Subject: Re: [bitcoin-dev] Denial of Service using Package Relay

From: Andrew Chow <[email-masked]>
Date: Thu, 06 Jul 2023 17:24:47 +0000

On 07/06/2023 12:22 PM, alicexbt via bitcoin-dev wrote:
 > 1) Register input in A
 > 2) Double spend same input with zero fee to your own address
 > 3) Register unconfirmed UTXO from 2 in B

Why would unconfirmed inputs be accepted in a coinjoin? That seems 
unsafe, regardless of package relay. The sender of the unconfirmed 
transaction can already replace it thereby pinning or otherwise 
invalidating the coinjoin, it doesn't need package relay.

Furthermore, the coordinator B shouldn't accept the unconfirmed UTXO 
from 2 because it doesn't even know about that unconfirmed transaction. 
It has zero fee, so it's not going to be relayed.

Conceivably a similar attack can already be done by simply registering 
the same UTXO with multiple coordinators anyways. This doesn't require 
package relay either.

***

Package relay should help coinjoins since any one of the participants 
can rebroadcast the coinjoin with a further CPFP if the coinjoin is 
below the minimum relay fee. Some of the upcoming package RBF proposals 
should also help by allowing other child transactions in the package to 
RBF the entire thing, thereby resolving the need to have everyone 
re-sign the coinjoin in order to RBF.

Andrew