Date: Wed, 10 Jun 2020 06:47:28 +0000
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Subject: Re: [bitcoin-dev] Question about PayJoin effectiveness
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Good morning again Mr. Lee,

> > I am trying to learn about payjoin. I have a couple concerns on its
> > effectiveness. Are my concerns valid or am I missing something?
> > concern 1
> > If it is known to be a payjoin transaction anyone could determine the
> > sender the recipient and amount right?
> > Lets assume that everyone has a single utxo because payjoin becomes com=
mon
> > use and payjoin consolidates utxos through "snowballing". If Alice has =
a
> > UTXO of 0.05 btc and Bob has a UTXO of 1.15 btc. Bob can be assumed to
> > have more balance because he is a merchant and his customers payjoin hi=
m
> > payments alot.


It is also helpful to remember that Bob cannot exist in isolation, and ther=
efore, Bob probably has:

* Employees.
* Suppliers.
* Shareholders.

For example, suppose Bob holds in reserve a 0.05 BTC UTXO in a holding wall=
et.

Then Bob takes the 1.16 UTXO it got from Alice and transfers 1.12 BTC to th=
e holding wallet:

    Bob merchant wallet 1.16 --___-- 1.17 Bob holding wallet
    Bob holding wallet  0.05 --   -- 0.04 Bob merchant wallet

The above looks exactly like one of the "customer pays Bob" transactions, b=
ut is in fact different.

Then Bob uses the holding wallet to pay out to employees, suppliers, and sh=
areholders, such as in a single large batched transaction, and then leaves =
behind another 0.05 BTC in the holding wallet (or some random small number =
of BTC) for the next time Bob has to pay to employees/suppliers/shareholder=
s.

So the transaction below:

    1.16 --___-- 1.17
    0.05 --   -- 0.04

*could* be interpreted as the 0.05 owner paying to the 1.16 owner, but in f=
act that is just Bob preparing the incoming funds from the merchant front-e=
nd for processing to send to its own liabilities.

Regards,
ZmnSCPxj