Date: Fri, 22 Mar 2019 16:05:13 +0000
To: "Kenshiro \\[\\]" <tensiam@hotmail.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Subject: Re: [bitcoin-dev] Payjoin privacy with the receiver of the
	transaction
Precedence: list

Good morning,

> >There's certainly some interesting about the idea of "pre-fragmenting" y=
our wallet utxo so you can make (or in payjoin: receive) payments with bett=
er privacy aspects.However, it's pretty unlikely to be practical for normal=
 users, as it'll generally result in pretty big and cost-ineffective transa=
ctions.
>
> For users that really want privacy it should not be a problem. When a wal=
let receive a high amount of btc (+100$ or another amount defined by the us=
er) it can automatically make a transaction to itself splitting the amount =
in several addresses. The amounts that are already small don't need to be s=
plitted again. Small amount addresses + Payjoin could give real privacy to =
bitcoin users. Users that don't want privacy could disable the "Private" mo=
de in the wallet and disable the auto-splitting feature.=C2=A0
>
> i.e.: you receive 1000$ in btc and the wallet make an automatic transacti=
on to itself to 10 addresses, 100$ each.
>

It seems to me, to interact somewhat with ZeroLink.

Under ZeroLink, post-mix UTXOs must not be combined.
(Basic Post-Mix Wallet Requirement: "Post-mix wallet MUST prevent joining i=
nputs together.")

The upshot of this, for practical use, is that as payments are done by the =
user, available coins become smaller and smaller.
And the maximum amount the user can pay with, is limited by the largest pos=
t-mix coin they have.

If a ZeroLink post-mix wallet were to split its UTXOs as soon as it got the=
m from the mix, then it would immediately find itself limiting the maximum =
amount the user could pay.
I suppose if the ZeroLink post-mix wallet had multiple post-mix coins, it c=
ould split one of them for the same purpose as above.

Another thought, is if a ZeroLink post-mix wallet could support a Payjoin, =
as either receiver or sender.
Naively, it seems to me to improve privacy to do so, as long as the ZeroLin=
k post-mix wallet only provides a single UTXO to the Payjoin, whether as re=
ceiver or sender.
For a ZeroLink post-mix wallet to a ZeroLink post-mix wallet Payjoin, this =
would typically result in a two-input, two-output transaction, with both pa=
rticipants having one input and one output each in the transaction, but dif=
ficult (?) for third parties to determine which input/output belongs to whi=
ch.

Now, if we suppose that both ZeroLink and Payjoin become commonly used, the=
n it is likely that two users using the same Chaumian CoinJoin mix transact=
ion will find that one needs to pay the other.
Thus hopefully it may become common for a Chaumian CoinJoin mix transaction=
 to have outputs that (directly or indirectly) merge into Payjoin two-input=
 two-output transactions.
This can then be used to allow a ZeroLink post-mix wallet some limited amou=
nt of merging its post-mix UTXOs.
For instance, if a ZeroLink post-mix wallet has a 0.25BTC and a 0.15BTC coi=
n, and needs to pay 0.3 BTC, it may very well simulate a Payjoin to itself,=
 and create a transaction (0.25, 0.15) -> (0.35, 0.05).
Then it can use the 0.35BTC output to pay the 0.3 BTC.

Possibly, anyway.

Regards,
ZmnSCPxj