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Why do you need users to sign in. Do they really want to sign in. What do they get out of it versus what do you get out of it

Using Sign in with Google just encourages use of Google. Is that really the best thing for users. Google is a privacy disaster

If you and your users truly both need authentication of each other why not let your users use x509 client certificates

You are probably already using x509 server certificates

The term "SDK" is synonymous with "unnecessary fluff". Its been that way since the 1990s