Identity for Mobile Applications

In this lesson, you will learn how to configure secure OpenID Connect (OIDC) flows that address the inherent security limitations of mobile environments.

The Mobile Context

Mobile applications are "public clients." Unlike a web server, you cannot securely hide a ClientSecret within your app’s binary, as it could be extracted by anyone inspecting the package. Because of this, traditional flows like the Authorization Code Flow with a secret are inappropriate. Instead, we use the Authorization Code Flow with PKCE (Proof Key for Code Exchange).

How PKCE Secures the Flow

PKCE mitigates the risk of an attacker intercepting the authorization code. It works by having the mobile app generate a dynamic, one-time secret for every authentication attempt.

1. Code Verifier: The app generates a high-entropy cryptographic random string.
2. Code Challenge: The app hashes this string (usually with SHA-256) and sends the result to the identity provider during the initial request.
3. Verification: When the app exchanges the authorization code for an access token, it sends the original Code Verifier. The identity provider hashes the verifier and ensures it matches the Code Challenge sent earlier.

Implementation Concept

In C#, when using libraries like IdentityModel.OidcClient, you don’t need to handle the low-level math, but you must ensure your client configuration enforces this handshake:

var options = new OidcClientOptions
{
    Authority = "https://your-identity-server.com",
    ClientId = "mobile-app-client-id",
    RedirectUri = "myapp://callback",
    // Ensure the flow is configured for PKCE
    Flow = OidcClientOptions.AuthenticationFlow.AuthorizationCode,
    Policy = new Policy { Discovery = { RequireKeySet = true } }
};

By enforcing this, you ensure that even if an attacker manages to intercept the authorization code (e.g., via a malicious app registering the same custom URL scheme), they cannot exchange it for a token because they lack the original Code Verifier generated only in your app's memory. Always use ASWebAuthenticationSession (iOS) or Custom Tabs (Android) to ensure the authentication happens in a browser context isolated from your app, preventing your code from ever "seeing" the user's password.

Key Takeaway: Never store client secrets in mobile binaries; always use the Authorization Code Flow with PKCE to cryptographically bind the authentication request to the specific device instance.