Add devtools/mkpoint

[Sjors]

Tool to dump a channel commitment point, useful for recovering funds.

It takes the following arguments:

• node id (the public key of the other peer)
• channel database id (see chan # entries in log)
• commit_num: 0 for a freshly opened channel

For some reason hsmd.c doesn't have a header file, so I'm confused how to properly call its functions.

Background: https://bitcoin.stackexchange.com/questions/90196/how-to-recover-funds-from-a-force-closed-lnd-lightning-channel

[Sjors]

From re-reading IRC I suspect I also need to mix in hsm_secret somewhere in this process.

[darosior]

For some reason hsmd.c doesn't have a header file, so I'm confused how to properly call its functions.

You can use it as a subdaemon (as lightningd does) and use the autogenerated wire functions to request sigs. See lightningd/hsm_control.h.

From re-reading IRC I suspect I also need to mix in hsm_secret somewhere in this process.

Yep, it contains the bip32 master key, but it's handled by hsmd.

[Sjors]

So what would be the right approach to dump a per-commitment point? Should I make devtools/mkpoint spin up an hsmd daemon and then talk to it? Or is it better to add a developer RPC call? Or a plugin?

[darosior]

I'd go for spinning up hsmd as a subdaemon, however that's the path I know the less about so I might be wrong.
A plugin would need a hook or notification, I don't think that fits your need here.
A dev RPC call might be a good solution too if the channel is still active, to use when lightningd is running. However, as I think I read on IRC, this was to recover the points directly from the database file ? In this case an independent tool seems more appropriate.

[Sjors]

The channel I need this for is long closed and pruned from listfunds. I assume it's not even in the database file, but it can still be reconstructed using the information I got from the logs.

[Sjors]

It works now! I probably don't have time or skills to clean up this PR though... @darosior would it make sense to combine this with #3177?

[darosior]

@Sjors I wonder for the same reasons I wonder moving @rustyrussell signmessage and checkmessage to hsmtools (cf #3177)..

EDIT: After talking about it with ZmnSCPxj and cdecker it does not seem to be a good idea. I think I'll integrate it to the hsmtools though.

[Sjors]

Closing in favor of #3186 which introduces the equivalent: tools/hsmtools dumpcommitments NODE_PUBKEY DB_INDEX COMMITMENT_INDEX