Problem Summary I'm experiencing a persistent invalid-credential error with Apple Sign-In on iOS despite having verified every aspect of the configuration over the past 6 months. The error occurs at the Firebase Authentication level after successfully receiving credentials from Apple. Error Message: Firebase auth error: invalid-credential - Invalid OAuth response from apple.com. Environment Platform: iOS (Flutter app) Firebase Auth: v5.7.0 Sign in with Apple: v6.1.2 Xcode: Latest version with capability enabled iOS Target: 13.0+ Bundle ID: com.harmonics.orakl What Actually Happens ✅ Apple Sign-In popup appears ✅ User can authenticate with Apple ID ✅ Apple returns credentials with identityToken ❌ Firebase rejects with invalid-credential error The error occurs at Firebase level, not Apple level. What I've Tried Created a brand new Apple Key (previous key was 6 months old) Tested with both App ID and Service ID in Firebase Completely reinstalled CocoaPods dependencies Verified nonce handling is correct (hashed to Apple, raw to Firebase) Activated Firebase Hosting and attempted to deploy .well-known file Checked Cloud Logging (no detailed error messages found) Disabled and re-enabled Apple Sign-In provider in Firebase Verified Return URL matches exactly Waited and retried multiple times over 6 months Questions Is the .well-known/apple-developer-domain-association.txt file required? If yes, how should it be generated? Firebase Hosting doesn't auto-generate it. Could there be a server-side caching/blacklist issue with my domain or Service ID after multiple failed attempts? Should the Apple Key be linked to the Service ID instead of the App ID? The key shows as linked to Z3NNDZVWMZ.com.harmonics.orakl (the App ID). Is there any way to get more detailed error logs from Firebase about why it's rejecting the Apple OAuth response? Could using a custom domain instead of .firebaseapp.com resolve the issue? Additional Context Google Sign-In works perfectly on the same app The configuration has been reviewed by multiple developers Error persists across different devices and iOS versions No errors in Xcode console except the Firebase rejection Any help would be greatly appreciated. I've exhausted all standard troubleshooting steps and documentation. Project Details: Bundle ID: com.harmonics.orakl Firebase Project: harmonics-app Team ID: Z3N....... code : // 1. Generate raw nonce final String rawNonce = _generateRandomNonce(); // 2. Hash with SHA-256 final String hashedNonce = _sha256Hash(rawNonce); // 3. Send HASHED nonce to Apple ✅ final appleCredential = await SignInWithApple.getAppleIDCredential( scopes: [AppleIDAuthorizationScopes.email, AppleIDAuthorizationScopes.fullName], nonce: hashedNonce, // Correct: hashed nonce to Apple ); // 4. Create Firebase credential with RAW nonce ✅ final oauthCredential = OAuthProvider("apple.com").credential( idToken: appleCredential.identityToken!, rawNonce: rawNonce, // Correct: raw nonce to Firebase ); // 5. Sign in with Firebase - ERROR OCCURS HERE ❌ await FirebaseAuth.instance.signInWithCredential(oauthCredential);