[bitcoin-dev] [Lightning-dev] RBF Pinning with Counterparties and Competing Interest

[Olaoluwa Osuntokun]

> So what is needed is to allow B to add fees to HTLC-Timeout:

Indeed, anchors as defined in #lightning-rfc/688 allows this.

>  * With `SIGHASH_NOINPUT` we can make the C-side signature
>  `SIGHASH_NOINPUT|SIGHASH_SINGLE` and allow B to re-sign the B-side
>  signature for a higher-fee version of HTLC-Timeout (assuming my cached
>  understanding of `SIGHASH_NOINPUT` still holds).

no_input isn't needed. With simply single+anyone can pay, then B can attach
a new input+output pair to increase the fees on their HTLC redemption
transaction. As you mention, they now enter into a race against this
malicious ndoe to bump up their fees in order to win over the other party.

If the malicious node uses a non-RBF signalled transaction to sweep their
HTLC, then we enter into another level of race, but this time on the mempool
propagation level. However, if there exists a relay path to a miner running
full RBF, then B's higher fee rate spend will win over.

-- Laolu

On Tue, Apr 21, 2020 at 9:13 PM ZmnSCPxj via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> Good morning Matt, and list,
>
>
>
> >     RBF Pinning HTLC Transactions (aka "Oh, wait, I can steal funds,
> how, now?")
> >     =============================
> >
> >     You'll note that in the discussion of RBF pinning we were pretty
> broad, and that that discussion seems to in fact cover
> >     our HTLC outputs, at least when spent via (3) or (4). It does, and
> in fact this is a pretty severe issue in today's
> >     lightning protocol [2]. A lightning counterparty (C, who received
> the HTLC from B, who received it from A) today could,
> >     if B broadcasts the commitment transaction, spend an HTLC using the
> preimage with a low-fee, RBF-disabled transaction.
> >     After a few blocks, A could claim the HTLC from B via the timeout
> mechanism, and then after a few days, C could get the
> >     HTLC-claiming transaction mined via some out-of-band agreement with
> a small miner. This leaves B short the HTLC value.
>
> My (cached) understanding is that, since RBF is signalled using
> `nSequence`, any `OP_CHECKSEQUENCEVERIFY` also automatically imposes the
> requirement "must be RBF-enabled", including `<0> OP_CHECKSEQUENCEVERIFY`.
> Adding that clause (2 bytes in witness if my math is correct) to the
> hashlock branch may be sufficient to prevent C from making an RBF-disabled
> transaction.
>
> But then you mention out-of-band agreements with miners, which basically
> means the transaction might not be in the mempool at all, in which case the
> vulnerability is not really about RBF or relay, but sheer economics.
>
> The payment is A->B->C, and the HTLC A->B must have a larger timeout (L +
> 1) than the HTLC B->C (L), in abstract non-block units.
> The vulnerability you are describing means that the current time must now
> be L + 1 or greater ("A could claim the HTLC from B via the timeout
> mechanism", meaning the A->B HTLC has timed out already).
>
> If so, then the B->C transaction has already timed out in the past and can
> be claimed in two ways, either via B timeout branch or C hashlock branch.
> This sets up a game where B and C bid to miners to get their version of
> reality committed onchain.
> (We can neglect out-of-band agreements here; miners have the incentive to
> publicly leak such agreements so that other potential bidders can offer
> even higher fees for their versions of that transaction.)
>
> Before L+1, C has no incentive to bid, since placing any bid at all will
> leak the preimage, which B can then turn around and use to spend from A,
> and A and C cannot steal from B.
>
> Thus, B should ensure that *before* L+1, the HTLC-Timeout has been
> committed onchain, which outright prevents this bidding war from even
> starting.
>
> The issue then is that B is using a pre-signed HTLC-timeout, which is
> needed since it is its commitment tx that was broadcast.
> This prevents B from RBF-ing the HTLC-Timeout transaction.
>
> So what is needed is to allow B to add fees to HTLC-Timeout:
>
> * We can add an RBF carve-out output to HTLC-Timeout, at the cost of more
> blockspace.
> * With `SIGHASH_NOINPUT` we can make the C-side signature
> `SIGHASH_NOINPUT|SIGHASH_SINGLE` and allow B to re-sign the B-side
> signature for a higher-fee version of HTLC-Timeout (assuming my cached
> understanding of `SIGHASH_NOINPUT` still holds).
>
> With this, B can exponentially increase the fee as L+1 approaches.
> If B can get HTLC-Timeout confirmed before L+1, then C cannot steal the
> HTLC value at all, since the UTXO it could steal from has already been
> spent.
>
> In particular, it does not seem to me that it is necessary to change the
> hashlock-branch transaction of C at all, since this mechanism is enough to
> sidestep the issue (as I understand it).
> But it does point to a need to make HTLC-Timeout (and possibly
> symmetrically, HTLC-Success) also fee-bumpable.
>
> Note as well that this does not require a mempool: B can run in
> `blocksonly` mode and as each block comes in from L to L+1, if HTLC-Timeout
> is not confirmed, feebump HTLC-Timeout.
> In particular, HTLC-Timeout comes into play only if B broadcast its own
> commitment transaction, and B *should* be aware that it did so --- there is
> still no need for mempool monitoring here.
>
>
> Now, of course this only delays the war.
> Let us now consider what C can do to ensure that the bidding war will
> happen eventually.
>
> * C can bribe a miner to prevent HTLC-Timeout from confirming between L
> and L+1.
>   * Or in other words, this is a censorship attack.
>     * The Bitcoin censorship-resistance model is that censored
> transactions can be fee-bumped, which attracts non-censoring miners to try
> their luck at mining and evict the censoring miner.
>       * Thus, letting B bump the fee on HTLC-Timeout is precisely the
> mechanism we need.
>       * This sets up a bidding war between C requesting miners to censor,
> vs. B requesting miners to confirm, but that only sets the stage for a
> second bidding war later between C and B, thus C is at a disadvantage: it
> has to bribe miners to censor continuously from L to L+1 *and* additional
> bribe miners to confirm its transaction after L+1, whereas B can offer its
> bribe as being something that miners can claim now without waiting after
> L+1.
>
>
>
> The issue of course is the additional output that bloats the UTXO set and
> requires another transaction to claim later.
> And if we have `SIGHASH_NOINPUT`, it seems to me that
> Decker-Russell-Osuntokun sidesteps this issue as well, as any timed-out
> HTLC can be claimed with a fee-bumpable transaction directly without
> RBF-carve-out.
> (As well, it seems to me that, if both nodes support doing so, a
> Poon-Dryja channel can be upgraded, without onchain activity, to a
> Decker-Russell-Osuntokun channel: sign a transaction spending the funding
> tx to a txo that has been set up as Decker-Russell-Osuntokun, do not
> broadcast that transaction, then revoke the latest Poon-Dryja commitment
> transactions, then switch the mechanism over to Decker-Russell-Osuntokun;
> you still need to monitor for previous Poon-Dryja commitment transactions,
> but HTLCs now sidestep the issue under discussion here.)
>
> Regards,
> ZmnSCPxj
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