On November 12, 2018, a new certificate for server-based Game Center Player ID verification will be provided via the publicKeyUrl property of generateIdentityVerificationSignature, and the previous certificate will expire.
The new X.509 certificate URL returned by publicKeyUrl will be https://static.gc.apple.com/public-key/gc-prod-4.cer. Both the new certificate and prior certificates use SHA256withRSA.
Make sure your app always retrieves and uses the current publicKeyURL value so it automatically uses the new certificate. If your app caches the certificate or hardcodes the certificate URL, it will require an update.