I posted this relevant text to the darkwallet mailing list.
Summary:
One party waits for a mix (by enabling mixing on a pocket). Another wants to mix on demand.
Allow waiting party to specify a mixing fee that will be paid to its change address by the other side.
I suggest this will create a mixing market and increase mixing volume.
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A proposal for an improvement to darkwallet's coinjoin system.
Darkwallet's coinjoin works with two parties. One party will wait around with bitcoins they wish to mix. Another party will connect with them whenever they want to do a transactions and the two parties will do a coinjoin. By analogy with exchange matching engines, I will call the first party the coinjoin maker and the second the coinjoin taker.
The idea is that there will be people acting as makers who will slowly mix their coins for the benefit of the takers who want to immediately mix.
Here is an example of a darkwallet coinjoin transaction
https://blockchain.info/tx/c38aac9910f327700e0f199972eed8ea7c6b1920e965f9cb48a92973e7325046As you can see, the change addresses can be linked with the inputs but the address with outputs of 0.01btc cannot be linked. Mixing is achieved.
The maker/taker model solves the problem of having to wait around for someone who wants to coinjoin exactly the same amount as you.
There are a few small problems with this. I will propose an improvement.
First, the only people motivated enough to mix will be people who already have desire privacy. So, for the example of someone wanting to privately buy contraception and hide the fact from their parents, mixing may result in their coins being mixed with drug money. It is easy to imagine this ecosystem being split between 'clean' coins mostly owned by investors and 'dirty' coins owned by 'undesirables' of society. It's easy to imagine a blacklist system being made which these two groups are seperated and cannot cross the barrier of blacklists, which would be highly damaging to bitcoin fungiblility.
Secondly, there is little incentive to act as a coinjoin maker when you could just be a coinjoin taker and get your mixing done without any waiting.
Thirdly, a small number of coinjoin makers leaves open the possiblity of three-letter agencies offering coinjoin making, and using that role to deanonymize darkwallet coinjoins.
Proposal: Pay the coinjoin makers. They will put up offers to do coinjoin along with a fee they ask. The coinjoin transaction would be built up in a similar way to the above example, with the coinjoin maker fee being added to the associated change address and taken away from the coinjoin taker's change address.
An implication of this is that darkwallet coinjoin mixing will become like an almost-riskless savings account. We already see that holders of bitcoin are willing to earn just 0.006% per day by lending btc on the bitfinex exchange, and that contains a substantial risk that bitfinex will go disappear or be hacked taking all the bitcoins with it.
Darkwallet coinjoin with a maker's fees would be far less risky, the bitcoin private keys would never leave the owner's computer. This would likely result in investors pouring in. The huge relative supply of bitcoins along with the drought of other worthy bitcoin investments is likely to drive down the fees of mixing as the investors bid each other down.
The coinjoin takers will have access to tens of thousands of clean, untainted bitcoins to mix with at a very low price. The makers will have access to a very low risk investment for their bitcoins. Bitcoin fungiblility as a whole will benefit because the entire economy and flow of money will be more interrelated, making blacklists completely unfeasable. The market for coinjoin makers may end up so deep and liquid that three-party or even four-party coinjoins may become managable to organise.
~Belcher
PGP fingerprint: 0A8B 038F 5E10 CC27 89BF CFFF EF73 4EA6 77F3 1129
