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Hello Blue Sky! Check out first post in my series on transparency systems. educatedguesswork.org/posts/transp...

Most real-world authentication protocols rely on some kind of trusted authentication service to attest to endpoint identities. The obvious problem here is the word "trusted"; if the authentication service misbehaves, then the whole system breaks down.

When paired with some fairly public failures by WebPKI certificate authorities, this creates an obvious problem.

The solution that the ecosystem has converged on is something called a "transparency system", and in the case of the WebPKI "certificate transparency". Instead of trying to prevent misbehavior by CAs, a transparency system tries to make it detectable.

A misissued certificate can be revoked, and, if investigation reveals improper practices, browsers might choose to distrust the CA, thus rendering all of its certificates invalid.

The challenge here is ensuring that CAs are actually publishing every certificate. If your concern is that the CA was intentionally misissuing, it might just choose not to publish the bogus certificate.

The first step is to have the client (i.e., the browser) check that the certificate was published, thus hopefully forcing the CA to publish it. But now we have to confront the definition of "publish". How do we know the CA published to everyone?