This post is by Mike Jones from Mike Jones: self-issued
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I highly recommend a piece that my friend Vittorio Bertocci wrote on the relationship between OAuth 2.0 and sign-in/federation protocols. While OAuth 2.0 can be used to sign in users and the term “OAuth” is often bandied about in identity contexts, as he points out, there’s a lot of details to fill in to make that possible. That’s because OAuth 2.0 is a resource authorization protocol – not an authentication protocol.
Read his post for a better understanding of how OAuth 2.0 relates to sign-in protocols, including a useful discussion of how OpenID Connect fills in the gaps to enable people to sign in with OAuth 2.0 in an interoperable manner.