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@canadian_btc Wasabi Questions

Aug 21st, 2020 (edited)
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  1. > Can I mix CoinJoin change outputs together?
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  3. Currently, by default, Wasabi's guarantees coins coming to the wallet to be disconnected from CJ-d coins. If you need more guarantees about separation between coins coming to the wallet, then
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  5. 1. Practice labeling and don't enqueue coins together you don't want to expose common ownership with.
  6. 2. Or just use separate wallets, Wasabi has possibly the handiest wallet management system.
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  8. Failing to do 1 and 2 and still wanting to get some weak guarantees for the above condition, then it's slightly better to merge in coinjoins than in normal transactions.
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  10. > What if my Anonymity Set is 200, so the original coins are well mixed? Make any difference?
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  12. Short answer: Yes, it makes a difference.
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  14. For the long answer, maybe I'd reformulate the question to "What are the limitations of Wasabi anonset?"
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  16. The idea was that if you mix with 10 people, then you get 10 anonset. Straightforward, right? It is, but when transactions start building on top of each other, then it becomes problematic. Mathematically speaking there are 2 ways to go:
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  18. 1. Calculate k-anonymity (https://dataprivacylab.org/dataprivacy/projects/kanonymity/paper3.pdf) Take a mixed coin and try to find the number of possible sources of that coin. Even if the mix was only with 10 people, 1 of those 10 people are at least remixing and thus the previous mix that let's say happened with 10 more people are also possible sources. So the anonset would be 10+10, which is 20. Except if that previous mix also had a remixer, which also adds more anonset, thus mathematically speaking the final anonset of a mixed UTXO would be all the incoming bitcoins of all the Wasabi transactions ever happened, because that's how interconnected these mixes are. I suppose you can see why mathematical k-anonymity is super misleading, so we cannot go with that metric, so what can we go with?
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  20. 2. Probabilities. Take a UTXO in our first mix. The probability of it belonging to my input is 1/10. If I remixed twice then it's 1/100. If I remixed thrice, then it's 1/1_000. This is a nice metric, we could've gone with this metric and call it a day, but there's an issue here, too. Wasabi mixes aren't very clean. You often end up mixing with the same people and some other minor issues, so this metric would be still misleading.
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  22. So what did we decide on? Well, let's lowball it and stupidly add together anonsets after each and every mix: I mix once, I got anonset 10, I mix twice, I got anonset 10, that's 20 in sum. Thus Wasabi anonset isn't quite a mathematical metric as you can see the mathematical metrics are terribly misleading the users and fixing them is impossible with the blockchain information a client has, so the Wasabi anonset is basically just a user feedback that was immaturely called anonset, because that worked out quite well for a single transaction, but didn't think what we'll do for multiple transactions. This is something I'll remove/replace/change when implementing WabiSabi (https://github.com/zkSNACKs/WabiSabi/)
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  24. One important note here is that the metrics above are calculated for Bitcoin UTXOs, so don't confuse them for metrics that are calculated for transactions themselves.
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  26. > Is it better or worse to mix change with new non-joined UTXOs?
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  28. Change and new non-joined UTXOs should be considered as the same thing. They're the ugly red coins in your wallet. So the answer to this is the same as to the answer to your first question: "Can I mix CoinJoin change outputs together?"
  29. Now, a more interesting question would be, and maybe that's what you intended to ask in the first place that "how about exposing common ownership between mixed coins and un-mixed coins?"
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  31. And the newbie answer is: that's the single worst thing you can do as that instantly cancels the benefits of the mix. That's a good rule of thumb and keep that in your mind until you're ready for the advanced answer: if the red coins aren't coming from the same source as the mixed coins, then it's beneficial to mix together green and red coins as that throws off bockchain anal. However Wasabi currently does not have any visual indicator how to figure out if they're coming from the same source or not, in fact, even the labels are stripped. More, an early design decision was to not try to exploit this notion at all as Wasabi has no idea if there's a common source of coins before the coins arrived to the wallet and by not factoring in that possibility breaking rule of thumb just doesn't worth the risk, so let's keep ourselves to that instead of trying to exploit this small thing as it's prone to miscalculation and error. What should you do with your ugly red coins? There's no technical solution Wasabi currently has. Donate them or wait for WabiSabi: https://github.com/zkSNACKs/WabiSabi/
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