But, far more difficult and expensive the more the victim remixes.
Exactly. Unlike the WabiSabi and JoinMarket protocols which allow you to remix as much as you want, remixing in Whirlpool is permissioned: You have to trust the coordinator will eventually choose you.
Wouldn't it be concerning if it was only one premixer? One remixer isn't going to harm.
- If the other four premixed inputs belong to an attacker, then they know the respective remixed UTXO, which isn't crucial in and of itself; it'd be if they could work out the remixed input's past.
- If less than four premixed inputs belong to an attacker, then they know even less than that.
In other words, it can harm if the remixer has only participated in attacker's coinjoins where there was only one remixer. [/quote]
I was just providing an example transaction that didn't fit your initial description, it's not a factor in this topic specifically.
I was talking about Samourai's coordinator.
Right. The cost of the coordinator fee does not provide a defense against a coordinator Sybil attacking their own round since they are paying themselves. Mining fees do provide a defense against coordinators Sybil attacking their own round since they are consumed by an external party.
I would encourage you to read about how JoinMarket addresses the possibility of Sybil attacks being conducted by remixers: