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From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
To: Adam Gibson <ekaggata@gmail.com>,
	Bitcoin Protocol Discussion
	<bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] bustapay BIP :: a practical sender/receiver coinjoin protocol
Date: Wed, 30 Jan 2019 08:34:46 +0000	[thread overview]
Message-ID: <q_EImVoLLoTdTZI0kPb4olI3FFjIMx9Uj0O8acFefNbsMtU7K25wWz69Alm-jbwZ8SEV1U3Y6Re3705Xi2zQb5129MbtjEVE8dT_JtSSfmA=@protonmail.com> (raw)
In-Reply-To: <226c43d8-1fad-9d90-ba47-9230118e447d@gmail.com>

Good morning Adam,

> And I'm reminded that a related point is made by belcher in the gist
> comment thread iirc (after we discussed it on IRC): over time a
> "PayJoin-only" merchant doing the simplest thing - using a single utxo
> over and over again, will concentrate more and more funds into it, and
> inevitably violating UIH2 in an increasingly dramatic fashion
> (contributing a 100BTC utxo to a 0.1BTC payment etc.). Suggesting it's
> better if there's a mix of payjoin/non-payjoin.

To be pedantic: as I understand bustapay, it would still not violate UIH2 (unless I misunderstand UIH2).

Suppose the original transaction is: (0.05 payer, 0.07 payer) -> (0.1 payee, 0.02 payer)

Then bustapay with such a PayJoin-only merchant with 100BTC UTXO would give: (100 payee, 0.05 payer, 0.07 payer) -> (100.1 payee, 0.02 payer).
As I understand it, this technically does not violate UIH2.

It would still conceivably be interpreted as a payment of 100.1 BTC, from a payer who happens to have massively lopsided UTXOs being owned, but still does not violate UIH2.

However, if that 100.1 UTXO is subsequently used to pay a 100.3 payment, then that is used to pay a 100.7 payment, that strongly suggests such a naive PayJoin-only merchant.

Perhaps a simple heuristic against this would be:

1.  For every UTXO you own, flip a coin.
    If all of them come up heads, do not payjoin; just broadcast the original transaction.
2.  Else, randomly select a UTXO (value not care?) and payjoin with that UTXO.

However, I have no proper analysis of the blockchain, so --

Regards,
ZmnSCPxj


      reply	other threads:[~2019-01-30  8:34 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-30 20:24 [bitcoin-dev] bustapay BIP :: a practical sender/receiver coinjoin protocol rhavar
2018-09-10 12:30 ` Sjors Provoost
2018-09-10 15:49   ` rhavar
2019-01-25 14:47 ` Adam Gibson
2019-01-27  7:36   ` rhavar
2019-01-27 12:20     ` Adam Gibson
2019-01-27 19:24       ` rhavar
2019-01-27 19:42       ` James MacWhyte
2019-01-27 22:11         ` rhavar
2019-01-30  2:06           ` James MacWhyte
2019-01-30  2:46             ` rhavar
2019-01-30 20:58               ` James MacWhyte
2019-01-28  4:14     ` ZmnSCPxj
2019-01-28 13:19       ` Adam Gibson
2019-01-30  8:34         ` ZmnSCPxj [this message]

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