Subject: Re: [bitcoin-dev] bustapay BIP :: a practical sender/receiver coinjoin protocol

From: James MacWhyte <[email-masked]>
Date: Tue, 29 Jan 2019 18:06:30 -0800

James

On Sun, Jan 27, 2019 at 2:11 PM <[email-masked]> wrote:

>
> It isn't passed "back and forth so many times".
>

You are right, I got the wrong impression the first time I read it.

> This is an important anti-DoS/anti-spy tactic, as it proves the sender
> actually owns those inputs and if the protocol is not followed to
> completion, the transaction can be dumped on the network.
>

I'm not convinced this is a valid concern, at least not valid enough to add
extra complications to the process. The sender could still refuse to sign
the final transaction after they see the recipient's in-/outputs; "show me
yours and I'll show you mine" isn't much of a spy deterrent, and nothing
here prevents a DOS attack.

As an implementor, I would suggest keeping the protocol as simple as
possible. By dropping the signing in the first step, the recipient doesn't
need to maintain the ability to lookup and verify unspent outputs. It also
would enforce the increased privacy, which the sender obviously wants if
they are going down this path (in other words, either have the process
complete or fail -- don't give the recipient the ability to broadcast the
not-private transaction against the wishes of the sender).