Webserver certificate authentication with intermediate CAs

Thu, 13 Oct 2016 14:35:07 +0200

Authenticating HTTPS clients using TLS certificates has long been very easy to do. The hardest part, in fact, is to create a PKI infrastructure robust enough so that compromised (or otherwise no longer desirable) certificates cannot be used anymore. While setting that up can be fairly involved, it's all pretty standard practice these days.

But authentication using a private CA is laughably easy. With apache, you just do something like:

SSLCACertificateFile /path/to/ca-certificate.pem
SSLVerifyClient require

...and you're done. With the above two lines, apache will send a CertificateRequest message to the client, prompting the client to search its local certificate store for certificates signed by the CA(s) in the ca-certificate.pem file, and using the certificate thus found to authenticate against the server.

This works perfectly fine for setting up authentication for a handful of users. It will even work fine for a few thousands of users. But what if you're trying to set up website certificate authentication for millions of users? Well, in that case, storing everything in a single CA just doesn't work, and you'll need intermediate CAs to make things not fall flat on their face.

Unfortunately, the standard does not state what should happen when a client certificate is signed by an intermediate certificate, and the distinguished name(s) in the CertificateRequest message are those of the certificate(s) at the top of the chain rather than the intermediates. Previously, browsers would not just send out the client certificate, but also send along the certificate that it knew to have signed that client certificate, and so on until it found a self-signed certificate. With that, the server would see a chain of certificates that it could verify against the root certificates in its local trust store, and certificate verification would succeed.

It would appear that browsers are currently changing this behaviour, however. With the switch to BoringSSL, Google Chrome on the GNU/Linux platform no longer sent the signing certificates, and instead only sends the leaf certificate that it wants to use for certificate authentication. In the bug report, Ryan Sleevi explains that while the change was premature, the long term plans are for this to be the behaviour not just on GNU/Linux, but on Windows and macOS too. Meanwhile, the issue has been resolved for Chrome 54 (due to be released), but there's no saying for how long. As if that was not enough, the new version of Safari as shipped with macOS 10.12 has also stopped sending intermediate certificates, and expects the web server to be happy with just receiving the leaf certificates.

So, I guess it's safe to say that when you want to authenticate a client in a multi-level hierarchical CA environment, you cannot just hand the webserver a list of root certificates and expect things to work anymore; you'll have to hand it the intermediary certificates as well. To do so, you need to modify the SSLCACertificateFile parameter in apache configuration so that it not only contains the root certificate, but all the intermediate certificates as well. If your list of intermediate certificates is rather long, it may improve performance to use SSLCACertificatePath and the c_rehash tool to create a hashed directory with certificates rather than a PEM file, but in essense, that should work.

While this works, the problem with doing so is that now the DNs of all the intermediate certificates are sent along with the CertificateRequest message to the client, which (again, if your list of intermediate certificates is rather long) may result in performance issues. The fix for that is fairly easy: add a line like

SSLCADNRequestFile /path/to/root-certificates.pem

where the file root-certificates.pem contains the root certificates only. This file will tell the webserver which certificates should be announced in the CertificateRequest message, but the SSLCACertificateFile (or SSLCACertificatePath) configuration item will still be used for actual verification of the certificates in question. Note though that the root certificates apparently also seem to need to be available in the SSLCACertificatePath or SSLCACertificateFile configuration; if you do not do so, then authentication seems to fail, although I haven't yet found why.

I've set up a test page for all of those "millions" of certificates, and it seems to work for me. I've you're trying to use one of those millions of certificates against your own webserver or have a similar situation with a different set of certificates, you might want to make the above changes, too.