OpenID Connect is a profile of OAuth 2 that adds authentication and identity information. Sign in with Apple was a partial implementation of OpenID Connect, for

OpenID Connect is a profile of OAuth 2 that adds authentication and identity information. Sign in with Apple was a partial implementation of OpenID Connect, for sharing authentication as well as an email address and the user's name between Apple and other apps/services. As a partial implementation, there were both difficulties using existing third party libraries and known security issues in Apple's implementation. This was shown initially by manual inspection, then by Apple's implementation failing the freely available certification test suite published by the OpenID Foundation (I believe run by third parties, not Apple themselves). The OpenID Foundation published an open letter both to ask Apple to fix these issues, and to inform other operators that there were security issues with Apple's implementation. A complete implementation solves known security and interoperability issues.