Age key support

[Tomaszal]

As outlined in https://github.com/Mic92/ssh-to-age/issues/14, it would be great if it would be possible to use piv-agent to generate age keys in some way.

Currently the only way to get SSH keys from Yubikey seems to be age-plugin-yubikey. Unfortunately it doesn't seem to be able to use the PIV keys generated by piv-agent. While that itself isn't too big of a deal, it also seems that age-plugin-yubikey cannot run while piv-agent is active, as the PIV device is busy. That means that piv-agent would have to be stopped every time a file needs to be encrypted or decrypted, which is really inconvenient.

Depending on the outcome of https://github.com/Mic92/ssh-to-age/issues/14, it might be possible to get age keys from the SSH keys generated by piv-agent. However, I realised that piv-agent is already generating both SSH and GPG keys, so why not age? @str4d seems to acknowledge that some sort of an agent would be good to implement for age keys, and I have a hunch that piv-agent might already have a lot of the groundwork necessary for that.

[smlx]

Yes I'd like to implement this but the current specification for the plugin interface to age is still a work in progress https://github.com/C2SP/C2SP/pull/5

[Tomaszal]

Hi @smlx, it seems the specification has been merged last year, is this still something you would like to do? With age keys gaining support and git being able to use ssh keys for signing, it feels finally feasible to mostly avoid using pgp keys as a developer. It would be fantastic if piv-agent could provide age keys as imo it still has the best UX out of the physical security key SSH solutions (including yubikey-agent), but the GPG experience isn't great (at not a fault of this project, GPG is just a pain to work with in general), which would be possible to avoid with age.

[smlx]

Thanks for the bump, and I totally agree that it would be amazing to eliminate GPG from common development workflows.

age support is still on my TODO list, but not a super high priority for now because ideally we should keep the x25519 key in hardware. Yubikeys are getting PIV support for x25519 sometime soon so once I've got hardware with support I'll be able to implement this feature.

Yes technically piv-agent could use x25519 key files for this, but it just isn't very interesting because for me at least key files are only an emergency fallback mechanism.

[str4d]

Once I get access to a YubiKey with firmware 5.7.0+, I will add support for native age identities to age-plugin-yubikey, likely following the same pattern as I used for the P-256 slots (storing them in the reserved slots, making them identifiable in the self-signed certificate, and extracting PIN+touch policies from the YK cert extension). It would be great for piv-agent to be compatible with this!

It would also be good to know how piv-agent's existing P-256 YubiKey support is incompatible with age-plugin-yubikey. How are you generating and managing those?

[smlx]

I agree it would be nice for piv-agent to be compatible with age-plugin-yubikey. I haven't looked into how you are doing PIN/touch policies, but in piv-agent it is purely convention as described in https://github.com/smlx/piv-agent/blob/main/internal/securitykey/slotspec.go

The generation of certificates in piv-agent is mostly cribbed from the way yubikey-agent does it.

Could you elaborate a bit on how you manage PIN/touch policies in cert extensions for age-plugin-yubikey?

[Tomaszal]

Yubico says "security keys with firmware 5.7 will be available to purchase in late May 2024" in their latest press release. They also confirmed Ed25519 and X25519 PIV support in the new firmware.