Hello Apple Developer Community and Support,
Our team is encountering a critical and persistent issue with our backend integration of Sign In with Apple, and we are hoping for some insights or assistance.
Problem:
We consistently receive an "invalid_client" error (HTTP 400 status) when our backend service attempts to exchange the authorization code for tokens at Apple's https://appleid.apple.com/auth/token endpoint. The error message from Apple's response is simply {"error":"invalid_client"}.
Our Setup:
Client Application: An iOS native application.
Backend Service: A Go backend responsible for server-to-server token exchange and user management.
Sign In with Apple Flow: The iOS app initiates the Sign In with Apple flow, obtains an authorization code, and then passes this code to our backend for token exchange.
Extensive Troubleshooting Performed (No Success):
We have meticulously followed all official Apple documentation (including TN3107: Resolving Sign In with Apple Response Errors) and industry best practices. Here's a summary of our verification steps, all of which currently show correct configurations and parameters:
Backend client_secret JWT Construction:
We generate a client_secret JWT as required for server-to-server communication.
We've confirmed the claims in the generated JWT are correct:
iss (Issuer): Our Team ID (e.g., XXXXXXXXXX).
sub (Subject): Our Service ID (e.g., com.example.service.backendauth).
aud (Audience): https://appleid.apple.com.
kid (Key ID): The Key ID associated with our .p8 private key (e.g., YYYYYYYYYY).
We have performed rigorous verification of the .p8 private key content itself, ensuring no corruption, extra characters, or formatting issues in the environment variable. Our backend logs confirm it's parsing the correct PEM content.
Token Exchange Request Parameters:
The client_id parameter sent in the POST request to /auth/token is correctly set to our App Bundle ID (e.g., com.example.app.ios), as this is the identifier for which the code was originally issued.
The redirect_uri parameter sent in the POST request to /auth/token is precisely matched to a registered "Return URL" in our Apple Developer Portal (e.g., https://api.example.com:port/api/auth/callback?provider=apple).
Apple Developer Portal Configuration (Meticulously Verified):
App ID: Enabled for "Sign In with Apple".
Service ID: Enabled for "Sign In with Apple". Its "Primary App ID" is correctly linked to our App Bundle ID (e.g., com.example.app.ios). Its "Return URLs" exactly match our backend's redirect_uri (e.g., https://api.example.com:port/api/auth/callback?provider=apple).
Key: Our .p8 key has "Sign In with Apple" enabled. Crucially, in its configuration panel, the "Primary App ID" is correctly linked to our App Bundle ID (e.g., com.example.app.ios). We've ensured this key is specifically created for "Sign In with Apple" and not other services like APNs.
We have performed multiple full revocations and meticulous re-creations of the App ID, Service ID, and Key in the Apple Developer Portal, ensuring correct linkages and using new identifiers to bypass any potential caching issues.
Network & System Health Checks:
Network connectivity from our backend server to https://appleid.apple.com (port 443) has been confirmed as fully functional via ping and curl -v.
The incoming TLS handshake from our iOS client app to our backend server's callback URL (https://api.example.com:port/...) is successful and verified via openssl s_client -connect. There are no longer any TLS handshake errors (EOF).
Our backend server's system clock is accurately synchronized via NTP.
Request for Assistance:
Given that all our visible configurations, environment variables, and request parameters appear to be correct and align with Apple's documentation, and network connectivity is confirmed, we are at a loss for why the invalid_client error persists.
Based on TN3107, this error typically implies an issue with the client secret's signature or its validity for the given client_id. However, our logs confirm correct iss, sub, aud, and kid, and the private key content.
Has anyone encountered this persistent invalid_client error when all checks pass? Are there any less common configurations or troubleshooting steps we might be missing? Could this indicate a caching or propagation delay on Apple's servers, even after waiting periods?
Any insights or guidance would be greatly appreciated. We are prepared to provide detailed, anonymized logs and screenshots to Apple Developer Support privately if requested.
Thank you.
