Auth0 + Xamarin.Android认证问题：兑换代码时未授权错误

Let's break down the most common fixes for this unauthorized error when redeeming the authorization code in your Xamarin.Android app. Since you've already followed the tutorial and set up RS256 + OIDC Conformant, let's dive into the often-overlooked details that cause this issue:

• Double-check your Android app's package name and signing fingerprint in Auth0

  This is the #1 culprit for unauthorized errors in mobile Auth0 integrations.

  • Head to your Auth0 application dashboard → Settings → Applications → Your Xamarin.Android app entry
  • Under Allowed Android Origins, confirm the entry matches exactly: package:<your-app-package-name> (e.g., package:com.yourcompany.yourapp)
  • For the signing fingerprint:
    1. Generate the SHA-1 fingerprint for your debug/release keystore using keytool -list -v -keystore <your-keystore-path> -alias <your-alias>
    2. Paste it into the Signing Certificates section in Auth0. Note: Debug and release builds use different keystores, so add both if you're testing with debug builds.

• Verify PKCE implementation (critical for Authorization Code Flow)
  Even if your code matches the tutorial, double-check these details:

  • Your app generates a valid code_verifier and code_challenge before starting the auth flow
  • The code_challenge_method is set to S256 (not plain text)
  • When redeeming the code, you're sending the exact same code_verifier that was used to generate the challenge. A common mistake is regenerating it instead of storing it temporarily during the flow.

• Validate Allowed Callback URLs
  Ensure your Auth0 app's Allowed Callback URLs includes the correct scheme for your Xamarin.Android app. It should follow this format:

  <your-auth0-domain>://<your-app-package-name>/android/<your-app-package-name>/callback
  

  Example: dev-xxxxxx.auth0.com://com.yourcompany.yourapp/android/com.yourcompany.yourapp/callback
  Replace placeholders with your actual Auth0 domain and app package name.

• Check Auth0 Application Type and Grant Types

  • Confirm your Auth0 app is set to Native type (not Regular Web or Single Page Application)
  • Under Settings → Advanced → Grant Types, make sure Authorization Code is enabled (it should be default for Native apps, but double-check to be safe)

• Test with a minimal debug setup
  To rule out code-side issues:

  1. Create a brand new Xamarin.Android project and follow the Auth0 tutorial step-by-step
  2. Use the exact package name and fingerprint you configured in Auth0
  3. Run the minimal app—if it works, compare your original app's code for accidental modifications to the auth flow logic

• Inspect network requests (if possible)
  Use a tool like Charles Proxy to capture the request to Auth0's /oauth/token endpoint when redeeming the code. Verify:

  • You're sending all required parameters: code, client_id, redirect_uri, code_verifier, grant_type=authorization_code
  • The redirect_uri matches exactly what was used in the initial authorization request
  • No extra headers or parameters are interfering with the request

内容的提问来源于stack exchange，提问作者pavel