How to Implement Single Sign-On Using Okta API in C#

Integrate Okta SDK in C# Project

• To start with Okta integration in C#, you should include the Okta SDK in your project by adding the NuGet package. Open the NuGet Package Manager Console and execute:

Install-Package Okta.Sdk

Configure Okta Client

• Create an `okta-appsettings.json` file to hold your Okta configurations. This should include your `OrgUrl`, `ClientId`, `ClientSecret`, and `AuthorizationServerId`.

{
  "Okta": {
    "OrgUrl": "https://{yourOktaDomain}",
    "ClientId": "{yourClientId}",
    "ClientSecret": "{yourClientSecret}",
    "AuthorizationServerId": "default"
  }
}

• Make sure to load these settings in your application startup using the `Microsoft.Extensions.Configuration` library.

Set Up Authentication Context

• Create a new service that implements Okta's authentication flow. Utilize dependency injection to create an Okta client using the configuration defined previously.

public class OktaAuthService
{
    private readonly IOktaClient _oktaClient;
    
    public OktaAuthService(IOktaClient oktaClient)
    {
        _oktaClient = oktaClient;
    }

    // Implement the method to authenticate users here
}

• Ensure to register the Okta client during the application startup:

services.AddSingleton<IOktaClient>(new OktaClient(new OktaClientConfiguration
{
    OrgUrl = configuration["Okta:OrgUrl"],
    Token = configuration["Okta:ClientSecret"],
}));

Generate Authorization URL

• To initiate the SSO process, generate an authorization URL that redirects the user to Okta for authentication. Okta uses OAuth/OpenID Connect for this.

public string GenerateAuthorizationUrl()
{
    var authorizeUrl = new UriBuilder(_oktaClient.Configuration.OrgUrl)
    {
        Path = "/oauth2/default/v1/authorize",
        Query = "client_id={yourClientId}&response_type=code&scope=openid&redirect_uri={yourRedirectUri}&state={state}&nonce={nonce}"
    };
    
    return authorizeUrl.ToString();
}

• This URL directs the user to Okta's login, and upon successful login, the user is redirected back with an authorization code in the query string.

Exchange Authorization Code for Tokens

• After receiving the authorization code, your application must exchange it for an access token. Use the Okta SDK to handle this OAuth flow.

public async Task<IOAuthTokenResponse> ExchangeCodeForTokenAsync(string authorizationCode, string redirectUri)
{
    var tokenRequest = new AuthorizeRequest
    {
        GrantType = "authorization_code",
        Code = authorizationCode,
        RedirectUri = redirectUri,
        ClientId = _oktaClient.Configuration.ClientId,
        ClientSecret = _oktaClient.Configuration.ClientSecret,
    };

    return await _oktaClient.GetOAuthTokensAsync(tokenRequest);
}

• This completes the login process, and you can now use the tokens to authenticate API requests or retrieve user info.

Verify the ID Token

• Once you have the ID token, verify its signature and claims. This step ensures that the token is authentic and has not expired.

public bool VerifyIdToken(string idToken)
{
    var handler = new JwtSecurityTokenHandler();
    var token = handler.ReadJwtToken(idToken);

    // Perform validation checks on the token (e.g., signature and expiration)
    return token.ValidTo > DateTime.UtcNow;
}

• Use libraries such as `System.IdentityModel.Tokens.Jwt` and `Microsoft.IdentityModel.Tokens` to validate the token signature and claims.

Logout Implementation

• To log out the user, redirect them to the Okta sign-out URL, which terminates the session within Okta.

public string GetLogoutUrl()
{
    return $"{_oktaClient.Configuration.OrgUrl}/oauth2/default/v1/logout?id_token_hint={idToken}&post_logout_redirect_uri={yourLogoutRedirectUri}";
}

• Ensure the ID token is valid and the post-logout redirect URI is configured in your Okta application settings.

By following these steps, you will have a comprehensive implementation of Single Sign-On using the Okta API in C#. Adjust the configurations and implementations as needed based on your application's specific requirements and infrastructure.