关的认证处理。如果没有定义会怎么样？仔细看上面这块源码，只有当AuthenticationScheme不为空时才会做认证，否则一旦在Controller打上鉴权标签[Authorize]，将会直接返回401，所以我们必须指定自己的Scheme。

　　那么我们在哪里指定我们的Scheme类似呢？我们先返回到ConfigureService的AddJwtBearer，使用过的朋友们肯定知道，这里获取的Scheme是我们在ConfigureService通过Addxxx scheme指定的Scheme类型。这里我们是使用JWT的

　　在这里指定了TOptions 为JwtBearerOptions，而THandler为JwtBearerHandler。

public virtual AuthenticationBuilder AddScheme<TOptions, THandler>(
      string authenticationScheme,
      string displayName,
      Action<TOptions> configureOptions)
      where TOptions : AuthenticationSchemeOptions, new()
      where THandler : AuthenticationHandler<TOptions>
    {
      return this.AddSchemeHelper<TOptions, THandler>(authenticationScheme, displayName, configureOptions);
    }


    private AuthenticationBuilder AddSchemeHelper<TOptions, THandler>(
      string authenticationScheme,
      string displayName,
      Action<TOptions> configureOptions)
      where TOptions : class, new()
      where THandler : class, IAuthenticationHandler
    {
      this.Services.Configure<AuthenticationOptions>((Action<AuthenticationOptions>) (o => o.AddScheme(authenticationScheme, (Action<AuthenticationSchemeBuilder>) (scheme =>
      {
        scheme.HandlerType = typeof (THandler);
        scheme.DisplayName = displayName;
      }))));
      if (configureOptions != null)
        this.Services.Configure<TOptions>(authenticationScheme, configureOptions);
      this.Services.AddTransient<THandler>();
      return this;
    }

　　注意这里TOptions 是需要继承AuthenticationSchemeOptions的，在这里是JwtBearerOptions，而THandler是AuthenticationHandler<TOptions>类型的Handler，在这里是JwtBearerHandler。

　　我们回到Scheme的分析继续往下，首先看一下AuthenticationScheme的定义　　

public class AuthenticationScheme
  {
    /// <summary>Constructor.</summary>   
    public AuthenticationScheme(string name, string displayName, Type handlerType)
    {
      if (name == null)
        throw new ArgumentNullException(nameof (name));
      if (handlerType == (Type) null)
        throw new ArgumentNullException(nameof (handlerType));
      if (!typeof (IAuthenticationHandler).IsAssignableFrom(handlerType))
        throw new ArgumentException("handlerType must implement IAuthenticationHandler.");
      this.Name = name;
      this.HandlerType = handlerType;
      this.DisplayName = displayName;
    }

    /// <summary>The name of the authentication scheme.</summary>
    public string Name { get; }

    /// <summary>
    /// The display name for the scheme. Null is valid and used for non user facing schemes.
    /// </summary>
    public string DisplayName { get; }

    /// <summary>
    /// The <see cref="T:Microsoft.AspNetCore.Authentication.IAuthenticationHandler" /> type that handles this scheme.
    /// </summary>
    public Type HandlerType { get; }
  }

　　在这里可以看到，如果要使用Aspnet Core自身的认证体系，需先注册Scheme，并且该Scheme必须指定一个类型为IAuthenticationHandler的Handler，否则会抛出异常。（这个其实在AddxxxScheme的时候已经指定了AuthenticationHandler）

　　我们再看一下IAuthenticationSchemeProvider的GetRequestHandlerSchemesAsync方法做了什么

public virtual Task<IEnumerable<AuthenticationScheme>> GetRequestHandlerSchemesAsync()
{
   return Task.FromResult<IEnumerable<AuthenticationScheme>>((IEnumerable<AuthenticationScheme>) this._requestHandlers);
}

　　这东西返回了_requestHandlers，这是什么？看代码

public class AuthenticationSchemeProvider : IAuthenticationSchemeProvider
  {
    private readonly object _lock = new object();
    private readonly AuthenticationOptions _options;
    private readonly IDictionary<string, AuthenticationScheme> _schemes;
    private readonly List<AuthenticationScheme> _requestHandlers;

    /// <summary>
    /// Creates an instance of <see cref="T:Microsoft.AspNetCore.Authentication.AuthenticationSchemeProvider" />
    /// using the specified <paramref name="options" />,
    /// </summary>   
    public AuthenticationSchemeProvider(IOptions<AuthenticationOptions> options)
      : this(options, (IDictionary<string, AuthenticationScheme>) new Dictionary<string, AuthenticationScheme>((IEqualityComparer<string>) StringComparer.Ordinal))
    {
    }

    /// <summary>
    /// Creates an instance of <see cref="T:Microsoft.AspNetCore.Authentication.AuthenticationSchemeProvider" />
    /// using the specified <paramref name="options" /> and <paramref name="schemes" />.
    /// </summary>   
    protected AuthenticationSchemeProvider(
      IOptions<AuthenticationOptions> options,
      IDictionary<string, AuthenticationScheme> schemes)
    {
      this._options = options.Value;
      IDictionary<string, AuthenticationScheme> dictionary = schemes;
      if (dictionary == null)
        throw new ArgumentNullException(nameof (schemes));
      this._schemes = dictionary;
      this._requestHandlers = new List<AuthenticationScheme>();
      foreach (AuthenticationSchemeBuilder scheme in this._options.Schemes)
        this.AddScheme(scheme.Build());
    }

　　public virtual void AddScheme(AuthenticationScheme scheme)
    {
      if (this._schemes.ContainsKey(scheme.Name))
        throw new InvalidOperationException("Scheme already exists: " + scheme.Name);
      lock (this._lock)
      {
        if (this._schemes.ContainsKey(scheme.Name))
          throw new InvalidOperationException("Scheme already exists: " + scheme.Name);
        if (typeof (IAuthenticationRequestHandler).IsAssignableFrom(scheme.HandlerType))
          this._requestHandlers.Add(scheme);
        this._schemes[scheme.Name] = scheme;
      }
    }
.....
}

　　这东西就是把我们在认证注册服务中指定的scheme，通过解析出的AuthenticationSchemeProvider 的构造函数加载来的，进而返回一系列的List<AuthenticationScheme>，OK拿到这些scheme之后有什么用呢？这里引出了我们的第二个对象AuthenticationHandlerProvider，下面我们来了解一下。　　

IAuthenticationHandlerProvider

　　我们看到，AuthenticationMiddleware中用到了IAuthenticationHandlerProvider的GetHandlerAsync方法，那我们先看一下这个方法的作用

public class AuthenticationHandlerProvider : IAuthenticationHandlerProvider
  {
    private Dictionary<string, IAuthenticationHandler> _handlerMap = new Dictionary<string, IAuthenticationHandler>((IEqualityComparer<string>) StringComparer.Ordinal);

    /// <summary>Constructor.</summary>
    public AuthenticationHandlerProvider(IAuthenticationSchemeProvider schemes)
    {
      this.Schemes = schemes;
    }

    /// <summary>
    /// The <see cref="T:Microsoft.AspNetCore.Authentication.IAuthenticationHandlerProvider" />.
    /// </summary>
    public IAuthenticationSchemeProvider Schemes { get; }

    /// <summary>Returns the handler instance that will be used.</summary>   
    public async Task<IAuthenticationHandler> GetHandlerAsync( HttpContext context, string authenticationScheme)
    {
      if (this._handlerMap.ContainsKey(authenticationScheme))
        return this._handlerMap[authenticationScheme];
      AuthenticationScheme schemeAsync = await this.Schemes.GetSchemeAsync(authenticationScheme);
      if (schemeAsync == null)
        return (IAuthenticationHandler) null;
      IAuthenticationHandler handler = (context.RequestServices.GetService(schemeAsync.HandlerType) ?? ActivatorUtilities.CreateInstance(context.RequestServices, schemeAsync.HandlerType)) as IAuthenticationHandler;
      if (handler != null)
      {
        await handler.InitializeAsync(schemeAsync, context);
        this._handlerMap[authenticationScheme] = handler;
      }
      return handler;
    }
  }　

　　在创建Handler的时候，是先从AuthenticationScheme中获取，如果不存在则通过ActivatorUtilities创建。获取到Handle后，将会放在_handlerMap字典里面， 当下次获取Handler的时候，将直接从缓存中获取。

IAuthenticationService

　　这个对象是在AuthenticationMiddleware中最后才用到的，而且是基于HttpContext的扩展被调用

public static class AuthenticationHttpContextExtensions
{
    public static Task<AuthenticateResult> AuthenticateAsync(this HttpContext context, string scheme) =>
        context.RequestServices.GetRequiredService<IAuthenticationService>().AuthenticateAsync(context, scheme);

  ....          
}

　　这里主要调用了IAuthenticationService的AuthenticateAsync方法，看一下这个方法做了什么

public class AuthenticationService : IAuthenticationService
{
    public IAuthenticationSchemeProvider Schemes { get; }
    public IAuthenticationHandlerProvider Handlers { get; }
    public IClaimsTransformation Transform { get; }

    public virtual async Task<AuthenticateResult> AuthenticateAsync(HttpContext context, string scheme)
    {
        if (scheme == null)
        {
            var scheme = (await this.Schemes.GetDefaultAuthenticateSchemeAsync())?.Name;
            if (scheme == null)
                throw new InvalidOperationException($"No authenticationScheme was specified, and there was no DefaultAuthenticateScheme found.");
        }

        var handler = await Handlers.GetHandlerAsync(context, scheme);
        if(handler == null)
            throw await this.CreateMissingHandlerException(scheme);
        AuthenticateResult result = await handler.AuthenticateAsync();
        if (result != null && result.Succeeded)           
            return AuthenticateResult.Success(new AuthenticationTicket(await Transform.TransformAsync(result.Principal), result.Properties, result.Ticket.AuthenticationScheme));

        return result;
    }
}

　　这里其实就是我们在前面讲的根据Scheme获取对应的AuthenticationHandler，然后调用AuthenticateAsync()方法，这个方法调用了核心方法HandleAuthenticateOnceAsync，然后再调用HandleAuthenticateAsync()这个核心的认证方法。

　　从上图看到这个HandleAuthenticateAsync是个抽象方法，我们的子类都需要实现这个方法的动作，基于本文的例子，我们看一下JwtBearerHandler的一个实际认证。

public class JwtBearerHandler : AuthenticationHandler<JwtBearerOptions>
{
    protected override async Task<AuthenticateResult> HandleAuthenticateAsync()
    {
      JwtBearerHandler jwtBearerHandler = this;
      string token = (string) null;
      object obj;
      AuthenticationFailedContext authenticationFailedContext;
      int num;
      try
      {
        MessageReceivedContext messageReceivedContext = new MessageReceivedContext(jwtBearerHandler.Context, jwtBearerHandler.Scheme, jwtBearerHandler.Options);
        await jwtBearerHandler.Events.MessageReceived(messageReceivedContext);
        if (messageReceivedContext.Result != null)
          return messageReceivedContext.Result;
        token = messageReceivedContext.Token;
        if (string.IsNullOrEmpty(token))
        {
          string header = (string) jwtBearerHandler.Request.Headers["Authorization"];
          if (string.IsNullOrEmpty(header))
            return AuthenticateResult.NoResult();
          if (header.StartsWith("Bearer ", StringComparison.OrdinalIgnoreCase))
            token = header.Substring("Bearer ".Length).Trim();
          if (string.IsNullOrEmpty(token))
            return AuthenticateResult.NoResult();
        }
        if (jwtBearerHandler._configuration == null && jwtBearerHandler.Options.ConfigurationManager != null)
        {
          OpenIdConnectConfiguration configurationAsync = await jwtBearerHandler.Options.ConfigurationManager.GetConfigurationAsync(jwtBearerHandler.Context.RequestAborted);
          jwtBearerHandler._configuration = configurationAsync;
        }
        TokenValidationParameters validationParameters1 = jwtBearerHandler.Options.TokenValidationParameters.Clone();
        if (jwtBearerHandler._configuration != null)
        {
          string[] strArray = new string[1]
          {
            jwtBearerHandler._configuration.Issuer
          };
          TokenValidationParameters validationParameters2 = validationParameters1;
          IEnumerable<string> validIssuers = validationParameters1.get_ValidIssuers();
          object obj1 = (validIssuers != null ? (object) validIssuers.Concat<string>((IEnumerable<string>) strArray) : (object) null) ?? (object) strArray;
          validationParameters2.set_ValidIssuers((IEnumerable<string>) obj1);
          TokenValidationParameters validationParameters3 = validationParameters1;
          IEnumerable<SecurityKey> issuerSigningKeys = validationParameters1.get_IssuerSigningKeys();
          IEnumerable<SecurityKey> securityKeys = (issuerSigningKeys != null ? issuerSigningKeys.Concat<SecurityKey>((IEnumerable<SecurityKey>) jwtBearerHandler._configuration.get_SigningKeys()) : (IEnumerable<SecurityKey>) null) ?? (IEnumerable<SecurityKey>) jwtBearerHandler._configuration.get_SigningKeys();
          validationParameters3.set_IssuerSigningKeys(securityKeys);
        }
        List<Exception> exceptionList = (List<Exception>) null;
        foreach (ISecurityTokenValidator securityTokenValidator in (IEnumerable<ISecurityTokenValidator>) jwtBearerHandler.Options.SecurityTokenValidators)
        {
          if (securityTokenValidator.CanReadToken(token))
          {
            SecurityToken securityToken;
            ClaimsPrincipal claimsPrincipal;
            try
            {
              claimsPrincipal = securityTokenValidator.ValidateToken(token, validationParameters1, ref securityToken);
            }
            catch (Exception ex)
            {
              jwtBearerHandler.Logger.TokenValidationFailed(ex);
              if (jwtBearerHandler.Options.RefreshOnIssuerKeyNotFound && jwtBearerHandler.Options.ConfigurationManager != null && ex is SecurityTokenSignatureKeyNotFoundException)
                jwtBearerHandler.Options.ConfigurationManager.RequestRefresh();
              if (exceptionList == null)
                exceptionList = new List<Exception>(1);
              exceptionList.Add(ex);
              continue;
            }
            jwtBearerHandler.Logger.TokenValidationSucceeded();
            TokenValidatedContext validatedContext = new TokenValidatedContext(jwtBearerHandler.Context, jwtBearerHandler.Scheme, jwtBearerHandler.Options);
            validatedContext.Principal = claimsPrincipal;
            validatedContext.SecurityToken = securityToken;
            TokenValidatedContext tokenValidatedContext = validatedContext;
            await jwtBearerHandler.Events.TokenValidated(tokenValidatedContext);
            if (tokenValidatedContext.Result != null)
              return tokenValidatedContext.Result;
            if (jwtBearerHandler.Options.SaveToken)
              tokenValidatedContext.Properties.StoreTokens((IEnumerable<AuthenticationToken>) new AuthenticationToken[1]
              {
                new AuthenticationToken()
                {
                  Name = "access_token",
                  Value = token
                }
              });
            tokenValidatedContext.Success();
            return tokenValidatedContext.Result;
          }
        }
        if (exceptionList == null)
          return AuthenticateResult.Fail("No SecurityTokenValidator available for token: " + token ?? "[null]");
        authenticationFailedContext = new AuthenticationFailedContext(jwtBearerHandler.Context, jwtBearerHandler.Scheme, jwtBearerHandler.Options)
        {
          Exception = exceptionList.Count == 1 ? exceptionList[0] : (Exception) new AggregateException((IEnumerable<Exception>) exceptionList)
        };
        await jwtBearerHandler.Events.AuthenticationFailed(authenticationFailedContext);
        return authenticationFailedContext.Result == null ? AuthenticateResult.Fail(authenticationFailedContext.Exception) : authenticationFailedContext.Result;
      }
      catch (Exception ex)
      {
        obj = (object) ex;
        num = 1;
      }
      if (num == 1)
      {
        Exception ex = (Exception) obj;
        jwtBearerHandler.Logger.ErrorProcessingMessage(ex);
        authenticationFailedContext = new AuthenticationFailedContext(jwtBearerHandler.Context, jwtBearerHandler.Scheme, jwtBearerHandler.Options)
        {
          Exception = ex
        };
        await jwtBearerHandler.Events.AuthenticationFailed(authenticationFailedContext);
        if (authenticationFailedContext.Result != null)
          return authenticationFailedContext.Result;
        Exception source = obj as Exception;
        if (source == null)
          throw obj;
        ExceptionDispatchInfo.Capture(source).Throw();
        authenticationFailedContext = (AuthenticationFailedContext) null;
      }
      obj = (object) null;
      token = (string) null;
      AuthenticateResult authenticateResult;
      return authenticateResult;
    }
}

　　这个方法有点长，主要是从Request.Headers里面获取Authorization的Bearer出来解析，再在AddJwtBearer中传入的委托参数JwtBearerOptions的TokenValidationParameters属性作为依据进行对比来进行认证是否通过与否。

　　总结

　　本文对 ASP.NET Core 的认证流程做了一个源码分析流程介绍，由于是源码分析篇，所以可能会比较枯燥和苦涩难懂。在后面的真正使用过程中，然后再结合本篇的一个总结流程，相信大家会逐渐开朗。

1. 在Startup类中的ConfigureServices方法通过添加AddAuthentication注册我们最主要的三个对象AuthenticationService, AuthenticationHandlerProvider, AuthenticationSchemeProvider
2. 通过AddAuthentication返回的AuthenticationBuilder 通过AddJwtBearer（或者AddCookie）来指定Scheme类型和需要验证的参数
3. 在Startup类中的Configure方法通过添加UseAuthentication注册认证中间件
4. 在认证过程中，通过AuthenticationSchemeProvider获取正确的Scheme，在AuthenticationService中通过Scheme和AuthenticationHandlerProvider获取正确的AuthenticationHandler，最后通过对应的AuthenticationHandler的AuthenticateAsync方法进行认证流程