Leveraging OAuth2 Authorize Endpoint for Single Sign-On (SSO) with CSharp

Why did the C# developer get locked out of the SSO system?

Because they couldn't find the right key for the AuthCode!

Single Sign-On (SSO) is a critical feature in modern web applications, offering users a seamless and secure way to access multiple services with one set of credentials. OAuth2, a robust and widely-adopted authorization framework, provides the backbone for implementing SSO. In this article, we’ll explore how to use a standard OAuth2 authorize endpoint to enable SSO, detailing the key parameters involved in the process and providing C# examples to illustrate the implementation.

What is OAuth2?

OAuth2 is an open standard for access delegation, commonly used as a way to grant websites or applications limited access to a user’s information without exposing passwords. It is widely used by companies such as Google, Facebook, and GitHub to allow users to share information about their accounts with third-party applications.

OAuth2 Grant Types

There are several OAuth2 grant types, but for SSO, the Authorization Code Grant is the most relevant. This grant type is designed for applications where the authorization server can interact with the user’s web browser.

How OAuth2 Authorization Works for SSO

1. User Initiates Authentication: The user tries to access a protected resource on the client application.
2. Redirect to Authorization Server: The client application redirects the user to the OAuth2 authorization server.
3. User Authenticates: The user logs in to the authorization server.
4. Authorization Code Issued: Upon successful authentication, the authorization server redirects the user back to the client application with an authorization code.
5. Token Exchange: The client application exchanges the authorization code for an access token.
6. Access Granted: The access token is used to request resources from the resource server.

Key Parameters of the OAuth2 Authorize Endpoint

When redirecting the user to the authorization server, the client application must include several parameters in the request URL. Here’s a breakdown of these parameters:

• response_type: Indicates the type of response desired. For SSO using the authorization code grant, this is set to code.
• client_id: The client identifier issued to the client during the registration process.
• redirect_uri: The URL to which the authorization server will send the user after authorization.
• scope: A space-delimited list of permissions that the application is requesting.
• state: An opaque value used to maintain state between the request and the callback. It is used to prevent CSRF attacks.
• code_challenge and code_challenge_method: Used in PKCE (Proof Key for Code Exchange) to enhance security (optional but recommended).

Example Authorization Request

Here’s an example of what the URL might look like when redirecting the user to the authorization server:

https://authorization-server.com/auth
?response_type=code
&client_id=YOUR_CLIENT_ID
&redirect_uri=https://your-application.com/callback
&scope=openid profile email
&state=xyzABC123
&code_challenge=HASHED_CODE_VERIFIER
&code_challenge_method=S256

Step-by-Step SSO Implementation Using OAuth2 in C

1. Configure Your OAuth2 Client: Register your application with the authorization server to obtain a client ID and client secret.
2. Redirect User to Authorization Endpoint: When the user attempts to access a protected resource, redirect them to the authorization server with the necessary parameters.
3. Handle Authorization Code: After the user authenticates, they will be redirected back to your specified redirect_uri with an authorization code.
4. Exchange Authorization Code for Access Token: Your application will make a server-side request to the token endpoint to exchange the authorization code for an access token.
5. Access Protected Resources: Use the access token to make authenticated requests to protected resources on behalf of the user.

Example C# Code for Redirecting to Authorization Endpoint

public IActionResult SignIn()
{
    var authorizationUrl = "https://authorization-server.com/auth";
    var clientId = "YOUR_CLIENT_ID";
    var redirectUri = "https://your-application.com/callback";
    var scope = "openid profile email";
    var state = "xyzABC123"; // Generate a unique state value for each request
    var codeChallenge = "HASHED_CODE_VERIFIER"; // Generate a code challenge
    var codeChallengeMethod = "S256";

    var url = $"{authorizationUrl}?response_type=code&client_id={clientId}&redirect_uri={Uri.EscapeDataString(redirectUri)}&scope={Uri.EscapeDataString(scope)}&state={state}&code_challenge={codeChallenge}&code_challenge_method={codeChallengeMethod}";

    return Redirect(url);
}

Example C# Code for Exchanging Authorization Code for Access Token

public async Task<IActionResult> Callback(string code, string state)
{
    var tokenEndpoint = "https://authorization-server.com/token";
    var clientId = "YOUR_CLIENT_ID";
    var clientSecret = "YOUR_CLIENT_SECRET";
    var redirectUri = "https://your-application.com/callback";
    var codeVerifier = "ORIGINAL_CODE_VERIFIER"; // The original code verifier used to generate the code challenge

    var client = new HttpClient();

    var requestBody = new List<KeyValuePair<string, string>>
    {
        new KeyValuePair<string, string>("grant_type", "authorization_code"),
        new KeyValuePair<string, string>("code", code),
        new KeyValuePair<string, string>("redirect_uri", redirectUri),
        new KeyValuePair<string, string>("client_id", clientId),
        new KeyValuePair<string, string>("client_secret", clientSecret),
        new KeyValuePair<string, string>("code_verifier", codeVerifier)
    };

    var requestContent = new FormUrlEncodedContent(requestBody);

    var response = await client.PostAsync(tokenEndpoint, requestContent);
    var responseContent = await response.Content.ReadAsStringAsync();

    // Handle the response (e.g., deserialize the token response and store the tokens)

    return Ok(responseContent); // or Redirect to a protected resource
}

Implementing SSO with OAuth2 involves a series of well-defined steps and parameters that ensure secure and seamless user authentication across multiple applications. By understanding and correctly utilizing the OAuth2 authorize endpoint and incorporating the provided C# examples, developers can provide users with a streamlined login experience while maintaining robust security standards.

Whether you are developing a new application or integrating with existing identity providers, mastering OAuth2 for SSO is a crucial skill that can significantly enhance user experience and security.