Feedback-ID: i525146e8:Fastmail
Date: Tue, 10 Jan 2023 05:03:16 -0500
From: Peter Todd <pete@petertodd.org>
To: alicexbt <alicexbt@protonmail.com>
Message-ID: <Y7035Edqoq8CK+nl@petertodd.org>
References: <Y7ySzDjzx5eDjOH9@petertodd.org>
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Cc: bitcoin-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] Why Full-RBF Makes DoS Attacks on Multiparty
 Protocols Significantly More Expensive
Precedence: list


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On Tue, Jan 10, 2023 at 09:19:39AM +0000, alicexbt wrote:
> Hi Peter,
>=20
> > ## How Full-RBF Mitigates the Double-Spend DoS Attack
> >=20
> > Modulo tx-pinning, full-rbf mitigates the double-spend DoS attack in a =
very
> > straightforward way: the low fee transaction is replaced by the higher =
fee
> > transaction, resulting in the latter getting mined in a reasonable amou=
nt of
> > time and the protocol making forward progress.
>=20
> Asking this question based on a [discussion on twitter][0]. How would you=
 get extra sats to increase the fees?

You're misunderstanding the issue. There is no need for extra sats to incre=
ase
fees. Coinjoin transactions already have fees set at a level at which you'd
expect them to be mined in a reasonable amount of time. Full-RBF ensures th=
at,
modulo tx pinning, either the coinjoin gets mined, or any double-spend has =
to
have a high enough feerate that it will be mined in a reasonable amount of =
time
as well.

> It seems this would be possible with Joinmarket, Wasabi and even joinstr =
although things would get worse for Whirlpool. Whirlpool coinjoin transacti=
ons do not signal BIP 125 RBF so they were not replaceable earlier

Bringing up Whirlpool here is silly. Everyone knows Samourai has made, at b=
est,
some rather insane technical decisions. Quite likely downright malicious wi=
th
their xpub collection. Their opinion isn't relevant. Cite reputable sources.

Anyway, Wasabi would like to move to making coinjoins opt-in to RBF. Though
full-rbf may come sooner; for technical reasons opt-in RBF is ugly to imple=
ment
now as activation needs to be coordinated accross all clients:

https://github.com/zkSNACKs/WalletWasabi/issues/9041#issuecomment-1376653020

> however attacker would be able to perform DoS attacks now by double spend=
ing their inputs used in coinjoin.

As I explained, attackers can already do this with or without full-rbf simp=
ly
by picking the right time to broadcast the double spend. It's not an effect=
ive
attack anyway: with a UTXO you can already hold up a coinjoin round by simp=
ly
failing to complete stage #2 of the coinjoin. Actually doing a double-spend
simply guarantees that you're spending money on it. It's only effective with
low-fee double-spends in the absence of full-rbf.

> [0]: https://twitter.com/dammkewl/status/1599692908860706818

This tweet is nuts. Eg "Gives well connected mining pools an added advantag=
e"
is simply false. Full-RBF does the exact opposite.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

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