Subject: Re: [bitcoin-dev] Coinjoin with less steps using ALL|ANYONECANPAY

From: Ben Carman <[email-masked]>
Date: Mon, 22 May 2023 22:51:49 +0000

The problem with using ALL|ANYONECANPAY is that you cannot verify beforehand that the other inputs are the inputs you want added to the transaction.

Some examples of bad things that could happen:

  *   Coordinator adds its own inputs, you still get your outputs but effectively paid fees for no privacy gain
  *   The inputs added could be paying at a lower fee rate than expected, causing the tx to take longer than what you paid for
  *   Different input types or amount are added so you no longer have the same uniformity across the inputs
  *   (if you care) An input from a sanctioned address is added, causing you to get "tainted" coins.

This is the code in ln-vortex that verifies the psbt on the client side if you are curious

https://github.com/ln-vortex/ln-vortex/blob/master/client/src/main/scala/com/lnvortex/client/VortexClient.scala#L616

Best,

benthecarman

________________________________
From: bitcoin-dev <[email-masked]> on behalf of alicexbt via bitcoin-dev <[email-masked]>
Sent: Monday, May 22, 2023 7:51 AM
To: Bitcoin Protocol Discussion <[email-masked]>
Subject: [bitcoin-dev] Coinjoin with less steps using ALL|ANYONECANPAY

Hi Bitcoin Developers,

I recently experimented with different sighash flags, PSBTs and realized ALL|ANYONECANPAY could be used to reduce some steps in coinjoin.

Steps:

- 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.

Proof of Concept (Aice and Bob): https://gitlab.com/-/snippets/2542297

Tx: https://mempool.space/testnet/tx/c6dd626591dca7e25bbd516f01b23171eb0f2b623471fcf8e073c87c1179c492

I plan to use this in joinstr if there are no major drawbacks and it can even be implemented by other coinjoin implementations.

/dev/fd0
floppy disk guy

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