[noise] Lightning protocol Noise variation

[Trevor Perrin]

On Thu, Jan 9, 2020 at 12:03 PM Janus Troelsen <ysangkok at gmail.com> wrote:
>
> Alex suggested I bring up the variation of key rotation on this list,
> to probe the community. As I have linked in the PR, and if I
> understand correctly, one lightning-rfc spec writer claims that
> Lightning uses the key rotation it does (with two chaining keys)
> because it was developed before key rotation was added to Noise.

I think that's right (the Lightning mechanism pre-dates the Noise
rekey mechanism).

Glancing at it again, it looks fine (because HKDF is a PRF it can be
understood as just applying a chain of PRFs to the secure key CK).

I do think the "official" Noise rekey is simpler and more efficient,
since it doesn't store an extra chaining key in each direction or use
expensive HKDFs for each rekey.

Trevor