Re: [bitcoin-dev] bitcoin-dev Digest, Vol 96, Issue 58

From: ecurrencyhodler <ecurrencyhodler@gmail•com>
To: bitcoin-dev@lists•linuxfoundation.org
Subject: Re: [bitcoin-dev] bitcoin-dev Digest, Vol 96, Issue 58
Date: Thu, 25 May 2023 09:06:31 -0700	[thread overview]
Message-ID: <CAKFmrWM7DXEnsSg0FWbwFR8GUhky+xDE+SsOFeF=O7LsqnT0xQ@mail.gmail.com> (raw)
In-Reply-To: <mailman.17.1685016003.20381.bitcoin-dev@lists.linuxfoundation.org>

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Date: Wed, 24 May 2023
From: ecurrencyhodler<ecurrencyhodler@gmail•com>
To: Bitcoin Protocol Discussion
        <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Ark: An Alternative Privacy-preserving
        Second  Layer Solution (ecurrencyhodler)

Thanks David for your write up. Clear, concise, and super understandable as
always.

It sounds like to me that ARK is using PSBT's + covenants to secure
unconfirmed txns.

I see how this significantly reduces onchain footprint. But wouldn't this
also have a massive impact at scale to the mempool filling it with
unconfirmed txns? Couple concerns I have are:

   1. It would create an ever growing fee environment within the 10-minute
   block. Therefore it would be possible for the first settlement txn in the
   first 5 seconds to get pushed out of the mempool by later settlement txns
   closer to the 10 minute mark.
   2. Mempools would have to drastically increase their size to capture all
   the 5-second settlement unconfirmed txns and default bitcoin core nodes
   would get flooded.

Thanks again burak for your ARK proposal. There seems to be a good amount
of excitement about what this could unlock in the future which is quite
refreshing to see.

On Thu, May 25, 2023 at 5:01 AM <
bitcoin-dev-request@lists•linuxfoundation.org> wrote:

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> Today's Topics:
>
>    1. Re: Ark: An Alternative Privacy-preserving Second Layer
>       Solution (adiabat)
>    2. Re: Ark: An Alternative Privacy-preserving Second Layer
>       Solution (David A. Harding)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 24 May 2023 16:20:35 -0400
> From: adiabat <rx@awsomnet•org>
> To: Bitcoin Protocol Discussion
>         <bitcoin-dev@lists•linuxfoundation.org>
> Subject: Re: [bitcoin-dev] Ark: An Alternative Privacy-preserving
>         Second  Layer Solution
> Message-ID:
>         <CAKEeUhg1qeZOv-Lk8SSTxdkgfSee_E6_4fwNV=
> hfwsxLgwWkUw@mail•gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> Hi - thanks for the Ark write up; I have a bunch of questions but here's 2:
>
> ---
> Q1:
> "Pool transactions are created by ark service providers perpetually
> every 5 seconds"
>
> What exactly happens every 5 seconds?  From the 15.44.21-p-1080.png
> diagram [1], a pool transaction is a bitcoin transaction, with all the
> inputs coming from the ASP.  My understanding is that every 5 seconds,
> we progress from PoolTx(N) to PoolTx(N+1).  Does the ASP sign a new
> transaction which spends the same ASP funding inputs as the previous
> pool transaction, which is a double spend or fee bump?  Or does it
> spend the outputs from the previous PoolTx?
>
> In other words, does PoolTx(2) replace PoolTx(1) RBF-style, spending
> the same inputs (call this method A), or does PoolTx(2) spend an
> output Of Pooltx(1) such that PoolTx(1) must be confirmed in order for
> PoolTx(2) to become valid (method B)?  Or are they completely separate
> transactions with unconflicting inputs (method C)?
>
> When the ASP creates a pool transaction, what do they do with it?  Do
> they broadcast it to the gossip network?  Or share it with other pool
> participants?
>
> With method A, if the ASP shares pool transactions with other people,
> there Doesn't seem to be any way to ensure which PoolTx gets
> confirmed, invalidating all the other ones.  They're all valid so
> whichever gets into a block first wins.
>
> With method B, there seems to be a large on-chain load, with ~120
> chained transactions trying to get in every block. This wouldn't play
> nicely with mempool standardness and doesn't seem like you could ever
> "catch up".
>
> With method C, ASPs would need a pretty large number of inputs but
> could recycle them as blocks confirm.  It would cost a lot but maybe
> could work.
>
> ---
> Q2:
>
> The other part I'm missing is: what prevents the ASP from taking all
> the money?  Before even getting to vTXOs and connector outputs, from
> the diagram there are only ASP inputs funding the pool transaction.
> If the pool transaction is confirmed, the vTXOs are locked in place,
> since the vTXO output cannot be changed and commits to all
> "constrained outs" via OP_CTV.  If the pool transaction is
> unconfirmed, the ASP can create & sign a transaction spending all ASP
> funding inputs sending the money back to the ASP, or anywhere else.
> In this case, users don't have any assurance that their vTXO can ever
> turn into a real UTXO; the ASP can "rug-pull" at any time, taking all
> the money in the pool.  Adding other inputs not controlled by the ASP
> to the transaction wouldn't seem to fix the problem, because then any
> user removing their inputs would cancel the whole transaction.
>
> More detail about how these transactions work would be appreciated, thanks!
>
> -Tadge
>
> [1]
> https://uploads-ssl.webflow.com/645ae2e299ba34372614141d/6467d1f1bf91e0bf2c2eddef_Screen%20Shot%202023-05-19%20at%2015.44.21-p-1080.png
>
>
> ------------------------------
>
> Message: 2
> Date: Wed, 24 May 2023 13:02:40 -1000
> From: "David A. Harding" <dave@dtrt•org>
> To: Burak Keceli <burak@buraks•blog>, Bitcoin Protocol Discussion
>         <bitcoin-dev@lists•linuxfoundation.org>
> Subject: Re: [bitcoin-dev] Ark: An Alternative Privacy-preserving
>         Second Layer Solution
> Message-ID: <3c6c3b8b562bb56bbb855dc2b2b71f78@dtrt•org>
> Content-Type: text/plain; charset=US-ASCII; format=flowed
>
> Hi Burak,
>
> Thanks for this really interesting protocol!  I tend to analyze
> complicated ideas like this by writing about them in my own words, so
> I've pasted my summary of your idea to the end of this email in case
> it's useful, either to other people or to you in helping understand my
> one concern.
>
> My concern is the same one I think Olaoluwa Osuntokun mentioned on
> Twitter[1] and (less clear to me) might be related to ZmnSCPxj's
> concern[2]:
>
> It seems to me that receiving a payment on the protocol, including
> conditional payments using HTLC, PTLC, or Anchor-TLC, requires waiting
> for the transaction containing that payment to confirm to a sufficient
> depth (e.g., I'd wait 6 blocks for small payments and longer for huge
> payments).  Am I missing something?
>
> My summary of how I think that part of the protocol works is in the
> sections labeled "Make an unconditioned payment" and "Make a conditional
> payment" below.  In short, it's clear to me how the service provider and
> the customer can make instant atomic swaps with each other---they can
> either spend instantly cooperatively, or they have to wait for a
> timeout.  But how can a receiver of funds be assured that they will
> actually get those funds unless there's already a timelock and
> cooperative spend path placed on those funds?
>
> -Dave
>
> Rough initial summary of Ark protocol:
>
> Alice runs an Ark service provider.  Every 5 seconds, she broadcasts a
> new unconfirmed onchain transaction that pays three outputs (the
> three Cs):
>
> 1. *Change Output:* money not used for the other two Cs that gets sent
>     back to the the transaction creator.
>
> 2. *Connector Output:* an output that will be used in a future
>     transaction created by Alice as protection against double spends.
>
> 3. *Commitment Output:* a CTV-style commitment to a set of outputs that
>     can be published later in a descendant transaction (alternatively,
>     the commitment output may be spent unilaterally by Alice after 4
>     weeks).
>
> Bob wants to deposit 1 BTC with Alice.  He sends her an unsigned PSBT
> with an input of his and a change output.  She updates the PSBT with a
> commitment output that refunds Bob the 1 BTC and a connector output with
> some minimum value.  They both sign the PBST and it is broadcast.  We'll
> ignore fees in our examples, both onchain transaction fees and fees paid
> to Alice.
>
>  From here, there are several things that Bob can do:
>
> - *Unilaterally withdraw:* Bob can spend from the commitment output to
>    put his refund onchain.  The refund can only be spent after a 24-hour
>    time delay, allowing Bob to optionally come to an agreement with Alice
>    about how to spend the funds before Bob can spend them unilaterally
>    (as we'll see in a moment).  For example, the script might be[3]:
>
>      pk(B) && (older(1 day) || pk(A))
>
> - *Collaboratively withdraw:* as seen above, Bob has the ability to come
>    to a trustless agreement with Alice about how to spend his funds.
>    They can use that ability to allow Bob to trade his (unpublished) UTXO
>    for a UTXO that Alice funds and broadcasts.  For example:
>
>      - Alice creates an unsigned PSBT that uses as one of its inputs the
>        connector from Bob's deposit transaction.  This will ensure that
>        any attempt by Bob to double-spend his deposit transaction will
>        invalidate this withdrawal transaction, preventing Bob from being
>        able to steal any of Alice's funds.
>
>          Also included in Alice's unsigned PSBT is another connector
>          output plus the output that pays Bob his 1 BTC.
>
>      - Bob receives Alice's unsigned PSBT and creates a separate PSBT
>        that includes his unpublished UTXO as an input, giving its value
>        to Alice in an output.  The PSBT also includes as an input the
>        connector output from Alice's PSBT.  This will ensure that any
>        attempt by Alice to double spend her transaction paying him will
>        invalidate his transaction paying her.
>
>      - Bob signs his PSBT and gives it to Alice.  After verifying it,
>        Alice signs her PSBT and broadcasts it.
>
> - *Collaboratively trade commitments:* as mentioned, the commitment
>    output that pays Bob may be claimed instead by Alice after 4 weeks, so
>    Bob will need to either withdraw or obtain a new commitment within
> that
>    time.  To trade his existing commitment for a new commitment looks
>    similar to the collaborative withdrawal procedure but without the
>    creation of an immediately-spendable onchain output:
>
>      - Alice creates an unsigned PSBT that uses as one of its inputs the
>        connector from Bob's deposit transaction, again preventing double
>        spending by Bob.  Alice also includes a new connector and a new
>        commitment that again allows Bob to later claim 1 BTC.
>
>      - Bob receives Alice's PSBT and creates a PSBT transferring his
>        existing commitment to her, with the new connector again being
>        included as an input to ensure atomicity.
>
>      - Bob signs; Alice signs and broadcasts.
>
> - *Make an unconditioned payment:* using the mechanisms described above,
>    it's possible to make either an onchain payment or an offchain
>    payment---just have Carol receive the new output or commitment rather
>    than Bob.  That payment would have no conditions (except its
>    atomicity).
>
> - *Make a conditional payment:* imagine that Carol knows a secret (e.g.
>    a preimage) that Bob is willing to pay for.
>
>       - Alice creates an unsigned PSBT depending on the connector from
>         Bob's deposit transaction and creating a new connector.  The PSBT
>         includes an output paying Carol (either onchain or via a
>         commitment) with an HTLC, allowing Carol to claim the funds if
> she
>         reveals the secret or allowing Bob to claim the funds after a
>         timeout.
>
>       - Bob receives Alice's PSBT and creates a PSBT transferring his
>         existing commitment to her with the HTLC condition attached and,
>         again, with connectors being used to ensure atomicity.
>
>       - Bob signs; Alice signs and broadcasts.
>
>       - Carol can settle her HTLC by either revealing the secret onchain
>         or by trading her commitment containing the HTLC clause for a
>         commitment from Alice that doesn't contain the clause (which
>         Alice will only accept by learning the secret, since Alice has
>         to settle with Bob).  Alice can then either settle onchain or
>         trade commitments with Bob after giving him the secret.
>
> - *Do nothing for 4 weeks:* if Bob does nothing for four weeks, Alice
>    can claim the funds from the commitment output (i.e., takes his
>    money).
>
>      If Bob did actually do something, and if every other user who also
>      had an unpublished output in the commitment transaction did
>      something, then they all exchanged their portion of the funds in
>      this output to Alice, so Alice can now claim all of those funds
>      onchain in a highly efficient manner.
>
> Regarding the connector outputs, although all of the examples above show
> Alice directly spending from the connector output in Bob's deposit
> transaction, atomicity is also ensured if Alice spends from any output
> descended from Bob's connector output.  Connector outputs from different
> deposits can be used as inputs into the same transaction, merging their
> histories.  This allows all operations made by Alice to be fully atomic,
> ensuring that she doesn't lose any money during a reorg of any length.
>
> Users are not so well protected during reorgs, e.g. if Bob double-spends
> a transaction whose funds were later used in a payment to Carol, then
> Carol loses the money.  For this reason, Alice will probably want to
> prove to users that no funds they receive in a payment derive from any
> deposit less than safe_confirmation_depth blocks.
>
> [1] https://twitter.com/roasbeef/status/1661266771784126464
>
> [2]
>
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-May/021710.html
>
> [3]
>
> https://min.sc/#c=pk%28B%29%20%26%26%20%28older%281%20day%29%20%7C%7C%20pk%28A%29%29
>
>
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> End of bitcoin-dev Digest, Vol 96, Issue 58
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