Date: Tue, 23 May 2023 12:34:03 +0000
To: Ben Carman <benthecarman@live.com>
From: alicexbt <alicexbt@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Coinjoin with less steps using ALL|ANYONECANPAY
Precedence: list

Hi Ben,

Thanks for the feedback.

> -   Coordinator adds its own inputs, you still get your outputs but effec=
tively paid fees for no privacy gain

What will be the incentive for a coordinator to add its inputs in coinjoin?=
 Is this possible without ALL|ANYONECANPAY as well? Even if there is an inc=
entive its unlikely to work in joinstr as there is no centralized coordinat=
or. Multiple common relays are used to coordinate a coinjoin round.

> -   The inputs added could be paying at a lower fee rate than expected, c=
ausing the tx to take longer than what you paid for
> -   Different input types or amount are added so you no longer have the s=
ame uniformity across the inputs

> This is the code in ln-vortex that verifies the psbt on the client side i=
f you are curious
>=20
> https://github.com/ln-vortex/ln-vortex/blob/master/client/src/main/scala/=
com/lnvortex/client/VortexClient.scala#L616

These 2 are important things and could be managed with client side validati=
on by keeping min-max amounts for inputs in a round and disallow different =
types of inputs. Thanks for sharing the code that validates PSBT.

Joinstr will also use NIP38/48 channels for coinjoin rounds so that only pa=
rticipants in a coinjoin round are aware of details.

/dev/fd0
floppy disk guy

Sent with Proton Mail secure email.

------- Original Message -------
On Tuesday, May 23rd, 2023 at 4:21 AM, Ben Carman <benthecarman@live.com> w=
rote:


> The problem with using ALL|ANYONECANPAY is that you cannot verify beforeh=
and that the other inputs are the inputs you want added to the transaction.
>=20
> Some examples of bad things that could happen:
>=20
>=20
> -   Coordinator adds its own inputs, you still get your outputs but effec=
tively paid fees for no privacy gain
> -   The inputs added could be paying at a lower fee rate than expected, c=
ausing the tx to take longer than what you paid for
> -   Different input types or amount are added so you no longer have the s=
ame uniformity across the inputs
> -   (if you care) An input from a sanctioned address is added, causing yo=
u to get "tainted" coins.
>    =20
>=20
> This is the code in ln-vortex that verifies the psbt on the client side i=
f you are curious
>=20
> https://github.com/ln-vortex/ln-vortex/blob/master/client/src/main/scala/=
com/lnvortex/client/VortexClient.scala#L616
>=20
>=20
> Best,
>=20
> benthecarman
>=20
>=20
>=20
> From: bitcoin-dev <bitcoin-dev-bounces@lists.linuxfoundation.org> on beha=
lf of alicexbt via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
> Sent: Monday, May 22, 2023 7:51 AM
> To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
> Subject: [bitcoin-dev] Coinjoin with less steps using ALL|ANYONECANPAY
>=20
> Hi Bitcoin Developers,
>=20
> I recently experimented with different sighash flags, PSBTs and realized =
ALL|ANYONECANPAY could be used to reduce some steps in coinjoin.
>=20
> Steps:
>=20
> - Register outputs.
> - One user creates a signed PSBT with 1 input, all registered outputs and=
 ALL|ANYONECANPAY sighash flag. Other participants keep adding their inputs=
 to PSBT.
> - Finalize and broadcast the transaction.
>=20
> Proof of Concept (Aice and Bob):=C2=A0https://gitlab.com/-/snippets/25422=
97
>=20
> Tx: https://mempool.space/testnet/tx/c6dd626591dca7e25bbd516f01b23171eb0f=
2b623471fcf8e073c87c1179c492
>=20
> I plan to use this in joinstr if there are no major drawbacks and it can =
even be implemented by other coinjoin implementations.
>=20
> /dev/fd0
> floppy disk guy
>=20
> Sent with Proton Mail secure email.
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