Verify "long" chains

[woodruffw]

From #910:

Given the root is present in the trusted root file, I would prefer we still validate it. The intermediate being shipped in the trust root is more of an optimization, letting us avoid distributing a CRL if we need to revoke the intermediate cert. The PKI root of trust is supposed to be the Fulcio root cert.

Makes sense! In that case, we can simulate the "long chain" building by passing only the Fulcio root into the trust store and including the intermediate via the intermediates parameter to X509StoreContext. That will ensure we never terminate before the self-signed TA, regardless of OpenSSL flags like X509_V_FLAG_PARTIAL_CHAIN.

Originally posted by @woodruffw in #910 (comment)

[haydentherapper]

sigstore/protobuf-specs#245 now clarifies this.

[woodruffw]

Awesome! I'll open a PR for this today.

[woodruffw]

I'm looking into this now, and it's moderately complicated by the fact that we now (as of #910) have multiple chains to consider. Given that, I'm going to go with the ~X509_V_FLAG_PARTIAL_CHAIN approach for now and revisit the more general approach if sigstore/protobuf-specs#249 is accepted 🙂

[woodruffw]

Opened #924 with the fix.