Re: [Lightning-dev] Waiting SIGHASH_ANYPREVOUT and Packing Packages
Hi Michael,
> Browsing quickly through Greg's piece, a lot of the reasoning is based on
FOSS experience from Linux/Juniper, which to the best of my knowledge are
centralized software projects ?
> That is Greg's point. If Linux doesn't look further than the current
> version and the next version with a BDFL (Linus) a decentralized
> project like Bitcoin Core is going to struggle even more with longer
> term roadmaps.
I was far more inclined to recall the unsolved problems for Lightning/L2s
(pre-signed feerate/tx-pinnings) than calling out strong solutions to them.
I believe problem spaces are quite something stable in engineering/science,
at least until they're formalized differently. But even coming to consensus
on the existence of problems and a shared perception of the severity of
them can take a long time. In fact, it might even be the hardest step in a
decentralized ecosystem like Bitcoin.
And I fill in on the low-relevance of roadmaps, real development is a
continuous zigzag. If we look in the past and take the transaction
malleability issue, I think we can observe it took multiple proposals (bip
62, normalized txid, sighash_noinput, ...), of which we're even
implemented in Core, before to finally settle on segwit. Though I would say
lessons were drawn about shortcomings of every transient proposal.
> I think it is important to discuss what order changes should be
> attempted but I agree with David that putting specific future version
> numbers on changes is speculative at best and misleading at worst. The
> record of previous predictions of what will be included in particular
> future versions is not strong :)
I recognize it wasn't delicate to put exact version numbers, though note
multiple, alternative versions numbers were deliberately proposed for each
specific change and timelines given in terms of years, more as an invite
to open a discussion on such changes and where/when they could take place,
that in anyway a finite, consistent deployment proposal.
Further, I still believe it would be cool to have a bit more coordination
when Core implements sophisticated mechanisms designed for downstream
support, in the sense of feedback exchanged across projects all along their
release schedules. For e.g, with package-relay, as a Lightning team it's
likely you will have to rework your tx-broadcast module which might take a
few good weeks of review and test. Though, coming to this best practice
(imho) across the different Bitcoin layers might take years and that's
perfectly fine, we'll see what emerges :)
> What was making sense when you had like ~20 Bitcoin dev with 90% of the
technical knowledge doesn't scale when you have multiple second-layers
specifications
> It is great that we have a larger set of contributors in the ecosystem
> today than back in say pre 2017. But today that set of contributors is
> spread widely across a number of different projects that didn't exist
> pre 2017. Changes to Core are (generally) likely to be implemented and
> reviewed by current Core contributors as Lightning implementation
> developers (generally) seem to have their hands full with their own
> implementations.
Well I strongly believe that the Core review process is open to anyone :) ?
If some upper layers contributors are generously offering their time to
share back their experiences, especially during the design phase of
software features, I hope we might be on path to deliver better stuff.
Further, that's a more personal note, I'm worried long-term about
layer-monoculture cropping up in the ecosystem, a concern echoing the
history of Internet development [0].
> I think we can get the balance right by making progress on this
> (important) discussion whilst also maintaining humility that we don't
> know exact timelines and that getting things merged into Core relies
> on a number of people who have varying levels of interest and
> understanding of L2 protocols.
Yes, as answers to my post are showing, I might have lacked patience in
this case :/ Sometimes, it's hard to gauge your own cognitive dissonance on
topics.
Cheers,
Antoine
[0] See "Interactions between Layers" in "General Architectural and Policy
Considerations", RFC 3426
Le lun. 21 juin 2021 à 06:20, Michael Folkson a
écrit :
> I don't want to divert from the topic of this thread ("Waiting
> SIGHASH_ANYPREVOUT and Packing Packages"), we can set up a separate
> thread if we want to discuss this further. But just a couple of
> things.
>
> > Browsing quickly through Greg's piece, a lot of the reasoning is based
> on FOSS experience from Linux/Juniper, which to the best of my knowledge
> are centralized software projects ?
>
> That is Greg's point. If Linux doesn't look further than the current
> version and the next version with a BDFL (Linus) a decentralized
> project like Bitcoin Core is going to struggle even more with longer
> term roadmaps.
>
> I think it is important to discuss what order changes should be
> attempted but I agree
Re: [Lightning-dev] Waiting SIGHASH_ANYPREVOUT and Packing Packages
I don't want to divert from the topic of this thread ("Waiting
SIGHASH_ANYPREVOUT and Packing Packages"), we can set up a separate
thread if we want to discuss this further. But just a couple of
things.
> Browsing quickly through Greg's piece, a lot of the reasoning is based on
> FOSS experience from Linux/Juniper, which to the best of my knowledge are
> centralized software projects ?
That is Greg's point. If Linux doesn't look further than the current
version and the next version with a BDFL (Linus) a decentralized
project like Bitcoin Core is going to struggle even more with longer
term roadmaps.
I think it is important to discuss what order changes should be
attempted but I agree with David that putting specific future version
numbers on changes is speculative at best and misleading at worst. The
record of previous predictions of what will be included in particular
future versions is not strong :)
> What was making sense when you had like ~20 Bitcoin dev with 90% of the
> technical knowledge doesn't scale when you have multiple second-layers
> specifications
It is great that we have a larger set of contributors in the ecosystem
today than back in say pre 2017. But today that set of contributors is
spread widely across a number of different projects that didn't exist
pre 2017. Changes to Core are (generally) likely to be implemented and
reviewed by current Core contributors as Lightning implementation
developers (generally) seem to have their hands full with their own
implementations.
I think we can get the balance right by making progress on this
(important) discussion whilst also maintaining humility that we don't
know exact timelines and that getting things merged into Core relies
on a number of people who have varying levels of interest and
understanding of L2 protocols.
On Mon, Jun 21, 2021 at 9:13 AM Antoine Riard wrote:
>
> Hi Dave,
>
> > That might work for current LN-penalty, but I'm not sure it works for
> eltoo.
>
> Well, we have not settled yet on the eltoo design but if we take the later
> proposal in date [0], signing the update transaction with SIGHGASH_ANYPREVOUT
> lets you attach non-interactively a single-party controlled input at
> broadcast-time. Providing the input amount is high enough to bump the
> transaction feerate over network mempools, it should allow the tx to
> propagate across network mempools and that way solve the pre-signed feerate
> problem as defined in the post ?
>
> > If Bitcoin Core can rewrite the blind CPFP fee bump transaction
> > to refer to any prevout, that implies anyone else can do the same.
> > Miners who were aware of two or more states from an eltoo channel would
> > be incentivized to rewrite to the oldest state, giving them fee revenue
> > now and ensuring fee revenue in the future when a later state update is
> > broadcast.
>
> Yep, you can add a per-participant key to lockdown the transaction and avoid
> any in-flight malleability ? I think this is discussed in the "A Stroll
> through Fee-Bumping Techniques" thread.
>
> > If the attacker using pinning is able to reuse their attack at no cost,
> > they can re-pin the channel again and force the honest user to pay
> > another anyprevout bounty to miners.
>
> This is also true with package-relay where your counterparty, with a better
> knowledge of network mempools, can always re-broadcast a CPFP-bumped
> malicious package ? Under this assumption, I think you should always be ready
> to bump our honest package.
>
> Further, for the clarity of the discussion, can you point to which pinning
> scenario you're thinking of or if it's new under SIGHASH_ANYPREVOUT, describe
> it ?
>
> > Repeat this a bunch of times and the honest user has now spent more on fees
> > than their balance from the
> closed channel.
>
> And sadly, as this concern also exists in case of a miner-harvesting attack
> against LN nodes, a concern that Gleb and I expressed more than a year ago in
> a public post [1], a good L2 client should always upper bound its fee-bumping
> reserve. I've a short though-unclear note on this notion of fee-bumping upper
> to warn other L2 engineers in "On Mempool Funny Games against Multi-Party
> Funded Transactions"
>
> Please read so:
>
> "A L2 client, with only a view of its mempool at best, won't understand why
> the transaction doesn't confirm and if it's responsible for the
> fee-bumping, it might do multiple rounds of feerate increase through CPFP,
> in vain. As the fee-bumping algorithm is assumed to be known if the victim
> client is open source code, the attacker can predict when the fee-bumping
> logic reaches its upper bound."
>
> Though thanks for the recall! I should log dynamic-balances in RL's
> `ChannelMonitorUpdate` for our ongoing implementation of anchor, updating my
> TODO :p
>
> > Even if my analysis above is wrong, I would encourage you or Matt or
> someone to write up this anyprevout idea in more detail and distribute
> it before you promote
Re: [Lightning-dev] Waiting SIGHASH_ANYPREVOUT and Packing Packages
Hi Dave,
> That might work for current LN-penalty, but I'm not sure it works for
eltoo.
Well, we have not settled yet on the eltoo design but if we take the later
proposal in date [0], signing the update transaction with
SIGHGASH_ANYPREVOUT lets you attach non-interactively a single-party
controlled input at broadcast-time. Providing the input amount is high
enough to bump the transaction feerate over network mempools, it should
allow the tx to propagate across network mempools and that way solve the
pre-signed feerate problem as defined in the post ?
> If Bitcoin Core can rewrite the blind CPFP fee bump transaction
> to refer to any prevout, that implies anyone else can do the same.
> Miners who were aware of two or more states from an eltoo channel would
> be incentivized to rewrite to the oldest state, giving them fee revenue
> now and ensuring fee revenue in the future when a later state update is
> broadcast.
Yep, you can add a per-participant key to lockdown the transaction and
avoid any in-flight malleability ? I think this is discussed in the "A
Stroll through Fee-Bumping Techniques" thread.
> If the attacker using pinning is able to reuse their attack at no cost,
> they can re-pin the channel again and force the honest user to pay
> another anyprevout bounty to miners.
This is also true with package-relay where your counterparty, with a better
knowledge of network mempools, can always re-broadcast a CPFP-bumped
malicious package ? Under this assumption, I think you should always be
ready to bump our honest package.
Further, for the clarity of the discussion, can you point to which pinning
scenario you're thinking of or if it's new under SIGHASH_ANYPREVOUT,
describe it ?
> Repeat this a bunch of times and the honest user has now spent more on
fees than their balance from the
closed channel.
And sadly, as this concern also exists in case of a miner-harvesting attack
against LN nodes, a concern that Gleb and I expressed more than a year ago
in a public post [1], a good L2 client should always upper bound its
fee-bumping reserve. I've a short though-unclear note on this notion of
fee-bumping upper to warn other L2 engineers in "On Mempool Funny Games
against Multi-Party Funded Transactions"
Please read so:
"A L2 client, with only a view of its mempool at best, won't understand why
the transaction doesn't confirm and if it's responsible for the
fee-bumping, it might do multiple rounds of feerate increase through CPFP,
in vain. As the fee-bumping algorithm is assumed to be known if the victim
client is open source code, the attacker can predict when the fee-bumping
logic reaches its upper bound."
Though thanks for the recall! I should log dynamic-balances in RL's
`ChannelMonitorUpdate` for our ongoing implementation of anchor, updating
my TODO :p
> Even if my analysis above is wrong, I would encourage you or Matt or
someone to write up this anyprevout idea in more detail and distribute
it before you promote it much more.
That's a really fair point, as a lot of the reasoning was based on private
discussion with Matt. Though as SIGHASH_ANYPREVOUT isn't advocated for
community consensus and those things take time, should just take a few
hours of my time.
> Even if every protocol based on presigned transactions can magically
allow dynamically adding inputs and modifying outputs for fees, and we
also have a magic perfect transaction replacement protocol,
"“Any sufficiently advanced technology is indistinguishable from magic.”
Arthur C. Clarke
Wit apart, that might be the outcome with careful bitcoin protocol
development, where technical issues are laid out in a best effort (of
mine!) and spread to the Bitcoin community on the most public bitcoin
communication channel ?
And humbly, on all those L2 issues I did change my opinion, as I've written
so much explicitly in this thread post by pointing to an older post of mine
("Advances in Bitcoin Contracting : Uniform Policy and Package Relay").
This reversal, partially motivated by a lot of discussion with folks,
including yourself, initiated since at least mid last year.
> package relay is still fundamentally useful for CPFP fee bumping very low
> feerate transactions received from an external party. E.g. Alice pays
> Bob, mempool min feerates increase and Alice's transaction is dropped,
> Bob still wants the money, so he submits a package with Alice's
> transaction plus his own high feerate spend of it.
I think this point would be a reverse of our p2p design where we are now
making the sender responsible for the receiver quality of its mempool
feerate ? This question has never been clear up during the years-long
discussion of package-relay design [1].
Though referring to the thread post and last week's transaction-relay
workshop, I did point out that package-relay might serve in the long-term
as a mempool-sync mechanism to prevent potential malicious mempool
partitions [2].
> Package relay is a clear improvement now, and one I
Re: [Lightning-dev] Waiting SIGHASH_ANYPREVOUT and Packing Packages
On Fri, Jun 18, 2021 at 06:11:38PM -0400, Antoine Riard wrote: > 2) Solving the Pre-Signed Feerate problem : Package-Relay or > SIGHASH_ANYPREVOUT > > For Lightning, either package-relay or SIGHASH_ANYPREVOUT should be able to > solve the pre-signed feerate issue [3] > > [...] > > [3] I don't think there is a clear discussion on how SIGHASH_ANYPREVOUT > solves pinnings beyond those LN meetings logs: > https://gnusha.org/lightning-dev/2020-06-08.log For anyone else looking, the most relevant line seems to be: 13:50 < BlueMatt> (sidenote: sighash_no_input is *really* elegant here - assuming a lot of complicated logic in core to do so, you could imagine blind-cpfp-bumping *any* commitment tx without knowing its there or which one it is all with one tx...in theory) That might work for current LN-penalty, but I'm not sure it works for eltoo. If Bitcoin Core can rewrite the blind CPFP fee bump transaction to refer to any prevout, that implies anyone else can do the same. Miners who were aware of two or more states from an eltoo channel would be incentivized to rewrite to the oldest state, giving them fee revenue now and ensuring fee revenue in the future when a later state update is broadcast. If the attacker using pinning is able to reuse their attack at no cost, they can re-pin the channel again and force the honest user to pay another anyprevout bounty to miners. Repeat this a bunch of times and the honest user has now spent more on fees than their balance from the closed channel. Even if my analysis above is wrong, I would encourage you or Matt or someone to write up this anyprevout idea in more detail and distribute it before you promote it much more. > package-relay sounds a reasonable, temporary "patch". Even if every protocol based on presigned transactions can magically allow dynamically adding inputs and modifying outputs for fees, and we also have a magic perfect transaction replacement protocol, package relay is still fundamentally useful for CPFP fee bumping very low feerate transactions received from an external party. E.g. Alice pays Bob, mempool min feerates increase and Alice's transaction is dropped, Bob still wants the money, so he submits a package with Alice's transaction plus his own high feerate spend of it. Package relay is a clear improvement now, and one I expect to be permanent for as long as we're using anything like the current protocol. > # Deployment timeline > > So what I believe as a rough deployment timeline. I don't think it's appropriate to be creating timelines like this that depend on the work of a large number of contributors who I don't believe you've consulted. For details on this point of view, please see https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-July/014726.html Stuff will get done when it gets done. -Dave signature.asc Description: PGP signature ___ Lightning-dev mailing list Lightning-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
Re: [Lightning-dev] Waiting SIGHASH_ANYPREVOUT and Packing Packages
> That's a question I hope we'll gather feedback during next Thursday's transaction relay workshops. As someone kindly pointed out to me, workshop is happening Tuesday, June 22th. Not Thursday, mistake of mine :/ Le ven. 18 juin 2021 à 18:11, Antoine Riard a écrit : > Hi, > > It's a big chunk, so if you don't have time browse parts 1 and 2 and share > your 2 sats on the deployment timeline :p > > This post recalls some unsolved safety holes about Lightning, how > package-relay or SIGHASH_ANYPREVOUT can solve the first one, how a mempool > hardening can solve the second one, few considerations on package-relay > design trade-offs and propose a rough deployment timeline. > > 1) Lightning Safety Holes : Pre-Signed Feerate and Tx-Pinning (to skip if > you're a LN dev) > > As of today, Lightning is suffering from 2 safety holes w.r.t to > base-layer interactions, widely discussed among ln devs. > > The first one, the pre-signed feerate issue with future broadcasted > time-sensitive transactions is laid out clearly in Matt Corallo's "CPFP > Carve-Out Fee-Prediction Issues in Contracting Applications (eg Lightning)" > [0]. This issue might provoke loss of funds, even in non-adversarial > settings, i.e a Lightning routing hub not being able to settle backward > onchain a successful HTLC during occurrences of sudden mempool congestion. > > As blockspace demand increases with an always growing number of > onchain/offchain bitcoin users, coupling effects are more likely to happen > and this pre-signed feerate issue is going to become more urgent to solve > [1]. For e.g, few percentiles of increases in feerate being overpriced by > Lightning routing hubs to close "fractional-reserve" backed anchor > channels, driving mempools congestions, provoking anchor channels > fee-bumping reserves becoming even more under-provisioned and thus close > down, etc. > > The second issue, malicious transaction pinnings, is documented in Bastien > Teinturier's "Pinning Attacks" [2]. AFAIK, there is a rough consensus among > devs on the conceptual feasibility of such a class of attacks against a LN > node, though so far we have not seen them executed in the wild and I'm not > aware of anyone having realized them in real-world conditions. Note, there > is a variety of attack scenarios to consider which is function of a wide > matrix (channel types, LN implementation's `update_fee` policy, LN > implementation's `cltv_delta` policy, mempool congestion feerate groups, > routing hubs or end nodes) Demoing against deployed LN implementations with > default settings has been on my todo for a while, though a priori One > Scenario To Exploit Them All doesn't fit well. > > Side-note, as a LN operator, if you're worried about those security risks, > you can bump your `cltv_delta`/`cltv_expiry_delta` to significantly coarse > the attacks. > > I think there is an important point to underscore. Considering the state > of knowledge we have today, I believe there is no strong interdependency > between solving pre-signed feerate and tx-pinning with the same mechanism > from a safety/usability standpoint. Or last such mechanism can be deployed > by stages. > > 2) Solving the Pre-Signed Feerate problem : Package-Relay or > SIGHASH_ANYPREVOUT > > For Lightning, either package-relay or SIGHASH_ANYPREVOUT should be able > to solve the pre-signed feerate issue [3] > > One of the interesting points recalled during the first transaction relay > workshops was that L2s making unbounded security assumptions on > non-normative tx-relay/mempool acceptance rules sounds a wrong direction > for the Bitcoin ecosystem long-term, and more prone to subtle bugs/safety > risks across the ecosystem. > > I did express the contrary, public opinion a while back [4]. That said, I > start to agree it's wiser ecosystem-wise to keep those non-normatives rules > as only a groundwork for weaker assumptions than consensus ones. Though it > would be nice for long-term L2s stability to consider them with more care > than today in our base-layer protocol development process [4] > > On this rational, I now share the opinion it's better long-term to solve > the pre-signed feerate problem with a consensus change such as > SIGHASH_ANYPREVOUT rather than having too much off-chain coins relying on > the weaker assumptions offered by bitcoin core's tx-relay/mempool > acceptance rules, and far harder to replicate and disseminate across the > ecosystem. > > However, if SIGHASH_ANYPREVOUT is Things Done Right(tm), should we discard > package-relay ? > > Sadly, in the worst-case scenario we might never reach consensus again > across the ecosystem and Taproot is the last softfork. Ever :/ *sad violons > and tissues jingle* > > With this dilemma in mind, it might be wise for the LN/L2 ecosystems to > have a fall-back plan to solve their safety/usability issues and > package-relay sounds a reasonable, temporary "patch". > > Even if package-relay requires serious engineering effort in Bitcoin
