Quote from: graphite on April 29, 2024, 04:21:45 AM

I thought joinmarkets were still pretty easy to unmix.

When you look at a single JoinMarket coinjoin, you can often (but not always) determine common input ownership for the participants and track their change. However, this isn't a limitation that ever requires you to compromise your privacy because you can use the taker role to construct a coinjoin where you have only one input and one (equal sized) output.

Here's an example of a JoinMarket sweep transaction: https://mempool.space/tx/9c6479472e1f3b5861c86bc904d11814e715a84c21aa8d0dd0861e635555451f

You can tell this is a sweep instead of an outgoing payment because there are 9 inputs, 9 equal sized outputs, and 8 change outputs that can be easily connected to maker inputs. The taker's input was from bc1q2pcrqv8jshjalxdan9hdm6qsh0njtm7jxydct9, but there is no trace of his output since it could be any of the 9 values for 0.03846033 BTC.

Quote from: graphite on April 29, 2024, 04:21:45 AM

This is the same problem with samurai wallets whirlpool right?

Yes, common input ownership and toxic change are the same problems inherent to Whirlpool coinjoins as well. The main advantage JoinMarket has is ability to coinjoin arbitrary amounts, so your toxic coins never have to touch each other. Whirlpool is limited to coinjoining fixed amounts (0.5, 0.05, 0.01 and 0.001), so every transaction you send or receive will require you to forfeit up to 100k sats in unmixable change in order to keep full privacy.

The second advantage JoinMarket has over Whirlpool is that Whirlpool cripples the privacy of users by requiring a premix "tx0" transaction. The premix conclusively reveals all the consolidated inputs are owned by the same person and links it with toxic change that can be tracked in future transactions.

JoinMarket mitigates this privacy leak by skipping the premix transaction and consolidating inputs directly in the coinjoin transaction. Here's an example of a 5 person JoinMarket coinjoin that consolidates 297 inputs: https://mempool.space/tx/63b28f5e17e03fef27795e1ea7fbf821e2d5f072098ffcef3d27b8b2d23ca719

This makes it difficult to determine which inputs belonged to the participant that created the 33677 change output and which inputs belonged to the participant that created the 44384 sat change output.

Quote from: graphite on April 29, 2024, 04:21:45 AM

If theres only 1 taker and for example 5 makers in a joinmarket coinjoin it should be pretty easy to unmix them if you watch to see which output get used as inputs for more coinjoins then they are makers and if you can determine which outputs were makers then you can determine which output is the taker by process of elimination.

If 5 out of the 6 equal sized outputs created in the coinjoin end up remixing as makers in the future while the remaining equal sized output is spent in a unique way, the first guess someone would make is that the output that didn't remix was created by the taker. But, this is still a guess and not a 100% guarantee since the taker could switch roles to become a maker, and makers could decide to stop remixing and spend their coins. So, you could have scenarios where all 6 of these outputs remix, or all 6 of these outputs are spent, or any combination in between.