You make some interesting points in these comparisons. I will address a few things you say in the article at the end, here, but first I want to discuss some background related to aut-ct (and yes this is mostly already implicit in your article but for other readers, the following):

There's always in my mind two very distinct threats from Sybilling. The first is that your protocol might subtly (or grossly) depend on non-collusion of participants. For that you want one kind of Sybil resistance.

The second is the problem of free-entry, which can occur even if non-collusion is not a requirement. If anyone can join a protocol without real limits, then the resource usage implied when the number of protocol users increases by 1000x can screw up resource utilization. (Bitcoin itself deals with this beautifully through implicit PoW dependence of messages to be considered valid.)

My feeling when working on the idea of RIDDLE and then later aut-ct was that I was only really addressing or thinking about the 2nd of those two cases. It's not impossible to use a utxo ownership proof to address the 1st of the two above, but it's clearly much less strong, by default, than one would hope.

Just a pure "I own a utxo" imposes no, or negligible cost. As per the delving post, and in a few other places, I've noted you can impose an age and value restriction to bump up the cost. So it's not inconceivable but I suspect it's troublesome, and, it tends to fall into the "anything's better than nothing" category. While a fidelity bond with a public timeout is more convenient specifically because you don't need to have "prepared" it in advance, a long time (so same lockup cost, but in advance).

The actual problems with fidelity bonds are twofold: privacy headache from public utxo announcement, and expense (the expense part is unintuitive, but, if a value lock is to impose cost that *specifically prevents Sybils*, it's very valuable that it be counted super-linearly in the size of the utxo; otherwise, a high-net-worth Sybiler can split his amount into 100 pieces e.g. to get 100 valid entry tokens. Chris Belcher originally proposed and implemented a quadratic scaling [2], but we reduced that default exponent and made it configurable, precisely because the HNW Sybiler (or honest participant) can nearly guarantee participation. It's a whole can of worms, though).

I can see that being true only in one specific sense: that Wa(bi)Sabi coordinators can Sybil and thus link. Obviously in the default honest mode of operation of coordinator this is not true of such systems.

> Joinstr uses aut-ct as the primary mechanism for sybil resistance, however fidelity bonds can also be used with aut-ct. There is an initiator who creates the pool and adds sybil requirements to join the pool. Everyone (maker and takers) needs to provide the proof for a successful coinjoin.

When you say "fidelity bonds can be used with aut-ct" I *thought* you meant that: the anon set can be the anon-set of all timelocked UTXOs (that may or may not be fidelity bond type), but if you do that then of course you need to form the anon set based on some time-range, I guess? The problem I always had with this was how do we coordinate anything like this for the anon set to be big enough. Like, if you did it with aut-ct it would either have to be taproot or users publishing (where?) the pubkeys in order to make the tree. People need to actually create these timelocked utxos, so they need somehow to be told well in advance what the realistic parameters are for it. It seems very difficult practically to coordinate and then to get to decent privacy, also (in case you commit a big chunk of funds and then only 5 other people do it .. you were hoping 5000, now you have little or no privacy from a big cost. Just an example)

For me this example illustrates why Sybil-threat type 1 is not well defended by this kind of system. How much does that really cost? It's reasonable to answer "almost absolutely nothing", since you don't spend those coins, and while in a vague, abstract sense they are "locked" (you need some that lasted that long), a well funded attacker could always have that amount x10 or x100 sitting *somewhere*. Handwavy, but imo it's only if we want to prevent 1000 of those at once that it starts to be in some sense "costly". But a Sybil attack on a coinjoin does not need more than N-1 participants to Sybil an N-party join, so 1000 is way over the line.

On Tuesday, May 27, 2025 at 10:21:42 PM UTC-3 /dev /fd0 wrote:

Hi Bitcoin Developers,

I have written a post comparing the sybil resistance of joinmarket, joinstr and wabisabi. I did not include whirlpool in this post because its not used anymore. Although it won't be any different from wabisabi.

Its not a long post but written after doing a lot of research. The results show that joinmarket has good enough sybil resistance. However, joinstr provides better solution.

Feel free to share your feedback.

Link: https://uncensoredtech.substack.com/p/sybil-resistance-in-coinjoin-implementations

/dev/fd0
floppy disk guy