There is just one thing missing from this. Name Constraints.
This doesn't get brought up enough but a Name Constraint on a root cert lets you limit where the root cert can be signed to. So instead of this cert being able to impersonate any website on the internet, you ratchet it down to just the domain (or single website) that you want to sign for.
Browser support for it is pretty new, which is why it's so often missed. It only happened in mid/late 2023.
I've been shopping a talk since then about how to set up a name-constrained root certificate, and what it should look like. It's still hard! CFSSL is my go-to tool, and it doesn't have support. I had to fork it to make it work. OpenSSL has support, but it's configuration is like all OpenSSL configuration - Poorly documented and nonstandard, mixing INI objects and object-refs.
There is mainstream browser support for name constraints now?! That is huge, I had given up hoping for adoption progress already and was one of my major gripes regarding web stagnation.
> The name constraints extension, which MUST be used only in a CA certificate, indicates a name space within which all subject names in subsequent certificates in a certification path MUST be located. Restrictions apply to the subject distinguished name and apply to subject alternative names. Restrictions apply only when the specified name form is present. If no name of the type is in the certificate, the certificate is acceptable.
> Name constraints are not applied to self-issued certificates (unless the certificate is the final certificate in the path). (This could prevent CAs that use name constraints from employing self-issued certificates to implement key rollover.)
If this is now finally supported that's great. The issue was that for it to be useful it has to be marked critical / fail-closed, because a CA with ignored name constraint == an unrestricted CA. But if you make it critical, then clients who don't understand it will just fail. You can see how this doesn't help adoption.
Stuff like this is why I consider giving people a CA how to akin to a loaded gun. They almost invariably are not going to securely store the keys properly, set up CRLs, or manage their PKI in a safe manner.
Some of us are aware of the risks and choose to accept them. Last week I tried to analyze HTTPS traffic on my Linux machine using MITM to check what some programs were sending back home, but omg, it was a pain, I also partially failed. Some apps just ignore system certs and use their own. Tools like mitmproxy help (docs are lacking btw). I paid for both the devices and the software, shouldn't I be able to take a peek at what they are doing?
I certainly wouldn’t trust myself. Now if I could import a root cert and specify what domains to trust that would be another thing, and it seems browsers are starting to pay attention to name constraints which has not taken 20 odd years.
I’d rather be able to further constraint at the cert store though.
Doesn't matter. PKI for https is a solution in search of a problem.
In reality all it does is just validate domain name ownership, something that could have more easily been done with DKIM keys. We don't need certificate authorities.
I previously used openssl-based scripts to generate certificates to use for local development or applications on a private network. I have since moved to using the step CLI [1].
OpenSSL is powerful, but it's hard to figure out how to use correctly. Each command seems cryptic no matter how many times I use it.
The step CLI is a lot simpler, even though it has a few quirks: generating PKCS1 formatted private keys instead of the newer PKCS7 format, making every leaf certificate eligible to be either a server certificate or a client certificate, and absurdly low default certificate expirations.
I'm hosting my own internal CA using Hashicorp Vault and some ansible + CI. The root CA is valid for 20 years, intermediate CA 10 years, client certs three months.
Initial setup is a handful of commands interacting with Vault's CLI, from there, with CI in place, client certs are renewed automatically. Services are restarted / reloaded as well. Works flawlessly.
I should maybe write a (small) blog explaining how it works.
Readit News
This doesn't get brought up enough but a Name Constraint on a root cert lets you limit where the root cert can be signed to. So instead of this cert being able to impersonate any website on the internet, you ratchet it down to just the domain (or single website) that you want to sign for.
https://github.com/FiloSottile/mkcert/issues/131
https://github.com/FiloSottile/mkcert/pull/113
Hopefully Filippo revisits this now that it's broadly supported.
I've been shopping a talk since then about how to set up a name-constrained root certificate, and what it should look like. It's still hard! CFSSL is my go-to tool, and it doesn't have support. I had to fork it to make it work. OpenSSL has support, but it's configuration is like all OpenSSL configuration - Poorly documented and nonstandard, mixing INI objects and object-refs.
https://github.com/nh2/internal-contstrained-pki
> When generating a CA cert via caddy and putting that in the trust store, those private keys can also forge certificates for any other domain.
RFC5280 (2008) "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile" > Section 4.2.1.10 Name Constraints: https://datatracker.ietf.org/doc/html/rfc5280#section-4.2.1.... :
> The name constraints extension, which MUST be used only in a CA certificate, indicates a name space within which all subject names in subsequent certificates in a certification path MUST be located. Restrictions apply to the subject distinguished name and apply to subject alternative names. Restrictions apply only when the specified name form is present. If no name of the type is in the certificate, the certificate is acceptable.
> Name constraints are not applied to self-issued certificates (unless the certificate is the final certificate in the path). (This could prevent CAs that use name constraints from employing self-issued certificates to implement key rollover.)
I’d rather be able to further constraint at the cert store though.
https://www.hohnstaedt.de/
OpenSSL is powerful, but it's hard to figure out how to use correctly. Each command seems cryptic no matter how many times I use it.
The step CLI is a lot simpler, even though it has a few quirks: generating PKCS1 formatted private keys instead of the newer PKCS7 format, making every leaf certificate eligible to be either a server certificate or a client certificate, and absurdly low default certificate expirations.
1: https://github.com/smallstep/cli
openssl genrsa -out private.key 4096 && openssl req -new -key private.key -out signreq.csr -subj "/CN=FQDN" && openssl x509 -req -days 365 -in signreq.csr -signkey private.key -out cert.crt
But ideally everyone could just use something like mkcert to take care of this
Initial setup is a handful of commands interacting with Vault's CLI, from there, with CI in place, client certs are renewed automatically. Services are restarted / reloaded as well. Works flawlessly.
I should maybe write a (small) blog explaining how it works.