[feature] support GPG keys

[koba-e964]

Supporting GPG keys can be a way forward.

Honestly, I'm not entirely sure if I can implement it, but the spec should be already published and there are some tools that handle these keys out there: https://github.com/goark/gpgpdump

[wader]

Hey, yeap that might be useful, i don't really need it myself atm but i'm happy to assist if someone wants to give it a go.