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

From: ZmnSCPxj <[email-masked]>
Date: Thu, 23 Apr 2020 12:46:59 +0000

Good morning Matt,

> > -   C directly contacts miners with an out-of-band proposal to replace its transaction with an alternative that is much smaller and has a low fee, but much better feerate.
>
> Or they can just wait. For example in today’s mempool it would not be strange for a transaction at 1 sat/vbyte to wait a day but eventually confirm.

That introduces the possibility that the entire tree (with high total fee, remember) gets confirmed, so it would be better for C to replace it with an alternative to a different address C still controls, with a slightly better fee rate but smaller (no child transactions) and lower total fee, so an economically-rational C will make that effort (and if there are still other transactions in the mempool, an economically-rational miner will accept this proposal).

But in any case this is a minor detail and the attack will work either way.

>
> > -   Miners, being economically rational, accept this proposal and include this in a block.
> >
> > The proposal by Matt is then:
> >
> > -   The hashlock branch should instead be:
> > -   B and C must agree, and show the preimage of some hash H (hashlock branch).
> > -   Then B and C agree that B provides a signature spending the hashlock branch, to a transaction with the outputs:
> > -   Normal payment to C.
> > -   Hook output to B, which B can use to CPFP this transaction.
> > -   Hook output to C, which C can use to CPFP this transaction.
> > -   B can still (somehow) not maintain a mempool, by:
> > -   B broadcasts its timelock transaction.
> > -   B tries to CPFP the above hashlock transaction.
> > -   If CPFP succeeds, it means the above hashlock transaction exists and B queries the peer for this transaction, extracting the preimage and claiming the A->B HTLC.
>
> Note that no query is required. The problem has been solved and the preimage-containing transaction should now confirm just fine.

Ah, right, so it gets confirmed and the `blocksonly` B sees it in a block.

Even if C hooks a tree of low-fee transactions on its hook output or normal payment, miners will still be willing to confirm this and the B hook CPFP transaction without, right?

Regards,
ZmnSCPxj