Private CA with X.509 Name Constraints

david@systemoverlord.com (David Tomaschik) — Sun, 14 Jun 2020 00:00:00 +0000

I wanted to run a small private Certificate Authority for some of my internal services. Since these aren’t reachable from the internet, and some of them are on network segments without internet connectivity, using a public ACME CA like Let’s Encrypt was inconvenient. On the other hand, if I run my own private CA and the keys get compromised, it could be used to MITM all my internet traffic. While that’s unlikely to happen, I decided to look for a better option.

It turns out that the idea of a “limited purpose” Certificate Authority is not new. RFC 5280 provides for something called “Name Constraints”, which allow an X.509 CA to have a scope limited to certain names, including the parent domains of the certificates issued by the CA. For example, a host constraint of .example.com allows the CA to issue certificates for anything under .example.com, but not any other host. For other hosts, clients will fail to validate the chain.

This hasn’t always been supported by TLS libraries and browsers, but all current browsers do support Name Constraints. Consequently, this is an approach to narrow the risks associated with a CA compromise for hosts other than those covered by the constraints in the CA certificate.

PKI on System Overlord

Private CA with X.509 Name Constraints