Why does OpenSSL report google's certificate is "self-signed"?

[Michael Wojcik]

> From: openssl-users <openssl-users-bounces at openssl.org> On Behalf Of Viktor
> Dukhovni
> Sent: Wednesday, 31 March, 2021 10:31
> To: openssl-users at openssl.org
> Subject: Re: Why does OpenSSL report google's certificate is "self-signed"?
>
> It looks like Google includes a self-signed root CA in the wire
> certificate chain, and if no match is found in the trust store,
> you'll get the reported error.

What do people think about this practice of including the root in the chain?

As far as I can see, neither PKIX (RFC 5280) nor the CA/BF Baseline Requirements say anything about the practice, though I may have missed something. I had a vague memory that some standard or "best practice" guideline somewhere said the server should send the chain up to but not including the root, but I don't know what that might have been.

On the one hand, including the root doesn't help with path validation: either some certificate along the chain is a trust anchor already, in which case there's no need to include the root; or it isn't, in which case the peer has no reason to trust the chain.

On the other, it's useful for debugging, and perhaps for quickly finding whether the highest intermediate in the chain is signed by a trusted root if that intermediate is missing an AKID (though we'd hope that isn't the case).

I can also see an application deferring trust to the user in this case: "this chain ends in this root, which you don't currently trust, but maybe you'd like to add it?". Which doesn't seem like a great plan either -- and PKIX says trust anchors should be added using a trustworthy out-of-band procedure, which this is not -- but I suppose it's a conceivable use case.

--
Michael Wojcik