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

[David A. Harding]

On Mon, Apr 20, 2020 at 10:43:14PM -0400, Matt Corallo via Lightning-dev wrote:
> 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.

IIUC, the main problem is honest Bob will broadcast a transaction
without realizing it conflicts with a pinned transaction that's already
in most node's mempools.  If Bob knew about the pinned transaction and
could get a copy of it, he'd be fine.

In that case, would it be worth re-implementing something like a BIP61
reject message but with an extension that returns the txids of any
conflicts?  For example, when Bob connects to a bunch of Bitcoin nodes
and sends his conflicting transaction, the nodes would reply with
something like "rejected: code 123: conflicts with txid 0123...cdef".
Bob could then reply with a a getdata('tx', '0123...cdef') to get the
pinned transaction, parse out its preimage, and resolve the HTLC.

This approach isn't perfect (if it even makes sense at all---I could be
misunderstanding the problem) because one of the problems that caused
BIP61 to be disabled in Bitcoin Core was its unreliability, but I think
if Bob had at least one honest peer that had the pinned transaction in
its mempool and which implemented reject-with-conflicting-txid, Bob
might be ok.

-Dave
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