On Wed, Apr 22, 2020 at 03:03:29PM -0400, Antoine Riard wrote:
> > 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?
> 
> That's an interesting idea, but an attacker can create a local conflict in
> your mempool

You don't need a mempool to send a transaction.  You can just open
connections to random Bitcoin nodes directly and try sending your
transaction.  That's what a lite client is going to do anyway.  If the
pinned transaction is in the mempools of a significant number of Bitcoin
nodes, then it should take just a few random connections to find one of
those nodes, learn about the conflict, and download the pinned
transaction.

If that's not acceptable, you could find some other way to poll a
significant number of people with mempools, e.g. BIP35 mempool messages
or reusing the payment hash in a bunch of 1 msat probes to LN nodes who
opt-in to scanning their bitcoind's mempools for a corresponding
preimage.

-Dave