Ideally, given an on-chain backup, after the remote force-closes, we should be able to spend our anchor output,
to CPFP the remote commitment tx (assuming the channel used OPTION_ANCHORS).
To spend the anchor output, we need to be able to sign with the local funding_privkey.
Previously we derived the funding_key from the channel_seed (which comes from os.urandom).
Prior to anchors, there was no use case for signing with the funding_key given a channel backup.
Now with anchors, we should make its derivation deterministic somehow, in a way so that it can
be derived given just an on-chain backup.
- one way would be to put some more data into the existing OP_RETURN
- uses block space
- the OP_RETURNs can be disabled via "use_recoverable_channels"
- only the initiator can use OP_RETURNs (so what if channel is in incoming dir?)
- instead, new scheme for our funding_key:
- we derive the funding_privkey from the lnworker root secret (derived from our bip32 seed)
- for outgoing channels:
- lnworker_root_secret + remote_node_id + funding_tx_nlocktime
- for incoming channels:
- lnworker_root_secret + remote_node_id + remote_funding_pubkey
- a check is added to avoid reusing the same key between channels:
not letting to user open more than one channel with the same peer in a single block
- only the first 16 bytes of the remote_node_id are used, as the onchain backup OP_RETURNs only contain that
- as the funding_privkey cannot be derived from the channel_seed anymore, it is included in the
imported channel backups, which in turn need a new version defined
- a wallet db upgrade is used to update already stored imported cbs
- alternatively we could keep the imported cbs as-is, so no new version, no new funding_privkey field, as it is clearly somewhat redundant given on-chain backups can reconstruct it
- however adding the field seems easier
- otherwise the existing code would try to derive the funding_privkey from the channel_seed
- also note: atm there is no field in the imported backups to distinguish anchor channels vs static-remotekey channels
