Re: [bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-15 Thread ZmnSCPxj via bitcoin-dev
Good morning Antoine, Gleb, and list,

In some ways, CoinPool is really part of a swarm of ideas:

* CoinPool
* Multiparticipant (N > 2) channels
* Channel factories
* Nodelets

What CoinPool and multiparticipant channels buy us is better flexibility with 
forwarding.
For example, if we compare a multiparty channel to a channel factory, suppose 
there exists three entities A, B, and C in the multiparty construction.

In a channel factory, each entity has to decide how much of its liquidity to 
tie up in a channel with a specific other peer in the multiparty construction.
This limits the practical payment forwarding when integrated into the Lightning 
Network.

In a CoinPool, any of the entities can forward to any of the other entities, 
without tying their liquidity to a channel specifically with those entities.

However, in a CoinPool, once any of the entities goes offline, the entire 
CoinPool can no longer update.
This is in contrast with channel factories, where, if entity C goes offline, 
the channel between A and B remains useable for forwarding.
In other words, channel factories degrade gracefully.

Further, we already have a decent solution for liquidity redistribution: JIT 
Routing by Rene Pickhardt.
Thus the liquidity issue with channel factories are somewhat mitigated (and if 
all participants are online, they also have the option of redistributing 
channel funds *inside* the factory as well, not just JIT routing), while 
gaining graceful degradation of the factory.


Another is that pathfinding algorithms work best if graph edges are edges and 
not in fact some kind of twisted multi-edge that connects more than two nodes 
together.

On the other hand, the participants of a CoinPool could create a "virtual node" 
that is a MuSig of their individual keys, and report that as the "real" node on 
LN gossip (each of them pretending to have a large channel with that virtual 
node), so that the rest of the network only sees edges that link two nodes (and 
existing pathfinding algos still work seamlessly, never realizing that this 
node is actually a virtual node that represents a CoinPool).
This is basically them creating a sort of Nodelet node, which other nodes 
cannot make channels to, and which uses channels with the Nodelet node as 
proxies for the CoinPool as a whole.


Regards,
ZmnSCPxj
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Re: [bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-12 Thread Antoine Riard via bitcoin-dev
Hi ZmnSCPxj,

> I have not studied the proposal in close detail yet, but anyway, my main
takeaway roughly is:
>
> * The core of CoinPool is some kind of multiparticipant (N > 2) offchain
update mechanism (Decker-Wattenhofer or Decker-Russell-Osuntokun).
>   * The output at each state of the update mechanism is some kind of
splitting construction (which I have not studied in detail).
>   * At each update of the state, all participants must sign off on the
new state.

Overall, that's a really accurate description. I would add you can embed a
funding outpoint of any offchain protocol on the splitting construction,
modulo some timelocks shenanigans.

> In order to hide transfers from the elected WabiSabi server, participants
can maintain two coins in every state, and move coins randomly across the
two coins they own at each state update, in order to hide "real" transfers
from the elected server.

Yes I'm quite sure you can reuse WabiSabi as a communication channel
between participants, assuming you support tapscript and merkle branch
transports, and server build a tree. Generally, we tried to keep design as
flexible as we can to reuse privacy tools.

> Indeed, from what I can understand, in order to properly set up the
splitting transactions in the first place, at each state every participant
needs to know how much each other participant actually owns in the CoinPool
at that point in time.

Yes, that's part of future research, defining better *in-pool* observer.
Sadly, right now, even if you use mask construction inside, it's quite easy
to trace leaves by value weight. Of course, you can enforce equal-value
leaves, as for a regular onchain CoinJoin. I think it comes with a higher
onchain cost in case of pool breakage.

> That way, output addresses can be to fresh pseudonyms of the participant,
removing all linkages of participant to amount they own, and each
participant can maintain multiple outputs per state for their own purposes
and to mildly obscure exactly how much they own in total.

That's right that an in-pool observer may learn a link between an exit and
an onchain withdraw. There is a future optimization, if you can swap your
withdraw with an already onchain output, therefore breaking heuristics.

> We can do this by using `SIGHASH_ANYPREVOUT` to force whoever performs a
unilateral close of the CoinPool to pay the onchain fees involved, so that
it would have to be a good reason indeed to perform a unilateral close.

Absolutely, for the fee structure, as the withdraw output is at the
discretion of user, I was thinking some CPFP. There is maybe a better
solution, haven't spend that much on the exact adequate, incentives-align
mechanism beyond a "withdraw-must-pay-its-fees".

Thanks for the high-quality review, as usual ;)

Antoine

Le ven. 12 juin 2020 à 04:39, ZmnSCPxj  a écrit :

> Good morning Antoine and Gleb,
>
> I have not studied the proposal in close detail yet, but anyway, my main
> takeaway roughly is:
>
> * The core of CoinPool is some kind of multiparticipant (N > 2) offchain
> update mechanism (Decker-Wattenhofer or Decker-Russell-Osuntokun).
>   * The output at each state of the update mechanism is some kind of
> splitting construction (which I have not studied in detail).
>   * At each update of the state, all participants must sign off on the new
> state.
>
> It seems to me that it would be possible to use a [WabiSabi protocol](
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-June/017969.html)
> during negotiation of a new state.
>
> Now, WabiSabi is a client-server protocol.
> As all participants in the CoinPool are needed in order to ratify each new
> state anyway, they can simply elect one of their number by drawing lots, to
> act as server for a particular state update.
>
> Then the participants can operate as WabiSabi clients.
> Each participant registers the outputs they currently own in the current
> state, getting credentials that sum up to the correct value.
> Then, during the WabiSabi run, they can exchange credentials among the
> participants in order to perform value transfers inside the WabiSabi
> construction.
> Then, at output registration, they register new outputs to put in the next
> state of the CoinPool.
>
> In order to hide transfers from the elected WabiSabi server, participants
> can maintain two coins in every state, and move coins randomly across the
> two coins they own at each state update, in order to hide "real" transfers
> from the elected server.
>
> Then, after output registration, the participants ratify the new state by
> signing off on the new state and revoking the previous state, using the
> update mechanism.
>
> Of course, we should note that one desired feature for CoinPool in the
> original proposal is that a participant can exit, and the CoinPool would
> still remain valid, but only for the remaining participants.
>
> This is arguably a mild privacy leak: every other participant now knows
> how much that particular participant took

Re: [bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-12 Thread ZmnSCPxj via bitcoin-dev
Good morning Antoine,

By dropping the requirement that a participant can seamlessly leave the 
CoinPool, it allows participants to split up their coins among new aliases and 
to use a different identity for later claiming coins.
With WabiSabi, none of the other participants can get a mapping between 
current-state aliases and the actual participants.

Now, in order to authorize moving coins from an output on the current state to 
a new output on the next state, obviously the pool needs to get a signature 
from its current owner.
Ideally, we would not want to have to implement SCRIPT inside the CoinPool 
software.

And with Taproot, a pubkey can hide one or more SCRIPTs.
If we use pubkeys as the identities of owners of coins, then it allows an alias 
to encode a SCRIPT.

With the combination of both features, we can instantiate HTLCs (or, with 
`SIGHASH_ANYPREVOUT`, PTLCs) inside a CoinPool "alias" pubkey identity, 
allowing for interoperation with LN.

Now suppose I have 1.0 BTC in a CoinPool.
I want to make an HTLC with you (hashlocked to you, timelocked to me), for 0.5 
BTC.

I encode the HTLC SCRIPT, and put it into a Taproot whose internal pubkey is a 
MuSig of fresh identities of mine and yours.

Then, inside the CoinPool, I split my 1.0BTC to a 0.5BTC coin to a fresh 
identity of mine, and 0.5BTC to our HTLC Taproot.

If you can acquire the hash, you give it to me, and I am supposed to hand you a 
partial signature share to the HTLC Taproot that you can later complete and 
present to the CoinPool in the next update round in order to get the HTLC value.
If I do not hand you the signature share even after you hand the hash, you just 
drop the entire CoinPool onchain, instantiating the HTLC Taproot output 
onchain, and using the SCRIPT branch to claim using the hash you know.

If the timelock expires, I ask you to hand over your partial signature to the 
HTLC Taproot that I can later complete and present to the CoinPool in the next 
update round to recover the HTLC value.
If you do not hand over the signature share, I drop the CoinPool onchain, which 
instantiates the HTLC Taproot output onchain, and use the SCRIPT branch to 
claim using the timelock branch.

You can also ask to abort the HTLC "early", before the timelock expires, by 
handing over your partial signature to the HTLC Taproot, which I can later 
complete and present to the CoinPool in the next update round.
This is equivalent to `update_fail_htlc` in the current LN BOLT spec.

This allows operation of any SCRIPT, incidentally, without requiring that 
CoinPool software include a SCRIPT interpreter, only signature validation.
Any time an output absolutely needs a SCRIPT, we just drop the CoinPool onchain 
and let onchain handle the SCRIPT interpretation.

Regards,
ZmnSCPxj

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Re: [bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-12 Thread ZmnSCPxj via bitcoin-dev
Good morning Antoine,


> Yes, that's part of future research, defining better *in-pool* observer. 
> Sadly, right now, even if you use mask construction inside, it's quite easy 
> to trace leaves by value weight. Of course, you can enforce equal-value 
> leaves, as for a regular onchain CoinJoin. I think it comes with a higher 
> onchain cost in case of pool breakage.


Perhaps not necessarily.

An advantage of WabiSabi is I can pretend to be two or more participants.

For example, I can pretend to be "Alice" and "Bob", and pretend that "Alice" 
owes a life debt to "Bob".

At initial state setup, I put a 1.0 BTC coin as "Alice" and a 0.5 BTC coin as 
"Bob".

Now, at each state update I need to sign as "Alice" and "Bob".
However, after the first initial state, I can use a new persona "Bobby" to 
*own* my coins, even though I still have to sign as "Alice" and "Bob" in every 
state update.

What the other pool participants see is that the 1.0 BTC "Alice" coin and the 
0.5 BTC "Bob" coin are merged into the 1.5 BTC "Bobby" coin.
What they cannot be sure of is:

* "Alice" paid to "Bob", who is now pretending to be "Bobby".
* "Bob" paid to "Alice", who is now pretending to be "Bobby".
* "Alice" and "Bob" are the same person, and is also pretending to be "Bobby".

All the other participants know is that whoever owns the coin *now* is still 
part of the pool, but cannot be sure which participant *really* owns which 
coin, and whether participants are sockpuppets (which is why it should use 
n-of-n at each state update, incidentally).

In effect, it "imports" the possibility of PayJoin inside the CoinPool 
construction.



Regards,
ZmnSCPxj
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Re: [bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-12 Thread Antoine Riard via bitcoin-dev
Hi Jeremy,

For the records, I didn't know between Greg and you was at the origin of
payment pools. Thanks for your pioneer work here, obviously this draws
inspiration from OP_CTV use cases and Channel Factories works, even if we
picked up different assumptions and tried to address another set of issues.

With regards to scalability, I hit it on my own while inquiring
covenanted-Bitcoin contracts for international trade. I mentioned the
any-order issue on such multi-party complex contracts in a talk last summer
(https://github.com/ariard/talk-slides/blob/master/advanced-contracts.pdf).

> All of these channels can be constructed and set up non-interatively using
> CTV, and updated interactively. By default payments can happen with
minimal
> coordination of parties by standard lightning channel updates at the leaf
> nodes, and channels can be rebalanced at higher layers with more
> participation.

Side review note on OP_CTV: I think it would be great to define
non-interactivity better, namely at least between 3 phases: establishment,
operation, closing.

Even OP_CTV protocols assume interactivity at establishment, at least 1) to
learn payees pubkeys endpoint (and internal leaves pubkeys if you want
update at operation) 2) validate transaction tree correctness between
participants.

At operation, it depends if participants want to dynamically rebalance
value across channels or not. If you desire dynamically rebalancing, assume
internal leaves scriptpubkeys are (multisig-all OR OP_CTV'ed merkle_tree).
Using OP_CTV is a saving in message rounds for every constant expression
across tree updates.

At closing, depends again if participants have committed update keys or
not. If dynamic update, you can prune the whole tree and just commit final
balances onchain, either with a O(N) fan-out transaction (N outputs) or a
O(log(N)) congestion tree (N transactions).

So I would say the originality of a hashchain covenant like OP_CTV is to
provide onchain *immutability* (unforgeability?) of the offchain
transaction tree and thus provides instant finality to payees. You can get
the same semantic with off-chain covenant, pre-signed set of transactions,
assuming more communications rounds and performance hit.

That said, IMO, immutability comes with a security trade-off, namely if any
payout key committed in your OP_CTV tree gets compromised your funds are at
stake. And you can't update the tree anymore at the root to rotate keys. I
think this should be weighted by anyone designing covenant protocols,
especially vaults.

> I don't think the following requirement: "A
> CoinPool must satisfy the following *non-interactive any-order withdrawal*
> property: at any point in time and any possible sequence of previous
> CoinPool events, a participant should be able to move their funds from the
> CoinPool to any address the participant wants without cooperation with
> other CoinPool members." is desirable in O(1) space.

With current design (Pool_tx+Split_tx) it's O(2) space. Pool_tx is similar
to a commitment tx and thus enables off-chain novation of pool distribution.

> Let's be favorable to Accumulators and assume O(1), but keep in mind
constant may
> be somewhat large/operations might be expensive in validation for updates.

Using a Merkle Tree as an accumulator should be constant-size in space, but
likely it has to be O(log(N) in computation (N set elements). This overhead
in computation should be accounted for in accumulator sigops to avoid
network validation resources free-riding, but I think it's a better
trade-off minimizing chain footprint.

> So in this context, CTV Pool has a clear benefit. The last recipient can
> always clear in Log(N) time whereas in the accumulator pool, the last
> recipient has to wait much much longer. There's no asymptotic difference
in
> Tx Size, but I suspect that CTV is at least as good or cheaper since it's
> just one tx hash and doesn't depend on implementation.

Yes I agree CTV pool performs better in the worst-case scenario. In my
opinon what we should really look on is the probability of withdrawal
scenarios. I see 2 failure cases:
* a pool participant being offline, thus halting the pool
* a pool participant with external protocol requirement to fulfill, like a
HTLC to timeout onchain

With regards to 1) we assume that watchtower infra are likely to become
ubiquitous in the future (if you want a secure LN experience), so user
uptime should be near to 100%. Of course,  it's a new architecture which
comes with trade-offs, but interesting to explore.

With regards to 2) as of today channel-failure-rate (like unilateral close)
it's still quite important (30% IIRC) so it plays in favor of OP_CTV pool
but in the future I expect single-digit
therefore making CoinPool far more competitive. Do we envision protocol
more time-sensitive than LN in the future (atomic swaps...) ? Hard to gauge.

Do you see other ways to refine model, like integrating out-of-pool
liquidity needs rate ?

Note, I think f

Re: [bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-12 Thread ZmnSCPxj via bitcoin-dev
Good morning Antoine and Gleb,

I have not studied the proposal in close detail yet, but anyway, my main 
takeaway roughly is:

* The core of CoinPool is some kind of multiparticipant (N > 2) offchain update 
mechanism (Decker-Wattenhofer or Decker-Russell-Osuntokun).
  * The output at each state of the update mechanism is some kind of splitting 
construction (which I have not studied in detail).
  * At each update of the state, all participants must sign off on the new 
state.

It seems to me that it would be possible to use a [WabiSabi 
protocol](https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-June/017969.html)
 during negotiation of a new state.

Now, WabiSabi is a client-server protocol.
As all participants in the CoinPool are needed in order to ratify each new 
state anyway, they can simply elect one of their number by drawing lots, to act 
as server for a particular state update.

Then the participants can operate as WabiSabi clients.
Each participant registers the outputs they currently own in the current state, 
getting credentials that sum up to the correct value.
Then, during the WabiSabi run, they can exchange credentials among the 
participants in order to perform value transfers inside the WabiSabi 
construction.
Then, at output registration, they register new outputs to put in the next 
state of the CoinPool.

In order to hide transfers from the elected WabiSabi server, participants can 
maintain two coins in every state, and move coins randomly across the two coins 
they own at each state update, in order to hide "real" transfers from the 
elected server.

Then, after output registration, the participants ratify the new state by 
signing off on the new state and revoking the previous state, using the update 
mechanism.

Of course, we should note that one desired feature for CoinPool in the original 
proposal is that a participant can exit, and the CoinPool would still remain 
valid, but only for the remaining participants.

This is arguably a mild privacy leak: every other participant now knows how 
much that particular participant took out from the CoinPool.
Indeed, from what I can understand, in order to properly set up the splitting 
transactions in the first place, at each state every participant needs to know 
how much each other participant actually owns in the CoinPool at that point in 
time.

To hide how much each participant owns in the CoinPool from other participants, 
we would have to make unilateral closes expose all the current outputs, without 
trying to identify *which* participant exited the CoinPool, and thus preventing 
anyone else from figuring out exactly how much each *other* participant 
actually owns in the CoinPool on exit.
That way, output addresses can be to fresh pseudonyms of the participant, 
removing all linkages of participant to amount they own, and each participant 
can maintain multiple outputs per state for their own purposes and to mildly 
obscure exactly how much they own in total.

If we drop that feature (of being able to exit a participant without closing 
the *entire* CoinPool), of course, we need to mildly disincentivize a 
participant closing unilaterally for trivial reasons.
We can do this by using `SIGHASH_ANYPREVOUT` to force whoever performs a 
unilateral close of the CoinPool to pay the onchain fees involved, so that it 
would have to be a good reason indeed to perform a unilateral close.


Regards,
ZmnSCPxj
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Re: [bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-11 Thread Jeremy via bitcoin-dev
Stellar work Antoine and Gleb! Really excited to see designs come out on
payment pools.

I've also been designing some payment pools (I have some not ready code I
can share with you guys off list), and I wanted to share what I learned
here in case it's useful.

In my design of payment pools, I don't think the following requirement: "A
CoinPool must satisfy the following *non-interactive any-order withdrawal*
property: at any point in time and any possible sequence of previous
CoinPool events, a participant should be able to move their funds from the
CoinPool to any address the participant wants without cooperation with
other CoinPool members." is desirable in O(1) space. I think it's much
better to set the requirement to O(log(n)), and this isn't just because of
wanting to use CTV, although it does help.

Let me describe a quick CTV based payment pool:

Build a payment pool for N users as N/2 channels between participants
created in a payment tree with a radix of R, where every node has a
multisig path for being used as a multi-party channel and the CTV branch
has a preset timeout. E.g., with radix 2:

  Channel(a,b,c,d,e,f,g,h)
 /   \
   Channel(a,b,c,d)
Channel(e,f,g,h)
/
\/ \
Channel(a,b)Channel(c,d)  Channel(e,f)
Channel(g,h)


All of these channels can be constructed and set up non-interatively using
CTV, and updated interactively. By default payments can happen with minimal
coordination of parties by standard lightning channel updates at the leaf
nodes, and channels can be rebalanced at higher layers with more
participation.


Now let's compare the first-person exit non cooperative scenario across
pools:

CTV-Pool:
Wait time: Log(N). At each branch, you must wait for a timeout, and you
have to go through log N to make sure there are no updated states. You can
trade off wait time/fees by picking different radixes.
TXN Size: Log(N) 1000 people with radix 4 --> 5 wait periods. 5*4 txn size.
Radix 20 --> 2 wait periods. 2*20 txn size.

Accumulator-Pool:
Wait Time: O(1)
TXN Size: Depending on accumulator: O(1), O(log N), O(N) bits. Let's be
favorable to Accumulators and assumer O(1), but keep in mind constant may
be somewhat large/operations might be expensive in validation for updates.


This *seems* like a clear win for Accumulators. But not so fast. Let's look
at the case where *everyone* exits non cooperatively from a payment pool.
What is the total work and time?

CTV Pool:
Wait time: Log(N)
Txn Size: O(N) (no worse than 2x factor overhead with radix 2, higher
radixes dramatically less overhead)

Accumulator Pool:
Wait time: O(N)
Txn Size: O(N) (bear in mind *maybe* O(N^2) or O(N log N) if we use an
sub-optimal accumulator, or validation work may be expensive depending on
the new primitive)


So in this context, CTV Pool has a clear benefit. The last recipient can
always clear in Log(N) time whereas in the accumulator pool, the last
recipient has to wait much much longer. There's no asymptotic difference in
Tx Size, but I suspect that CTV is at least as good or cheaper since it's
just one tx hash and doesn't depend on implementation.

Another property that is nice about the CTV pool style is the bisecting
property. Every time you have to do an uncooperative withdrawal, you split
the group into R groups. If your group is not cooperating because one
person is permanently offline, then Accumulator pools *guarantee* you need
to go through a full on-chain redemption. Not so with a CTV-style pool, as
if you have a single failure among [1,2,3,4,5,6,7,8,9,10] channels (let's
say channel 8 fails), then with a radix 4 setup your next steps are:
[1,2,3,4,5,6,7,8,9,10]
[1,2,3,4,5,6,7,X,9,10]
[1,2,3,4] [5,6,7,X] [9,10]
[1,2,3,4] 5 6 7 X [9,10]

So you only need to do Log(N) chain work to exit the bad actor, but then it
amortizes! A future failure (let's say of 5) only causes 5 to have to close
their channel, and does not affect anyone else.

With an accumulator based pool, if you re-pool after one failure, a second
failure causes another O(N) work. So then total work in that case is
O(N^2). You can improve the design by making the evict in any order option
such that you can *kick out* a member in any order, that helps solve some
of this nastiness (rather than them opting to leave). But I'm unclear how
to make this safe w.r.t. updated states. You could also allow, perhaps, any
number of operators to simultaneously leave in a tx. Also not sure how to
do that.



Availability:
With CTV Pools, you can make a payment if just your immediate conterparty
is online in your channel. Opportunistically, if people above you are
online, you can make channel updates higher up in the tree which have
better timeout properties. You can also create new channels, binding
yourself to different parties if t

[bitcoin-dev] CoinPool, exploring generic payment pools for Fun and Privacy

2020-06-11 Thread Antoine Riard via bitcoin-dev
Hi list,

We (Gleb Naumenko + I) think that a wide range of second-layer protocols
(LN, vaults, inheritance, etc) will be used by average Bitcoin users. We
are interested in finding and addressing the privacy issues coming from the
unique fingerprints these protocols bring.

More specifically, we are interested in answering the following questions:
1. How bad are privacy leaks from on-chain txn of second-layer protocols
and how much is leaked via protocol-specific metadatas (LN domain names,
watchtowers, ...) ?
2. How to establish a list of Bitcoin fingerprints and their severity to
inform protocol designers and clarify threat models ?
3. What kind of sophisticated heuristics spies may use in the future ?
4. How to mitigate privacy leaks ? Should each protocol adopt a common
toolbox (scriptless scripts, taproot, ...) in its own way or should we
design a confidential-layer to wrap around all of them ?
5. How to make the solution usable (cheaper, easier to integrate, safer)
for a daily basis ?

We suggest CoinPool: a generic payment pool [0] as a solution to those
problems. Although the design we propose is somewhat a scaling solution, we
won't focus on this aspect. This work is rather an exploration of *how a
pool construction could serve as a TLS for Bitcoin, enhancing both on-chain
and off-chain privacy*.

### Motivation: cross-protocols privacy

It has always been a challenge to make the on-chain UTXO graph more
private. We all know the issues with cleartext amounts, the linkability of
inputs/outputs, and other metadatas. Combining with p2p-level spying
(transaction-to-IP mapping) or some other patterns leading to real-world
identities enable serious spying.

Protocols on top of Bitcoin (LN, vaults[1], complicated spending conditions
based on Miniscript, DLC [2] are even more vulnerable to spying because:
- each of them brings new unique fingerprint/metadata [3]
- known spying techniques against second-layer are currently limited to
trivial heuristics, but we can't assume spies will always this
unsophisticated

There is already a wiki list [4] attempting to cover all issues like that,
although maintaining it would be challenging considering privacy is a
moving target.

Let's consider this example: Alice is a well-known LN merchant with a node
tied to a domain name. She always directs the output of channel closing to
her vault address. If she has another vault address on-chain with the same
unique unlocking script (like a CSV timelock with a specific delta) this
can be leveraged to cluster her transactions. And since one of her
addresses is tied to a domain name, all her funds can now be linked to a
real-world identity.

In theory, one may use CoinJoin-like solutions to mask cross-protocol
on-chain transfers. Unfortunately, robust designs like CoinSwap depend on
timelocking coins, extensive use of the on-chain space, and paying fees to
provide sufficient privacy, as we explain further. These properties imply
we can't expect users to be using strong CoinSwaps by default.

That's why instead of specialized high-latency, high-chain-use
CoinJoin-style protocols, we propose CoinPool: a low-latency, generic
off-chain protocol used to be wrapped around any other protocol. CoinPool
is based on shared UTXO ownership. It may reasonably improve on-chain
privacy while avoiding latency and locked liquidity issues. CoinPool may
also reduce the on-chain use (thus, help to scale Bitcoin) if participants
cooperate sufficiently.

We do believe that CoinSwap and other CoinJoins are of interest, but we
have to consider the trade-offs and choose the best tool for a job to make
privacy usable with regards to user resources. We will compare CoinPool to
CoinSwap in more detail later in this write-up.

### Extra-motivation: on-chain scalability

Even though it's not the main focus of this proposal, we also want to
mention that since CoinPool is a payment pool, it helps with on-chain
scalability. More specifically:
1. Shared UTXO ownership allows to represent many outputs as one, reducing
the UTXO set in size.
2. The CoinPool design enables off-chain transfers within the pool, helping
to save the block space by committing fewer transactions on-chain.
3. CoinPool provides decent support for batching activities from different
users, also helping to have fewer individual transactions on-chain.

Since the CoinPool provides scalability benefits, users will be even
incentivized to join CoinPools due to the conservative chain resources
usage and such enjoy privacy as a side-effect.

### CoinPool design

A CoinPool must satisfy the following *non-interactive any-order
withdrawal* property: at any point in time and any possible sequence of
previous CoinPool events, a participant should be able to move their funds
from the CoinPool to any address the participant wants without cooperation
with other CoinPool members.

The state of a CoinPool is represented by one on-chain UTXO (a funding
multisig of all pool participants) and a set of transa