ssh-key: make decode_as a public API

[baloo]

When parsing ssh keys from the SSH agent protocol, you need to read the first element of the protocol (the ssh_key::Algorithm (encoded as a String) to make a decision whether what comes next is supposed to be parsed as an ssh_key::Certificate or as an ssh_key::private::KeypairData.

Because the content has already been consumed from the reader, and because you can't go backwards in the reader. This requires decode_as to be exposed on ssh_key::private::KeypairData.

[baloo]

I'd love to have this backported and released to a ssh-key/v0.6.6

[wiktor-k]

Great to see this filed! Thanks a lot @baloo 🙇

I think in general all these "first string ID (algorithm), then we decide what to do" should have a decode_as variants or maybe even all uses of KeypairData should be inspected. If SSH sends us certificates instead of keys during SSH agent protocol it may do so in other places too.

In other words the Credential type that was added to the SSH agent crate may as well be useful for other people and maybe it's place is here?

And if so the question is what to do with other types which are used in SSH agent protocol. Maybe they could also be moved here after they "stabilize" a bit?

This is all just ideas for far future but in the short-term it'd be really nice to have this merged. 🙏

Thanks for your time! 👋

[baloo]

In other words the Credential type that was added to the SSH agent crate may as well be useful for other people and maybe it's place is here?

I don't think it will be ever used outside the agent protocol. This is only used because encoding of private keys changes whether they are tied to certificates or not. I believe agent protocol is the only one that involves private keys.
I would leave it in agent until we find another use for it.

[tarcieri]

Are you suggesting a new method, or if not, can you link in the source code to the method you're talking about?

As far as I can tell, there is no decode_as method defined on ssh_key::private::KeypairData at present, only on ssh_key::public::KeyData.

[baloo]

This is used for parsing the ssh-agent protocol.

https://github.com/wiktor-k/ssh-agent-lib/pull/33/files#r1554798031

[tarcieri]

Aah sorry, catching up on PRs and I missed this was extracting a decode_as method