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January 17, 2017, 02:37:55 PM |
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Let me clarify some things.
It's true that all solutions with a limited anonymity set somehow suffer from sybil attacks. However, if the possible anonymity set is large enough, and the system is indeed used regugarly, then it's quite probably that there will be some honest users in it. If you ask me, both ValueShuffle and TumbleBit provide reasonable anonymity sets; however TumbleBit scales a little better.
Regarding fees: Yes, fees make sybil attacks harder, because the attacker has to pay them as well. But fees are not specific to TumbleBit. Both ValueShuffle and TumbleBit (and basically all other mixing solutions) require normal transactions fees, because they rely on some kind of Bitcoin transaction. If desired, you can artificially increase the fees to make sybil attacks more expensive (at the cost of higher fees for honest users).
Centralization: That's right, both TumbleBit and ValueShuffle require some form of central point. (In theory, you can do ValueShuffle without central point. That would then use P2P connections between the nodes, so I would not call it "on-chain". However, then you need techniques that make the protocol quite slow and this is not what you use in practice.) Everybody can set up such a central point and people can chose which one they would like to use. In both TumbleBit and ValueShuffle, users do not need to trust the central point except for the fact that it does not help sybil attacks by excluding honest users. However, honest users could publicly complain about that, and then it's possible to switch to a new central point.
A difference between TumleBit and ValueShuffle is that the central point in TumbleBit is a "tumbler", which is a server specificially designed for running TumbleBit. ValueShuffle just requires some form of broadcast communication, e.g., a modern IRC server supporting timestamps. That may make a difference in legal terms: Maybe you don't want to run a server made for anonymizing money, but running an IRC server should not get you in trouble in reasonable jurisdictions.
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