Delivery-date: Wed, 26 Mar 2025 11:25:00 -0700
Sender: bitcoindev@googlegroups.com
Date: Tue, 25 Mar 2025 05:52:07 -0700 (PDT)
From: jbesraa <jbesraa@gmail.com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Message-Id: <1c7130d4-cbac-4404-968c-9eb7b4e2e4cbn@googlegroups.com>
In-Reply-To: <450755f1-84c5-4f32-abe0-67087ae884d6n@googlegroups.com>
References: <450755f1-84c5-4f32-abe0-67087ae884d6n@googlegroups.com>
Subject: [bitcoindev] Re: UTXO probing attack using payjoin
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While the possibility of UTXO probing via Payjoin is a valid concern=20
regarding privacy, it's important to note that it might not always come=20
without cost for the attacker. The Payjoin recipient needs to validate the=
=20
initial request, ensuring the sender's inputs are broadcastable. This means=
=20
the recipient could, in practice, broadcast the initial transaction even if=
=20
the sender aborts the Payjoin. Furthermore, implementing strategies like=20
maintaining a set of 'seen inputs' can make such probing attempts more=20
easily detectable and less effective. While these measures don't eliminate=
=20
the privacy considerations entirely, they do highlight that recipients have=
=20
potential defenses and that probing isn't necessarily a risk-free endeavor=
=20
for the attacker.

On Tuesday, March 25, 2025 at 1:48:15=E2=80=AFPM UTC+2 /dev /fd0 wrote:

Hi everyone,=20

Sometimes we are curious and want to know about UTXOs in other wallets.=20
Payjoin allows you to do this and the recipient would never doubt it=20
because it's a privacy tool. It's possible to find UTXO in recipient's=20
wallet without sending any bitcoin. It's called UTXO probing attack and=20
described in BIP 77-78.

I have shared a demo with all the details in this [post][0]. I have used=20
bullbitcoin wallet for testing this because it was the only [wallet][1]=20
which supports payjoin v2 (send, receive) and testnet3.

I think users should be aware of this tradeoff and the information they=20
share with the sender in payjoin. Payjoin should only be used with trusted=
=20
senders.

[0]: https://uncensoredtech.substack.com/p/utxo-probing-attack-using-payjoi=
n
[1]: https://en.bitcoin.it/wiki/PayJoin_adoption

/dev/fd0
floppy disk guy

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While the possibility of UTXO probing via Payjoin is a valid concern=20
regarding privacy, it's important to note that it might not always come=20
without cost for the attacker. The Payjoin recipient needs to validate=20
the initial request, ensuring the sender's inputs are broadcastable.=20
This means the recipient could, in practice, broadcast the initial=20
transaction even if the sender aborts the Payjoin. Furthermore,=20
implementing strategies like maintaining a set of 'seen inputs' can make
 such probing attempts more easily detectable and less effective. While=20
these measures don't eliminate the privacy considerations entirely, they
 do highlight that recipients have potential defenses and that probing=20
isn't necessarily a risk-free endeavor for the attacker.<br /><br /><div><d=
iv dir=3D"auto">On Tuesday, March 25, 2025 at 1:48:15=E2=80=AFPM UTC+2 /dev=
 /fd0 wrote:<br /></div><blockquote style=3D"margin: 0px 0px 0px 0.8ex; bor=
der-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">Hi everyone, <b=
r /><br />Sometimes we are curious and want to know about UTXOs in other wa=
llets. Payjoin allows you to do this and the recipient would never doubt it=
 because it's a privacy tool. It's possible to find UTXO in recipient's wal=
let without sending any bitcoin. It's called UTXO probing attack and descri=
bed in BIP 77-78.<br /><br />I have shared a demo with all the details in t=
his [post][0]. I have used bullbitcoin wallet for testing this because it w=
as the only [wallet][1] which supports payjoin v2 (send, receive) and testn=
et3.<br /><br />I think users should be aware of this tradeoff and the info=
rmation they share with the sender in payjoin. Payjoin should only be used =
with trusted senders.<br /><br />[0]: <a href=3D"https://uncensoredtech.sub=
stack.com/p/utxo-probing-attack-using-payjoin" target=3D"_blank" rel=3D"nof=
ollow">https://uncensoredtech.substack.com/p/utxo-probing-attack-using-payj=
oin</a><br />[1]: <a href=3D"https://en.bitcoin.it/wiki/PayJoin_adoption" t=
arget=3D"_blank" rel=3D"nofollow">https://en.bitcoin.it/wiki/PayJoin_adopti=
on</a><br /><br />/dev/fd0<br />floppy disk guy</blockquote></div>

<p></p>

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