Verify the signature on a self-signed TA cert against it's own pubkey

[job]

X509_verify_cert() doesn't check the purported root certificate itself unless X509_V_FLAG_CHECK_SS_SIGNATURE is set.

The pubkey was compared against the TAL, so check that the signature is right as required by RFC 6487, section 7.2, additional condition 1, applied to self-issued certs.

The error check looks weird, but OpenSSL 3 broke yet another API.

With help from Theo Buehler and Claudio Jeker

[job]

The comment about error checking and OpenSSL 3 API is in relationship to https://github.com/openssl/openssl/issues/24575

We cannot rely on "return codes <= 0 indicate error", because "2" also could be an error. So we need to explicitly check whether it's 1 or not

[ydahhrk]

I'm angsting over this note:

X509_verify_cert() doesn't check the purported root certificate itself unless X509_V_FLAG_CHECK_SS_SIGNATURE is set.

I take it the reason why you're verifying the signature manually (as opposed to adding X509_V_FLAG_CHECK_SS_SIGNATURE) is because Fort skips X509_verify_cert() on trust anchors?

That short circuit is looking quite naive now that you've introduced me to X509_V_FLAG_CHECK_SS_SIGNATURE.

[botovq]

You could add the X509_V_FLAG_CHECK_SS_SIGNATURE if you prefer that. That's more expensive since then you check the TA's signature each time you call X509_verify_cert(). This isn't necessary if you only add trust anchors and other certs that you verified to your verified_certstack().

Not doing the 'naive' short-circuit wouldn't have helped without that CHEC_SS flag.

[ydahhrk]

Thank you