Add decryption agent functionality

[filosottile]

Add decryption agent functionality

[FiloSottile]

A (r)age plugin that uses yubikey-agent is on the roadmap :)

[yvesf]

original message replaced
Edit: Here was a draft how yubikey-agent could be extended to be used with age. Following #63 (comment) I had an e-mail exchange with @tv42. As he points out below I used some routines from his project and by mistake didn't put the correct attribution to him. I understood him in our conversation that correcting this alone is not in his interest. Since even with correct attribution my draft contain some lines of code with him (the actual enc/decryption part of file-keys), I'll not advertise it here any further.

[tv42]

The changes are more than "inspired" by my code.
See https://github.com/yvesf/yubikey-agent/blob/47e52502ea3e8f2b096cb888cd7149b0ac308b27/plugin.go#L198-L220
versus https://github.com/tv42/yubage/blob/49146f64d316f1c15baf247f3f4cba28a3e13b9b/internal/pivplug/recipient.go#L132-L163
and please follow https://github.com/tv42/yubage/blob/master/LICENSE

[yvesf]

I updated the relevant places with a copyright notice pointing to the source, the links in my first comments are updated accordingly. I'm sorry that I forgot the attribution to tv42 in these places. To my knowledge these three are all places where your work is used. @tv42 please let me know if you see more problems.

This licensing jumble is not really nice in the code (there is also some code copied from age itself but that could potentially be resolved easily by the author of age). I want state it very clear here too that this is not like a MR but just in a "works for me" state.

I'm not too eager to deal with license compatibility, copyright and such stuff. I'd rather retract the whole thing.

[IxDay]

Is there a repo we can checkout to assist or just keep track of the progress?