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Protocols26 February 2025·4 min read

OAuth 2.0 and OpenID Connect: The evolution from authorization to identity

It is easy to read a specification and conclude the problem is solved.

Paycux engineering

It is easy to read a specification and conclude the problem is solved. It is harder to sit with the twenty implementations that each read the same paragraph differently and still call themselves compliant.

This piece walks through how we think about it at Paycux, what we have changed our minds about, and where the sharp edges are.

Why OIDC emerged: The drive to offload identity to identity providers

Consider why oidc emerged: the drive to offload identity to identity providers. Normalization is the whole job. Two providers can be perfectly compliant and still disagree about what a group membership is, whether a logout is meaningful, and which attribute carries the email address.

Normalization is the whole job. Two providers can be perfectly compliant and still disagree about what a group membership is, whether a logout is meaningful, and which attribute carries the email address.

Using OAuth for authentication

Using OAuth for authentication deserves its own treatment. Normalization is the whole job. Two providers can be perfectly compliant and still disagree about what a group membership is, whether a logout is meaningful, and which attribute carries the email address.

Version the assumption, not just the code. Write down what you expect from the peer, and make the failure loud when it stops being true, because the alternative is a connection that half works for six weeks.

  • Validate signature, issuer, audience and expiry as four separate checks
  • Normalize provider attributes into one internal shape
  • Fail loudly when a peer stops meeting a documented assumption
  • Keep a fixture per provider so regressions surface in CI

Dealing with inconsistent profile endpoints

Consider dealing with inconsistent profile endpoints. Treat anything the other side sends as untrusted input until it has been through validation you wrote. Signature checks, issuer checks, audience checks and expiry checks are four separate decisions, and skipping any one of them is a real vulnerability rather than a theoretical one.

Treat anything the other side sends as untrusted input until it has been through validation you wrote. Signature checks, issuer checks, audience checks and expiry checks are four separate decisions, and skipping any one of them is a real vulnerability rather than a theoretical one.

Implementing multiple flows for one core task

Implementing multiple flows for one core task is where this gets concrete. Treat anything the other side sends as untrusted input until it has been through validation you wrote. Signature checks, issuer checks, audience checks and expiry checks are four separate decisions, and skipping any one of them is a real vulnerability rather than a theoretical one.

Normalization is the whole job. Two providers can be perfectly compliant and still disagree about what a group membership is, whether a logout is meaningful, and which attribute carries the email address.

Version the assumption, not just the code.

Managing security and maintenance challenges

Managing security and maintenance challenges is where this gets concrete. Treat anything the other side sends as untrusted input until it has been through validation you wrote. Signature checks, issuer checks, audience checks and expiry checks are four separate decisions, and skipping any one of them is a real vulnerability rather than a theoretical one.

Normalization is the whole job. Two providers can be perfectly compliant and still disagree about what a group membership is, whether a logout is meaningful, and which attribute carries the email address.

Why do OAuth 2.0 and OIDC fit together so well?

Consider why do oauth 2.0 and oidc fit together so well?. Version the assumption, not just the code. Write down what you expect from the peer, and make the failure loud when it stops being true, because the alternative is a connection that half works for six weeks.

Version the assumption, not just the code. Write down what you expect from the peer, and make the failure loud when it stops being true, because the alternative is a connection that half works for six weeks.

Shared infrastructure

That brings us to shared infrastructure. Normalization is the whole job. Two providers can be perfectly compliant and still disagree about what a group membership is, whether a logout is meaningful, and which attribute carries the email address.

Treat anything the other side sends as untrusted input until it has been through validation you wrote. Signature checks, issuer checks, audience checks and expiry checks are four separate decisions, and skipping any one of them is a real vulnerability rather than a theoretical one.

Where this leaves us

The pattern repeats across every system we have looked at: the hard part is not the mechanism, it is keeping the mechanism honest as the surrounding assumptions change.

If you are working through the same problem and want to compare notes, the docs cover the mechanics and the console shows the behaviour on your own data.

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