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

From: "David A. Harding" <[email-masked]>
Date: Thu, 23 Apr 2020 05:59:57 -0400

On Wed, Apr 22, 2020 at 03:53:37PM -0700, Matt Corallo wrote:
> if you focus on sending the pinning transaction to miner nodes
> directly (which isn't trivial, but also not nearly as hard as it
> sounds), you could still pull off the attack. 

If the problem is that miners might have information not available to
the network in general, you could just bribe them for that knowledge.
E.g. as Bob's refund deadline approaches and he begins to suspect that
mempool shenanigans are preventing his refund transaction from
confirming, he takes a confirmed P2WPKH UTXO he's been saving for use in
CPFP fee bumps and spends part of its value (say 1 mBTC) to the
following scriptPubKey[1],

    OP_SHA256 <hash_whose_preimage_bob_wants> OP_EQUAL

Assuming the feerate and the bribe amount are reasonable, any miner who
knows the preimage is incentivized to include Bob's transaction and a
child transation spending from it in their next block.  That child
transaction will include the preimage, which Bob will see when he
processes the block.

If any non-miner knows the preimage, they can also create that child
transaction.  The non-miner probably can't profit from this---miners can
just rewrite the child transaction to pay themselves since there's no
key-based security---but the non-miner can at least pat themselves on
the back for being a good Summaritan.  Again Bob will learn the preimage
once the child transaction is included in a block, or earlier if his
wallet is monitoring for relays of spends from his parent transaction.

Moreover, Bob can first create a bribe via LN and, in that case, things
are even better.  As Bob's deadline approaches, he uses one of his
still-working channels to send a bunch of max-length (20 hops?) probes
that reuse the earlier HTLC's <hash>.  If any hop along the path knows
the preimage, they can immediately claim the probe amount (and any
routing fees that were allocated to subsequent hops).  This not only
gives smaller miners with LN nodes an equal chance of claiming the
probe-bribe as larger miners, but it also allows non-miners to profit
from learning the preimage from miners.

That last part is useful because even if, as in your example, the
adversary is able to send one version of the transaction just to miners
(with the preimage) and another conflicting version to all relay nodes
(without the preimage), miners will naturally attempt to relay the
preimage version of the transaction to other users; if some of those
users run modified nodes that write all 32-byte witness data blobs to a
database---even if the transaction is ultimately rejected as a
conflict---then targetted relay to miners may not be effective at
preventing Bob from learning the preimage.

Obviously all of the above requires people run additional software to
keep track of potential preimages[2] and then compare them to hash
candidates, plus it requires additional complexity in LN clients, so I
can easily understand why it might be less desirable than the protocol
changes under discussion in other parts of this thread.  Still, with
lots of effort already being put into watchtowers and other
enforcement-assistance services, I wonder if this problem can be largely
addressed in the same general way.

-Dave

[1] Requires a change to standard relay and mining policy.
[2] Pretty easy, e.g.

    bitcoin-cli getrawmempool \
    | jq -r .[] \
    | while read txid ; do
      bitcoin-cli getrawtransaction $txid true | jq .vout[].scriptPubKey.asm
    done \
    | grep -o '\<[0-9a-f]\{64\}\>'