Dec 28, 2023

Trimming-down list of trusted TLS ca-certificates system-wide using a whitelist approach

It's no secret that Web PKI was always a terrible mess.

Idk of anything that can explain it better than Moxie Marlinspike's old "SSL And The Future Of Athenticity" talk, which still pretty much holds up (and is kinda hilarious), as Web PKI for TLS is still up to >150 certs, couple of which get kicked-out after malicious misuse or gross malpractice every now and then, and it's actually more worrying when they don't.

And as of 2023, EU eIDAS proposal stands to make this PKI much worse in the near-future, adding whole bunch of random national authorities to everyone's list of trusted CAs, which of course have no rational business of being there on all levels.

(with all people/orgs on the internet seemingly in agreement on that - see e.g. EFF, Mozilla, Ryan Hurst's excellent writeup, etc - but it'll probably pass anyway, for whatever political reasons)

So in the spirit of at least putting some bandaid on that, I had a long-standing idea to write a logger for all CAs that my browser uses over time, then inspect it after a while and kick <1% CAs out of the browser at least. This is totally doable, and not that hard - e.g. cerdicator extension can be tweaked to log to a file instead of displaying CA info - but never got around to doing it myself.

Update 2024-01-03: there is now also CertInfo app to scrape local history and probe all sites there for certs, building a list of root and intermediate CAs to inspect.

But recently, scrolling through Ryan Hurst's "eIDAS 2.0 Provisional Agreement Implications for Web Browsers and Digital Certificate Trust" open letter, pie chart on page-3 there jumped out to me, as it showed that 99% of certs use only 6-7 CAs - so why even bother logging those, there's a simple list of them, which should mostly work for me too.

I remember browsers and different apps using their own CA lists being a problem in the past, having to tweak mozilla nss database via its own tools, etc, but by now, as it turns out, this problem seem to have been long-solved on a typical linux, via distro-specific "ca-certificates" package/scripts and p11-kit (or at least it appears to be solved like that on my systems).

Gist is that /usr/share/ca-certificates/trust-source/ and its /etc counterpart have *.p11-kit CA bundles installed there by some package like ca-certificates-mozilla, and then package-manager runs update-ca-trust, which exports that to /etc/ssl/cert.pem and such places, where all other tools can pickup and use same CAs. Firefox (or at least my Waterfox build) even uses installed p11-kit bundle(s) directly and immediately. Those p11-kit bundles need to be altered or restricted somehow to affect everything on the system, only needing update-ca-trust at most - neat!

One problem I bumped into however, is that p11-kit tools only support masking specific individual CAs from the bundle via blacklist, and that will not be future-proof wrt upstream changes to that bundle, if the goal is to "only use these couple CAs and nothing else".

So ended up writing a simple script to go through .p11-kit bundle files and remove everything unnecessary from them on a whitelist-bases - ca-certificates-whitelist-filter - which uses a simple one-per-line format with wildcards to match multiple certs:

Baltimore CyberTrust Root # CloudFlare
ISRG Root X* # Let's Encrypt
GlobalSign * # Google
DigiCert *
Sectigo *
Go Daddy *
Microsoft *
USERTrust *

Picking whitelisted CAs from Ryan's list, found that GlobalSign should be added, and that it already signs Google's GTS CA's (so latter are unnecessary), while "Baltimore CyberTrust Root" seem to be a strange omission, as it signs CloudFlare's CA cert, which should've been a major thing on the pie chart in that eIDAS open letter.

But otherwise, that's pretty much it, leaving a couple of CAs instead of a hundred, and couple days into it so far, everything seem to be working fine with just those. Occasional "missing root" error can be resolved easily by adding that root to the list, or ignoring it for whatever irrelevant one-off pages, though this really doesn't seem to be an issue at all.

This is definitely not a solution to Web PKI being a big pile of dung, made as an afterthough and then abused relentlessly and intentionally, but I think a good low-effort bandaid against clumsy mass-MitM by whatever random crooks on the network, in ISPs and idiot governments.

One long-term issue with this approach though, is that if used at any scale, it further shifts control over CA trust from e.g. Mozilla's p11-kit bundle to those dozen giant CAs above, who will then realistically have to share their CAs with other orgs and groups (as they're the only ones on the CA list), ossifies them to be in control of Web PKI in the future over time, and makes "trusting" them meaningless non-decision (as you can't avoid that, even as/if/when they have to sign sub-CAs for whatever shady bad actors in secret).

To be fair, there are proposals and movements to remedy this situation, like Certificate Transparency and various cert and TLS policies/parameters' pinning, but I'm not hugely optimistic, and just hope that a quick fix like this might be enough to be on the right side of "you don't need to outrun the bear, just the other guy" metaphor.

Link: ca-certificates-whitelist-filter script on github (codeberg, local git)