[OAuth]基于DotNetOpenAuth实现Client Credentials Grant

Client Credentials Grant是指直接由Client向Authorization Server请求access token，无需用户（Resource Owner）的授权。比如我们提供OpenAPI让大家可以获取园子首页最新随笔，只需验证一下Client是否有权限调用该API，不需要用户的授权。而如果Client需要进行发布博客的操作，就需要用户的授权，这时就要采用Authorization Code Grant。

DotNetOpenAuth是当前做得做好的基于.NET的OAuth开源实现，项目网址：https://github.com/DotNetOpenAuth。

Client Credentials Grant的流程图如下（图片1来源，图片2来源）：

一、Client向Authorization Server请求access token

主要操作如下：

1. 由client_id和client_secret构建出credentials。

2. 将credentials以http basic authentication的方式发送给Authorization Server。

3. 从Authorization Server的响应中提取access token

Client的实现代码如下：

public async Task<ActionResult> SiteHome()
{
    var client_id = "m.cnblogs.com";
    var client_secret = "20140213";
    var credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes(client_id + ":" + client_secret));

    var httpClient = new HttpClient();
    httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", credentials);
    var httpContent = new FormUrlEncodedContent(new
    Dictionary<string, string>
    {
        {"grant_type", "client_credentials"}
    });

    var response = await httpClient.PostAsync("https://authserver.open.cnblogs.com/oauth/token", httpContent);

    var responseContent = await response.Content.ReadAsStringAsync();
    if (response.StatusCode == System.Net.HttpStatusCode.OK)
    {
        var accessToken = JObject.Parse(responseContent)["access_token"].ToString();
        return Content("AccessToken: " + accessToken);              
    }
    else
    {
        return Content(responseContent);
    }
}

二、Authorization Server验证Client，发放access token

主要操作如下：

1. Authorization Server通过IAuthorizationServerHost.GetClient()获取当前Client。

2. Authorization Server通过IClientDescription.IsValidClientSecret()验证当前Client。

3. 验证通过后，将access token包含在响应中发送给Client。

主要实现代码如下（基于ASP.NET MVC）：

1. Authorization Server中Client实体类的实现代码（关键代码是IsValidClientSecret()的实现）：

    public class Client : IClientDescription
    {
        public string Id { get; set; }

        public string Secret { get; set; }

        public Uri DefaultCallback
        {
            get { throw new NotImplementedException(); }
        }

        private ClientType _clientType;
        public ClientType ClientType
        {
            get { return _clientType; }
            set { _clientType = value; }
        }

        public bool HasNonEmptySecret
        {
            get { throw new NotImplementedException(); }
        }

        public bool IsCallbackAllowed(Uri callback)
        {
            throw new NotImplementedException();
        }

        public bool IsValidClientSecret(string secret)
        {
            return this.Secret == secret;
        }
    }

AuthorizationServerHost的代码（关键代码是GetClient()与CreateAccessToken()的实现）：

public class AuthorizationServerHost : IAuthorizationServerHost
{
    public static readonly ICryptoKeyStore HardCodedCryptoKeyStore = new HardCodedKeyCryptoKeyStore("...");

    public IClientDescription GetClient(string clientIdentifier)
    {
        return ServiceLocator.GetService<IClientService>().GetClient(clientIdentifier);
    }

    public AccessTokenResult CreateAccessToken(IAccessTokenRequest accessTokenRequestMessage)
    {
        var accessToken = new AuthorizationServerAccessToken
        {
            Lifetime = TimeSpan.FromHours(10),
            SymmetricKeyStore = this.CryptoKeyStore,
        };
        var result = new AccessTokenResult(accessToken);
        return result;
    }

    public AutomatedAuthorizationCheckResponse CheckAuthorizeClientCredentialsGrant(IAccessTokenRequest accessRequest)
    {
        //...
    }

    public AutomatedUserAuthorizationCheckResponse CheckAuthorizeResourceOwnerCredentialGrant
        (string userName, string password, IAccessTokenRequest accessRequest)
    {
        //...
    }        

    public DotNetOpenAuth.Messaging.Bindings.ICryptoKeyStore CryptoKeyStore
    {
        get { return HardCodedCryptoKeyStore; }
    }

    public bool IsAuthorizationValid(IAuthorizationDescription authorization)
    {
        return true;
    }

    public INonceStore NonceStore
    {
        get { return null; }
    }
}

三、Client通过access token调用Resource Server上的API

主要实现代码如下：

public async Task<ActionResult> HomePosts(string blogApp)
{
    //获取access token的代码见第1部分
    //...
    var accessToken = JObject.Parse(responseContent)["access_token"].ToString();
    httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
    response = await httpClient.GetAsync("https://api.open.cnblogs.com/blog/posts/sitehome");
    return Content(await response.Content.ReadAsStringAsync());               
}

四、Resource Server验证Client的access token，响应Client的API调用请求

主要实现代码如下（基于ASP.NET Web API）：

1. 通过MessageHandler统一验证access token

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.MessageHandlers.Add(new BearerTokenHandler());
    }
}

2. BearerTokenHandler的实现代码（来自DotNetOpenAuth的示例代码）：

public class BearerTokenHandler : DelegatingHandler
{
    protected override async System.Threading.Tasks.Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
    {
        if (request.Headers.Authorization != null && request.Headers.Authorization.Scheme == "Bearer")
        {
            var resourceServer = new DotNetOpenAuth.OAuth2.ResourceServer
                (new StandardAccessTokenAnalyzer
                (AuthorizationServerHost.HardCodedCryptoKeyStore));

                var principal = await resourceServer.GetPrincipalAsync(request, cancellationToken);
                HttpContext.Current.User = principal;
                Thread.CurrentPrincipal = principal;
        }

        return await base.SendAsync(request, cancellationToken);
    }
}

3. Web API的示例实现代码：

public class PostsController : ApiController
{
    [Route("blog/posts/sitehome")]
    public async Task<IEnumerable<string>> GetSiteHome()
    {
        return new string[] { User.Identity.Name };
    }
}

四、Client得到Resouce Server的响应结果

根据上面的Resouce Server中Web API的示例实现代码，得到的结果是：

["client:m.cnblogs.com"] 

小结

看起来比较简单，但实际摸索的过程是曲折的。分享出来，也许可以让初次使用DotNetOpenAuth的朋友少走一些弯路。

【参考资料】

The OAuth 2.0 Authorization Framework

Claim-based-security for ASP.NET Web APIs using DotNetOpenAuth

Implementing an API Key with DotNetOpenAuth