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
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