Authentication as a service to expose rest server to external services
I have a rest service which I'd like to expose to other companies' backend services. I already have Firebase which allows authentication but this is for an end user rather than external service.
Is there something similar to Firebase but for service to service communication where 3rd party users can manage keys?
Google Cloud Platform is preferable. I looked at Apigee from Google but it's heavyweight for just authentication.
Have you considered Google's CAS to create a private CA. Give third parties access to add/rollover certs in your private CA (should be possible, not sure). Then in your services, you can trust the issuer, namely your private CA. You can also use the certs to do fine-grained authorizations.