Forceclose: Signature must be zero for failed CHECK(MULTI)SIG operation

[Feyynt]

Hi, when I use the forceclose command some channels give me this error regarding the signature:

[INF] CHAN: Published TX a445b928e0603d0af0b1c23876adb975c442009aa7c73c1c08bbb9a09bf1918e, response: sendrawtransaction RPC error: {"code":-26,"message":"non-mandatory-script-verify-flag (Signature must be zero for failed CHECK(MULTI)SIG operation)"}

[guggero]

It seems like you're affected by an old bug of lnd that would sometimes produce an invalid commitment transaction signature. Unfortunately there isn't a lot you can do about that. Do you have any balance in those channels? Were they closed out by your peer in the meantime perhaps?

[Feyynt]

Yes, I have local balance inside. All of them are still open and the other peers are unreachable

[guggero]

Hmm, well, there is a way to re-sign those commitment transactions by turning them into PSBTs and re-signing just your part of the signature. But that's rather complicated and I haven't written it down anywhere. But I've done it twice before, manually. Unfortunately I'm very busy as always. How many sats are we talking about here?

If you want I can send you the transcript of my chat with the latest user I explained the process to, if you want to reconstruct it from there.

[Feyynt]

You are the best!
Sure, you can send it to me. I will try, thank you very much!

[guggero]

What's the best way to contact you directly?

I'm adding a 200k satoshi bounty for turning the transcript into a document that can be added to the docs of chantools. So if you're up for writing a HOWTO, I'm looking forward to the PR.

[Feyynt]

I just wrote you on Slack

[pinheadmz]

Encountered this today, didnt realize my lnd was so out of date! i was just testing on a private signet though, intentionally trying to get justiced in a test environment.

--> lnd --version
lnd version 0.15.99-beta commit=tlv/v1.1.0-66-gd468391d9
--> chantools -v
chantools version v0.12.2, commit 

@guggero out of curiosity is this bug disclosed or explained somewhere? Seems like an invalid signature could be a big problem.

[pinheadmz]

Ok after upgrading to lnd version 0.17.0-beta commit=fn/v1.0.2-58-g54c47a3fe and creating a new (test) channel and attempting a force close, I still got this signature failure. I didn't change the walletDB, just upgraded lnd.

[guggero]

@guggero out of curiosity is #31 (comment) disclosed or explained somewhere? Seems like an invalid signature could be a big problem.

I think I was speculating at that point, as I was never actually able to track down a specific bug.

In your test scenario, did you also update your peer? Or is that on the same (old) version?
And which commitment transaction are you publishing? I assume the one from the updated node?

[pinheadmz]

Remote node is "version": "0.17.3-beta commit=v0.17.3-beta" and has never been restarted. Local node started off 15.99-beta, created wallet, opened channel, attempted and failed force close with chantools. Then upgraded local to 17.0-beta without clearing out channel.db or wallet.db. Opened a new channel, backed up channel.db, sent LN payments, then failed again to forceclose with chantools.

I'm going to dig in a bit to the details today to figure out which sig is invalid. "Hopefully" its an issue with chantools right? Because otherwise that implies LND is either sending invalid sig, or allowing invalid sigs from remote or both!

[guggero]

Yeah, very likely a bug in chantools itself. This command wasn't updated in a while, so maybe it just needs some tweak to be compatible with anchor channels (which might have been used as the default because v0.17.3 chooses them by default and v0.15.99 supports them).