Subject: Re: [bitcoin-dev] OP_Expire and Coinbase-Like Behavior: Making HTLCs Safer by Letting Transactions Expire Safely

From: Antoine Riard <[email-masked]>
Date: Mon, 13 Nov 2023 02:18:16 +0000

Your two latest mails.

> The problem that OP_Expire aims to solve is the fact that Carol could
prevent
> Bob from learning about the preimage in time, while still getting a
chance to
> use the preimage herself. OP_Expire thoroughly solves that problem by
ensuring
> that the preimage is either broadcast in the blockchain in a timely
fashion, or
> becomes useless.

I respectfully disagree - There is a more general underlying issue for
outdated states in multi-party off-chain constructions, where any "revoked"
or "updated" consensus-valid state can be used to jam the latest off-chain
agreed-on, through techniques like replacement cycling or pinning.

> My suggestion of pre-signing RBF replacements, without anchor outputs,
and with
> all outputs rendered unspendable with 1 CSV, is clearly superior: there
are
> zero degrees of freedom to the attacker, other than the possibility of
> increasing the fee paid or broadcasting a revoked commitment. The latter
of
> course risks the other party punishing the fraud.

Assuming the max RBF replacement is pre-signed at 200 sats / vb, with
commitment transaction of ~268 vbytes and at least one second-stage HTLC
transaction of ~175 vbytes including witness size, a channel counterparty
must keep at worst a fee-bumping reserve of 35 268 sats, whatever payment
value. As of today, any payment under $13 has to become trimmed HTLCs.
Trimmed HTLCs are coming with their own wormhole of issues, notably making
them a target to be stolen by low-hashrate capabilities attackers [0].

[0]
https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-May/002714.html

> This does have the practical problem that a set of refund transactions
will
> also need to be signed for each fee variant. But, eg, signing 10x of each
isn't
> so burdensome. And in the future that problem could be avoided with
> SIGHASH_NOINPUT, or replacing the pre-signed refund transaction mechanism
with
> a covenant.

I think if you wish to be safe against fees griefing games between
counterparties, both counterparties have to maintain their own fee-bumping
reserves, which make channel usage less capital efficient, rather than
being drawn from a common reserve.

> Using RBF rather than CPFP with package relay also has the advantage of
being
> more efficient, as no blockspace needs to be consumed by the anchor
outputs or
> transactions spending them. Of course, there are special circumstances
where
> BIP125 rules can cause CPFP to be cheaper. But we can easily fix that, eg
by
> reducing the replacement relay fee, or by delta-encoding transaction
updates.

It is left as an exercise to the reader how to break the RBF approach for
LN channels as proposed.

> As SIGHASH_NOINPUT is desirable for LN-Symmetry, a softfork containing
both it
> and OP_Expire could make sense.

I think there is one obvious issue of pre-signing RBF replacements combined
with LN-symmetry, namely every state has to pre-commit to fee values
attached and such states might spend each other in chain. So now you would
need `max-rbf-replacement` *  `max-theoretical-number-of-states` of
fee-bumping reserves unless you can pipe fee value with some covenant
magic, I think.

> In existing anchor output transactions, this type of attack wouldn't work
as
> when broadcasting the transaction, Alice would be spending her anchor
output,
> which Bob can't double spend.

However Bob can double-spend Alice's commitment transaction with his own
commitment transaction and a CPFP, as long as it's a better ancestor
feerate and absolute fee package, then double-spend his own CPFP. Which is
exactly what my test is doing so I don't think your statement of saying
this type of advanced replacement cycling attack wouldn't work isn't
correct.

Best,
Antoine

Le mer. 8 nov. 2023 à 02:06, Peter Todd <[email-masked]> a écrit :

> On Wed, Nov 08, 2023 at 12:51:31AM +0000, Peter Todd via bitcoin-dev wrote:
> > > In a post-package relay world, I think this is possible. And that
> > > replacement cycling attacks are breaking future dynamic fee-bumping of
> > > pre-signed transactions concerns me a lot.
> >
> > Well the answer here is pretty clear: v3 package relay with anchors is
> broken.
>
> BTW a subtlety of this that may not be obvious is that in v3 package relay,
> with zero value outputs, the outputs must be spent in the same package.
> Thus
> _unlike_ existing anchor-using transactions, there would be only one anchor
> output on the commitment transaction.
>
> In existing anchor output transactions, this type of attack wouldn't work
> as
> when broadcasting the transaction, Alice would be spending her anchor
> output,
> which Bob can't double spend. But that doesn't work in v3, which intends to
> limit UTXO growth by requiring that anchors be spent in the same package.
> Thus
> unlike existing anchor outputs, an anchor would be truly a OP_1 output
> without
> a signature, and thus belong to either Alice nor Bob uniquely.
>
> --
> https://petertodd.org 'peter'[:-1]@petertodd.org
>