There should be a BOLT for internal key derivation

[rustyrussell]

c-lightning uses a dumb system where the secrets (including the delayed_payment_point_secret and payment_point_secret which aren't needed for normal channel operation) are derived from a single seed using HKDF.

Since we're going to change that, I looked at lnd's much saner BIP-32-style https://github.com/lightningnetwork/lnd/blob/master/keychain/derivation.go . While it's completely optional, it serves as a well-thought-out method for key generation, so I'd like to turn it into a BOLT, with test vectors.

This would make creation of a wallet-independent salvage tool simpler, too.

[sstone]

Interesting! We also implemented a simplistic BIP32-based derivation scheme for channel and node keys (with arbitrarily chosen paths).

[canndrew]

Can this be made into an official BOLT? I'm writing an implementation and I'm at the point now where I need to decide how to generate keys. I'd prefer my implementation to be consistent with other implementations (for recovery purposes, for instance). Is copying lnd the way to go?