James O'Beirne [ARCHIVE] / is on Nostr

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🗓️ Active from 2015-10-07 to 2023-03-06
💬 Posted a total of 34 messages
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2023-06-07 23:19:58

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📅 Original date posted:2023-03-06
🗒️ Summary of this message: Greg and AJ propose TLUV-ifying the OP_VAULT proposal to avoid recursive script execution and simplify the spec. This change would make OP_VAULT more native to Taproot primitives. The proposal also introduces new opcodes to facilitate consistent witness structure. The change does not affect usage or expressiveness but may allow an optional withdrawal authorization script in the future. Additionally, the proposal suggests reusing OP_CHECKTEMPLATEVERIFY for withdrawal target spends.
📝 Original message:I'm glad to see that Greg and AJ are forming a habit of hammering
this proposal into shape. Nice work fellas.
To summarize:
What Greg is proposing above is to in essence TLUV-ify this proposal.
I.e. instead of relying on hashed commitments and recursive script
execution (e.g. <trigger-sPK-hash> + later presentation of preimage
script for execution), OP_VAULT would instead move through its
withdrawal process by swapping out tapleaf contents according to
specialized rules. If this is opaque (as it was to me), don't fret -
I'll describe it below in the "mechanics" section.
The benefits of this TLUVification are
- we can avoid any nested/recursive script execution. I know the
recursive stuff rankles some greybeards even in spite of it being
bounded to a single call. I'm not sure I share the concern but
maintaining the status quo seems good.
- the spec is easier to reason about, more or less. The opcodes
introduced don't have variadic witness requirements, and each opcode
is only consumed in a single way.
- there's less general indirection. Instead of saying "okay, here's the
hash of the script I'm going to use to authorize trigger
transactions," we're just outright including the trigger auth script
in the tapleaf at the birth of the vault as regular 'ol script that is
evaluated before execution of the OP_VAULT instruction.
Similarly, instead of relying on an implicit rule that an OP_VAULT can
be claimed by a recovery flow, we're relying on a specific tapleaf that
facilitates that recovery with OP_VAULT_RECOVER, described below.
Basically, OP_VAULT would just be implemented in a way that feels
more native to Taproot primitives.
Greg also introduces different opcodes to facilitate consistent
witness structure, rather than the variable ones we have now
since OP_VAULT and OP_UNVAULT can each be spent in two different
contexts. I've changed those a little here; instead of the three general
ones Greg gave, we whittled it down to two: OP_VAULT and
OP_VAULT_RECOVER.
So I think that, barring significant implementation complexity - which
I'll find out about soon and don't expect - this is a good change to the
proposal. As Greg noted, it doesn't really change anything about the
usage or expressiveness... other than the fact that, as a bonus, it
might allow an optional withdrawal authorization script (i.e. trigger
output => final target), which could be useful if e.g. some kind of
size-limiting opcode (e.g. OP_TX_MAXSIZE or something) came around in
the future as a kind of pinning fix.
If that last bit lost you, don't worry - that is speculative, but the
point is that this rework composes well with other stuff.
# CTV use
Another thing that has dawned on us is that we might as well just reuse
OP_CHECKTEMPLATEVERIFY for withdrawal target spends. Ben Carmen and
others realized early on that you can synthesize CTV-like behavior by
spending to a 0-delay OP_UNVAULT output, so something CTVish has always
implicitly been a part of the proposal. But CTV is better studied and
basically as simple as the OP_UNVAULT spend semantics, so the thought is
that we might as well reuse all the existing work (and scrutiny) from
CTV.
As a concrete example, an issue with the existing proposal is that the
existing CTVish OP_UNVAULT behavior has txid malleability, since it
doesn't commit to nVersion or nLockTime or the input sequences. Using
CTV solves this issue. Otherwise we'd basically reinvent it - "something
something convergent evolution."
I think this is a satisfying development, because there's clearly demand
for CTV use in other contexts (DLC efficiency, e.g.), and if it's
required behavior for practical vaults, I think pulling in the existing
BIP-119 that's been worked over for years reduces the conceptual
surface area added by OP_VAULT.
# New mechanics of the proposal
So here I'm going to describe my rendering of Greg and AJ's suggestions.
## Required opcodes
- OP_VAULT: spent to trigger withdrawal
- OP_VAULT_RECOVER: spent to recover
- OP_CHECKTEMPLATEVERIFY: spent into final withdrawal target
Creating an initial deposit
---------------------------
For each vault, vaulted coins are spent to an output with the taproot
structure
taproot(internal_key, {$recovery_leaf, $trigger_leaf, ...})
where
internal_key =
unchanged from original proposal (some very safe recovery key)
$recovery_leaf =
[<opt.> <recovery> <auth>] <recovery sPK hash> OP_VAULT_RECOVER
$trigger_leaf =
<trigger> <auth> <script> <spend-delay> OP_VAULT
... =
other (optional) leaves in the taptree
Triggering a withdrawal request
-------------------------------
To trigger the start of the withdrawal process, an output of the above
form is spent with a witness that contains
- Taproot control block pointing to $trigger_leaf.
- <trigger-vout-idx>, indicating the trigger output which must abide
by the rules given below.
## Output structure
taproot(internal_key, {$recovery_leaf, $expr_withdraw, ...})
where
$recovery_leaf is preserved exactly
$expr_withdraw =
<spend-delay> OP_CSV OP_DROP <target-ctv-hash> OP_CTV
... is preserved exactly
(Spoiler: note here that the only thing that is changing is
s/expr_trigger/expr_withdrawl/ from the initial vault ouput.)
Of course $expr_withdraw *could* be prefixed by an optional "withdrawal
authorization" script, if some sensible use for that is found.
The validation rules are essentially unchanged from the existing
proposal:
- The total amount of all OP_VAULT inputs with matching $recovery_leaf
values must be reflected in output <trigger-vout-idx>
- <trigger-vout-idx> must correspond to an output that is identical to
the input taptree but with the spent tapleaf (OP_VAULT) swapped out
for the timelocked CTV constructed using <target-ctv-hash> and
<spend-delay> as extracted from the spent tapleaf
- internal_key is preserved
- the whole rest of the taptree is preserved
- (this is what ensures the parameters of the vault are forwarded)
All batching, fee management characteristics are the same.
Finalizing withdrawal
---------------------
Happens via script-path spend to $expr_withdraw, i.e. a timelocked
OP_CTV.
Recovery
--------
Can happen from any of the above outputs using the $recovery_leaf
script path in a way very similar to the existing OP_VAULT proposal.
---
To reiterate, all aspects of the existing OP_VAULT proposal are either
preserved or improved upon in terms of malleability reduction,
composability, and flexibility. So big thanks to AJ and Greg.
I'll undertake implementing these changes in the coming days to verify
that they are, as expected, workable.
James
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2023-06-07 23:15:33

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📅 Original date posted:2022-10-19
📝 Original message:I'm also very happy to see this proposal, since it gets us closer to having
a mechanism that allows the contribution to feerate in an "unauthenticated"
way, which seems to be a very helpful feature for vaults and other
contracting protocols.
One possible advantage of the sponsors interface -- and I'm curious for
your input here Greg -- is that with sponsors, assuming we relaxed the "one
sponsor per sponsoree" constraint, multiple uncoordinated parties can
collaboratively bump a tx's feerate. A simple example would be a batch
withdrawal from an exchange could be created with a low feerate, and then
multiple users with a vested interest of expedited confirmation could all
"chip in" to raise the feerate with multiple sponsor transactions.
Having a single ephemeral output seems to create a situation where a single
UTXO has to shoulder the burden of CPFPing a package. Is there some way we
could (possibly later) amend the ephemeral anchor interface to allow for
this kind of collaborative sponsoring? Could you maybe see "chained"
ephemeral anchors that would allow this?
On Tue, Oct 18, 2022 at 12:52 PM Jeremy Rubin via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:
> Excellent proposal and I agree it does capture much of the spirit of
> sponsors w.r.t. how they might be used for V3 protocols.
>
> The only drawbacks I see is they don't work for lower tx version
> contracts, so there's still something to be desired there, and that the
> requirement to sweep the output must be incentive compatible for the miner,
> or else they won't enforce it (pass the buck onto the future bitcoiners).
> The Ephemeral UTXO concept can be a consensus rule (see
> https://rubin.io/public/pdfs/multi-txn-contracts.pdf "Intermediate UTXO")
> we add later on in lieu of managing them by incentive, so maybe it's a
> cleanup one can punt.
>
> One question I have is if V3 is designed for lightning, and this is
> designed for lightning, is there any sense in requiring these outputs for
> v3? That might help with e.g. anonymity set, as well as potentially keep
> the v3 surface smaller.
>
> On Tue, Oct 18, 2022 at 11:51 AM Greg Sanders via bitcoin-dev <
> bitcoin-dev at lists.linuxfoundation.org> wrote:
>
>> > does that effectively mark output B as unspendable once the child gets
>> confirmed?
>>
>> Not at all. It's a normal spend like before, since the parent has been
>> confirmed. It's completely unrestricted, not being bound to any
>> V3/ephemeral anchor restrictions on size, version, etc.
>>
>> On Tue, Oct 18, 2022 at 11:47 AM Arik Sosman via bitcoin-dev <
>> bitcoin-dev at lists.linuxfoundation.org> wrote:
>>
>>> Hi Greg,
>>>
>>> Thank you very much for sharing your proposal!
>>>
>>> I think there's one thing about the second part of your proposal that
>>> I'm missing. Specifically, assuming the scenario of a v3 transaction with
>>> three outputs, A, B, and the ephemeral anchor OP_TRUE. If a child
>>> transaction spends A and OP_TRUE, does that effectively mark output B as
>>> unspendable once the child gets confirmed? If so, isn't the implication
>>> therefore that to safely spend a transaction with an ephemeral anchor, all
>>> outputs must be spent? Thanks!
>>>
>>> Best,
>>> Arik
>>>
>>> On Tue, Oct 18, 2022, at 6:52 AM, Greg Sanders via bitcoin-dev wrote:
>>>
>>> Hello Everyone,
>>>
>>> Following up on the "V3 Transaction" discussion here
>>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-September/020937.html
>>> , I would like to elaborate a bit further on some potential follow-on work
>>> that would make pinning severely constrained in many setups].
>>>
>>> V3 transactions may solve bip125 rule#3 and rule#5 pinning attacks under
>>> some constraints[0]. This means that when a replacement is to be made and
>>> propagated, it costs the expected amount of fees to do so. This is a great
>>> start. What's left in this subset of pinning is *package limit* pinning. In
>>> other words, a fee-paying transaction cannot enter the mempool due to the
>>> existing mempool package it is being added to already being too large in
>>> count or vsize.
>>>
>>> Zooming into the V3 simplified scenario for sake of discussion, though
>>> this problem exists in general today:
>>>
>>> V3 transactions restrict the package limit of a V3 package to one parent
>>> and one child. If the parent transaction includes two outputs which can be
>>> immediately spent by separate parties, this allows one party to disallow a
>>> spend from the other. In Gloria's proposal for ln-penalty, this is worked
>>> around by reducing the number of anchors per commitment transaction to 1,
>>> and each version of the commitment transaction has a unique party's key on
>>> it. The honest participant can spend their version with their anchor and
>>> package RBF the other commitment transaction safely.
>>>
>>> What if there's only one version of the commitment transaction, such as
>>> in other protocols like duplex payment channels, eltoo? What about multi
>>> party payments?
>>>
>>> In the package RBF proposal, if the parent transaction is identical to
>>> an existing transaction in the mempool, the parent will be detected and
>>> removed from the package proposal. You are then left with a single V3 child
>>> transaction, which is then proposed for entry into the mempool. In the case
>>> of another parent output already being spent, this is simply rejected,
>>> regardless of feerate of the new child.
>>>
>>> I have two proposed solutions, of which I strongly prefer the latter:
>>>
>>> 1) Expand a carveout for "sibling eviction", where if the new child is
>>> paying "enough" to bump spends from the same parent, it knocks its sibling
>>> out of the mempool and takes the one child slot. This would solve it, but
>>> is a new eviction paradigm that would need to be carefully worked through.
>>>
>>> 2) Ephemeral Anchors (my real policy-only proposal)
>>>
>>> Ephemeral Anchors is a term which means an output is watermarked as an
>>> output that MUST be spent in a V3 package. We mark this anchor by being the
>>> bare script `OP_TRUE` and of course make these outputs standard to relay
>>> and spend with empty witness data.
>>>
>>> Also as a simplifying assumption, we require the parent transaction with
>>> such an output to be 0-fee. This makes mempool reasoning simpler in case
>>> the child-spend is somehow evicted, guaranteeing the parent will be as well.
>>>
>>> Implications:
>>>
>>> a) If the ephemeral anchor MUST be spent, we can allow *any* value, even
>>> dust, even 0, without worrying about bloating the utxo set. We relax this
>>> policy for maximum smart contract flexibility and specification simplicity..
>>>
>>> b) Since this anchor MUST be spent, any spending of other outputs in the
>>> same parent transaction MUST directly double-spend prior spends of the
>>> ephemeral anchor. This causes the 1 block CSV timelock on outputs to be
>>> removed in these situations. This greatly magnifies composability of smart
>>> contracts, as now we can do things like safely splice directly into new
>>> channels, into statechains, your custodial wallet account, your cold
>>> wallet, wherever, without requiring other wallets to support arbitrary
>>> scripts. Also it hurts that 1 CSV time locked scripts may not be miniscript
>>> compatible to begin with...
>>>
>>> c) *Anyone* can bump the transaction, without any transaction key
>>> material. This is essentially achieving Jeremy's Transaction Sponsors (
>>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-September/018168.html)
>>> proposal without consensus changes. As long as someone gets a fully signed
>>> parent, they can execute a bump with minimal wallet tooling. If a
>>> transaction author doesn’t want a “sponsor”, do not include the output.
>>>
>>> d) Lightning Carve-out(
>>> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-October/002240.html)
>>> is superseded by this logic, as we are not restricted to two immediately
>>> spendable output scenarios. In its place, robust multi-party fee bumping is
>>> possible.
>>>
>>> e) This also benefits more traditional wallet scenarios, as change
>>> outputs can no longer be pinned, and RBF/CPFP becomes robust. Payees in
>>> simple spends cannot pin you. Batched payouts become a lot less painful.
>>> This was one of the motivating use cases that created the term “pinning” in
>>> the first place(
>>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-February/015717.html),
>>> even if LN/L2 discussion has largely overtaken it due to HTLC theft risks.
>>>
>>> Open Question(s):
>>>
>>>
>>> 1.
>>>
>>> If we allow non-zero value in ephemeral outputs, does this open up a
>>> MEV we are worried about? Wallets should toss all the value directly to
>>> fees, and add their own additional fees on top, otherwise miners have
>>> incentive to make the smallest utxo burn transaction to claim those funds.
>>> They just confirmed your parent transaction anyways, so do we care?
>>> 2.
>>>
>>> SIGHASH_GROUP like constructs would allow uncommitted ephemeral
>>> anchors to be added at spend time, depending on spending requirements.
>>> SIGHASH_SINGLE already allows this.
>>>
>>>
>>>
>>>
>>> Hopefully this gives people something to consider as we move forward in
>>> thinking about mempool design within the constraints we have today.
>>>
>>>
>>> Greg
>>>
>>> 0: With V3 transactions where you have "veto power" over all the inputs
>>> in that transaction. Therefore something like ANYONECANPAY is still broken.
>>> We need a more complex solution, which I’m punting for the sake of progress.
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev at lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev at lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
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2023-06-07 23:04:13

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📅 Original date posted:2022-02-10
📝 Original message:> It's not that simple. As a miner, if i have less than 1vMB of
transactions in my mempool. I don't want a 10sats/vb transaction paying
100000sats by a 100sats/vb transaction paying only 10000sats.
I don't understand why the "<1vMB in the mempool" case is even worth
consideration because the miner will just include the entire mempool in the
next block regardless of feerate.
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2023-06-07 23:04:11

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📅 Original date posted:2022-02-17
📝 Original message:> Is it really true that miners do/should care about that?
De facto, any miner running an unmodified version of bitcoind doesn't
care about anything aside from ancestor fee rate, given that the
BlockAssembler as-written orders transactions for inclusion by
descending ancestor fee-rate and then greedily adds them to the block
template. [0]
If anyone has any indication that there are miners running forks of
bitcoind that change this behavior, I'd be curious to know it.
Along the lines of what AJ wrote, optimal transaction selection is
NP-hard (knapsack problem). Any time that a miner spends deciding how
to assemble the next block is time not spent grinding on the nonce, and
so I'm skeptical that miners in practice are currently doing anything
that isn't fast and simple like the default implementation: sorting
fee-rate in descending order and then greedily packing.
But it would be interesting to hear evidence to the contrary.
---
You can make the argument that transaction selection is just a function
of mempool contents, and so mempool maintenance criteria might be the
thing to look at. Mempool acceptance is gated based on a minimum
feerate[1]. Mempool eviction (when running low on space) happens on
the basis of max(self_feerate, descendant_feerate) [2]. So even in the
mempool we're still talking in terms of fee rates, not absolute fees.
That presents us with the "is/ought" problem: just because the mempool
*is* currently gating only on fee rate doesn't mean that's optimal. But
if the whole point of the mempool is to hold transactions that will be
mined, and if there's good reason that txns are chosen for mining based
on fee rate (it's quick and good enough), then it seems like fee rate
is the approximation that should ultimately prevail for txn
replacement.
[0]:
https://github.com/bitcoin/bitcoin/blob/master/src/node/miner.cpp#L310-L320
[1]:
https://github.com/bitcoin/bitcoin/blob/master/src/txmempool.cpp#L1106
[2]:
https://github.com/bitcoin/bitcoin/blob/master/src/txmempool.cpp#L1138-L1144
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