From: ZmnSCPxj <ZmnSCPxj@protonmail•com>
To: Yuval Kogman <nothingmuch@woobling•org>,
Bitcoin Protocol Discussion
<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Non-equal value CoinJoins. Opinions.
Date: Sun, 29 Dec 2019 10:23:39 +0000 [thread overview]
Message-ID: <Ucl9pe26g2ECz-SRmXPV3WLxVR8PBOf0dnMR_aD8NwTqBNmq6e3a9hKJtwkYPJz7v_QUCxT_Y5X0w1VkvbiQZ6H3QJVcOtpUhNYTQ29rwFA=@protonmail.com> (raw)
In-Reply-To: <CAAQdECBqFKxAoZXCkWynN4wj5g8C9vzdhuEWk9b-BYqDW=us6g@mail.gmail.com>
Good morning Yuval,
> Additionally (though is a broader criticism of CoinJoin based privacy and not specific to unequal amounts, and in particular refers to ZmnSCPxj's assertion of 0 linkability) I am very worried that perspectives that focus on linkability information revealed by a single coinjoin transaction in isolation. This problem was alluded in the document, to but I don't see that it was addressed. Naively the post/pre mix transaction graph would seem to present a computationally much harder problem when looking at the combinatorics through the same lens, but reality it can also be used to place many constraints on valid partitions/sub-transaction assignments for a single transaction with equal amounts. The trivial example is post mix linking of outputs, but there are many other ways to draw inferences or eliminate possible interpretations of a single transaction based on its wider context, which in turn may be used to attack other transactions.
Indeed, this is a problem still of equal-valued CoinJoin.
In theory the ZeroLink protocol fixes this by strongly constraining user behavior, but ZeroLink is not "purely" implemented in e.g. Wasabi: Wasabi still allows spending pre- and post-mix coins in the same tx (ZeroLink disallows this) and any mix change should be considered as still linked to the inputs (though could be unlinked from the equal-valued output), i.e. returned to pre-mix wallet.
> Finally, the proof as well as its applicability seems suspect to me, since seems to involve trusting the server:
> "Since the distinct list [...] [is] kept on the server and not shared with the players"
> "The server knows the linkages of the commitments but does not participate as a verifier "
> "If there is a problem [...] each component is assigned to another player at random for verification"
> these 3 statements together seems to suggest the server is trusted to not use sybils in order the compromise privacy by participating in the verification process?
Equal-valued CoinJoins fix this by using a Chaumian bank, which constrains value transfers to specific fixed amounts.
Since an equal-valued CoinJoin uses a single fixed amount anyway, it is not an additional restriction.
CashFusion cannot use the same technique without dropping into something very much like an equal-valued CoinJoin.
Regards,
ZmnSCPxj
next prev parent reply other threads:[~2019-12-29 10:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-27 18:03 nopara73
2019-12-28 17:38 ` Ethan Heilman
2019-12-28 23:25 ` ZmnSCPxj
2020-02-22 18:01 ` nopara73
2019-12-29 3:31 ` Yuval Kogman
2019-12-29 9:57 ` Yuval Kogman
2019-12-29 10:23 ` ZmnSCPxj [this message]
2019-12-29 17:48 ` Yuval Kogman
2019-12-30 1:14 ` Lucas Ontivero
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