Add initial gpg support

[areese]

This PR adds support to use a Yubikey for GPG encryption and decryption.

There are still some large changes to be made based on the feedback in https://github.com/go-piv/piv-go/pull/138 and https://github.com/go-piv/piv-go/pull/137.

Once those are resolved, this code will be updated to reflect those.

[ericchiang]

Thanks, but I'm going to preemptively say there's no way this package will ever support GPG/PGP. A large motivating factor for this project was leveraging more reasonable signing primitives on Yubikeys. See: https://blog.gtank.cc/modern-alternatives-to-pgp/. I defiantly do not want to carry any code for the PGP app, and that's beyond the scope of piv-go.

In the future, it may be better to reach out to a maintainer before sending a large PR like this?

Closing, since yeah... I don't see this as compatible with this project. You may want to fork if this is something you're looking to publish

[areese]

That’s fine The effort for me to maintain an open fork or to upstream everything is the same cost.

I’ll separate out all the parts that allow me to use pgp support without rewriting the entire library to talk to the cards.

piv-go has all of the apdu primitives.

My issue is key size, I have constraints that require a 4096 bit key that the cards only support via pgp.

[areese]

I need 4096 bit hardware backed keys.

I use S/MIME for email, and something else for signing commits (except on GitHub because reasons)

[ericchiang]

Sure, I just want to make clear that "I defiantly do not want to carry any code for the PGP app" also applies to internal refactors for this kind of thing (I don't have unlimited review time for OSS). If you need to use or modify the smartcard code, please fork it

[areese]

That’s perfectly reasonable.

I’ll make this into a fork.

I already have one