In this commit, we extend the DerivePrivKey method to allow callers that
don't know the full KeyLocator information to attempt to derive a
private key via a brute force mechanism. If we don't now the full
KeyLoactor, then given the KeyFamily, we can walk down the derivation
path and compare keys one by one. In order to ensure we don' t enter an
infinite loop when given an unknown public key, we cap the number of
keys derived at 100k.
An upcoming feature to lnd that adds static channel backups will utilize
this feature, as we need to derive the shachain root given only the
public key and key family, as we don't currently store this KeyLocator
on disk.
In this commit, we fix a slight bug in the existing implementation of
DeriveNextKey for btcwallet. Before this commit, we would only set the
public key, and not also the derivation path. It's important that we
also set the path information, as in the near future we'll be using the
KeyDescriptors returned from this method to create static channel back
ups. With these static backups, the key alone may be insufficient to
re-derive the private key as we may need to fallback to brute forcing in
order to re-derive the key as it's possible we add new key families in
the future.
In this commit, we add a new package to lnd: the keychain package. The
role of this package is to house all the interfaces that lnd will use
to generate the various keys it needs to create contracts and operate
within the network. Additionally, we also use this package to define a
deterministic key derivation scheme, that can be implemented by any
software/hardware that partially understands BIP43-like derivation.
The first version (v0) of the keychain schema is very simple. We re-use
BIP43, with a slight twist. Re-using BIP43 lets us leverage all the
existing libraries out there as far as compatibility. The purpose used
is 1017, and the coin type, the particular coin type of the chain where
the keys are meant to be used. Within our schema, we define multiple
“key families”. Each key family is actually just manifested as a new
“account” within the BIP44/43 family. With this schema, if we have
static description of the keys used within a channel, given the seed,
and the “KeyLocator”, we can re-derive all keys necessary be able to
re-sign for the channel.
