Subject: [bitcoindev] Re: Sybil resistance in different coinjoin implementations

From: waxwing/ AdamISZ <[email-masked]>
Date: Sat, 7 Jun 2025 07:39:21 -0700 (PDT)

Hi fd0,

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):

I want to expand on something I said when we were discussing this on 
delving [1]:

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).

So given that, it's very natural to look for alternatives to, or finesses 
on, fidelity bond ideas. Which you do, here, so, coming to some parts of 
your article:

> Since WabiSabi is a coinjoin implementation based on centralized 
coordinator, a user must also trust the coordinator not to link inputs and 
outputs.

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 <https://github.com/AdamISZ/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.

Interesting idea to blend, but I am left considering an ambiguity:

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)

But reading further I see you were looking at it the other way; literally 
two separate things, both fidelity bonds, and aut-ct tokens.

> Everyone shares aut-ct proof that proves they own a P2TR UTXO worth 
0.1-0.2 BTC that is unspent until last block and aged more than 2016 blocks

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.

Cheers,
AdamISZ/waxwing

[1] 
https://delvingbitcoin.org/t/anonymous-usage-tokens-from-curve-trees-or-autct/862/3
[2] https://gist.github.com/chris-belcher/87ebbcbb639686057a389acb9ab3e25b

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
>

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