Add X509AuthorityKeyIdentifier to generated leaf certificates

[AronUJVARY]

Python starting at 3.13 defaults to using VERIFY_X509_STRICT in its ssl context (https://docs.python.org/3/library/ssl.html, https://gist.github.com/mdehling/350fc63d286a31b2653aef1362c6b0f5/). This leads to python rejecting the leaf certificates of dev-proxy, complaining about missing authority key identifier. Adding the X509AuthorityKeyIdentifierExtension fixes the issue.

[AronUJVARY]

@dotnet-policy-service agree company="Cinemo"

[waldekmastykarz]

Thanks a lot for this @AronUJVARY! 🙏 Great catch. Python 3.13 turning on VERIFY_X509_STRICT by default is exactly the kind of thing that silently breaks people, and you're right that our leaf certs should've had an AKI all along. Really appreciate you tracking it down and sending a fix.

Adding X509AuthorityKeyIdentifierExtension to the leaf is the right fix. There's one catch though: CreateFromCertificate(_ca, includeKeyIdentifier: true, ...) throws when the root doesn't have a Subject Key Identifier. We still load roots created by the previous (Titanium-based) engine, and those don't have an SKI, so for folks upgrading this would fail on every HTTPS request. That's also why the test helper needed the SKI added. The test passes now, but it hides the scenario rather than covering it.

Since this is going into v4, which is a major release, we can do this properly instead of working around old certs. Here's what I'd suggest:

• Treat roots without an SKI as invalid in TryLoadRoot, same as we do for expired/non-CA roots. We then regenerate the root and purge the leaf cache, which the code already handles. Even with an AKI on the leaf, strict mode also requires the CA to have an SKI, so this is needed for Python to work anyway. Optionally, also check that KeyUsage includes keyCertSign.
• Invalidate cached leaves without a matching AKI in TryLoadLeaf. Leaves are cached for up to a year, so without this, folks on v4 previews would keep getting old leaves that lack the AKI. Checking that the AKI matches the root's SKI also protects us against leaves signed by a different root.
• Tests: keep the SKI in the helper, and add tests that (1) a root without an SKI gets regenerated, (2) a cached leaf without an AKI gets re-created, and (3) the leaf's AKI key id matches the root's SKI.

With these changes the root is regenerated on upgrade, so people will need to trust it again. That's fine for v4, and I'll call it out in the release notes.

Would you be up for making these changes? If not, no worries at all, just let me know and we'll take it from here. Thanks again!