Firebase Authentication with Azure AD B2C

I have a native iOS application developed in xCode/Swift that integrates the Firebase Auth SDK for registering and login of new users. The Firebase Auth system should be linked to an Azure Active Directory B2C so that:

• new users that are created exist in both Firebase Auth and Azure AD B2C (I guess with same credentials?)
• when a user is logged in, he/she is logged-in both in the context of the Firebase Auth and Azure AD B2C so that secure call are possible on both platforms
• when users are deleted either on the Azure AD B2C or firebase Auth they should also be deleted on the other platform. For this I could maybe use the Firebase Functions Triggered on Auth events to sync with Azure AD B2C. For Azure AD B2C back to Firebase Auth I could foresee also a Firebase function that can be called (by Azure AD B2C) do delete a user.

After doing research I found that Firebase supports "custom tokens" (refer to https://firebase.google.com/docs/auth/admin/create-custom-tokens). Could this offer a solution? I do not see anything about creating new users, only authentication...

Me and my team developed a solution for this exact scenario around custom tokens. Our implementation run since a couple of months on Azure and we had no problem at all yet.

Here is the flow:

Notes:

1. Azure B2C can be replace by any STS.
2. The Azure function can be replace by any backend process.

Also, the good thing is that you don't have to maintain users in Firebase since you don't create Firebase users and only use a custom token so no need to delete users.