Lloyd Fournier [ARCHIVE] on Nostr: 📅 Original date posted:2020-12-14 📝 Original message: Hi Dave, Thanks for ...

📅 Original date posted:2020-12-14
📝 Original message:
Hi Dave,

Thanks for taking a read. You brought up really good points that need
addressing.

This is really cool! However, I don't understand why it's needed. Your
> goal seems to be for the sender to provide the commitment transaction
> and signatures before he learns whether the receiver actually needs
> them. That's just as easily accomplished by sending the data upfront in
> plain text.
>

To be clear: The goal is to offer a cooperative settlement transaction up
front to the (possibly) recovering party -- *not a commitment transaction*.
Note we cannot send a cooperative settlement tx up front for each
connection since they are not revocable -- the channel is over once it has
been received.

I admit I didn't properly consider just sending commitment transactions
over. This is probably because it exposes the recovering party to the
punishment mechanism and in my most recent line of research you *really*
don't want to do this. The idea I'm working with in revocable signature
based channels [1] is to make the node lose its static secret key if it
posts a revoked commitment tx. This means they could lose ALL funds from
ALL their channels with ALL their peers if they ever broadcast a single
revoked commitment transaction. This would be a very bad thing to happen
while you're trying to recover funds.

I agree with your core point that in LN as it exists today the security
assumption of both methods is that the adversary will be unable to
distinguish a connection attempt after data loss from an ordinary one. If
they can reliably do this then both methods can lead to loss of funds so
why bother with the wonky crypto?

I think there is a subtle reason why oblivious settlement signatures are
still preferable: it is difficult to provide a coherent UX for recovery
when just sending YOLO commitment transactions. It would be best if the
recovery UX was "hey user I've found 0.367 Bitcoin across these three
channels would you like to recover them?". The user can then accept this or
cancel the recovery process and go through some extra trouble to recover
their data (in practice, data is often not completely lost but recoverable
with some effort). This is how it could work using the oblivious settlement
txs I proposed.

Using YOLO commitment transactions this becomes "hey user I've found 0.523
Bitcoin across three channels" which may actually be more than you are owed
to entice you to shoot yourself in the foot. Even if it's exactly what you
are owed, when you confirm the recovery your next message might be "aaaand
it's gone" because one of the commit txs was revoked.

It seems difficult to recommend YOLO commitment transactions becoming the
standard way to recover funds. It could be preferable to the current system
but even that is up for debate I guess. I feel like I can recommend
oblivious settlements because (i) it's covert (like YOLO commitments txs
unlike current system) and (ii) it's "what you see is what you get" -- you
are guaranteed to recover the funds that you are presented with once you
finally trigger the recovery.

Although I do think oblivious settlement txs is a good idea I am working on
yet *another* recovery idea for doing peer provided encrypted backup and
full channel restoration in a way that provides another guarantee: If the
peer sends you an outdated encrypted backup (perhaps in the hope that it
has a revoked commitment tx in it) -- you can punish them immediately upon
receiving the backup (if you haven't actually had data loss).
Unfortunately, It looks like this will use more heavy cryptographic
primitives.

I think the challenge in either protocol above is deciding which peer
> goes first, because whoever sends the commitment transaction reveals
> what they think the current state is. Any node that refuses to go first
> can then be suspected of having lost data. BOLT2
> option_static_remotekey has this same problem, which is reasonably
> mitigated IMO by LN's penalty mechanism forcing any would-be thief to
> risk their own funds; this doesn't work for basic eltoo, though.
>

What is the story with option_static_remotekey? I am interested to know how
the negotiation of that option has a security issue but I don't see how it
could.

[1]: https://github.com/LLFourn/witness-asymmetric-channel

Cheers,

LL
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