Howdy, y'all --
In anticipation of trying to revive the OpenPGP compatibility patch created by Ignat Korchagin in 2016, I've assembled a simple test suite that can generate multiple builds of GRUB (different versions/patches/etc) and then test them with keys and signatures generated by multiple tools:
https://github.com/charles-dyfis-net/grub-openpgp-compat-test
Unfortunately, Ignat's original patch is not remotely trivial to square with the changes introduced with the verifier framework core (in 75a919e334f8514b6adbc024743cfcd9b88fa394), so it looks to me like the easiest approach will be starting from scratch, comparing the original code against the RFC. Hopefully the test framework will help -- but if anyone else has been doing work in this area already, I'd be very glad to hear of it before putting in more time!