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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.

The basic idea is to require every certificate has to be published, thus allowing sites -- or services on their behalf -- to detect (at least in principle) when a certificate has been misissued

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 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?

For instance, if the CA has it on their web site and sends it to clients but not to sites when they check, then the system breaks down.

The standard solution here is what's called a consensus system. Effectively, you compute a summary of all the published certificates (typically by assembling them into a Merkle hash tree).