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From: Billy Tetrud <billy.tetrud@gmail.com>
Date: Wed, 16 Jun 2021 17:58:22 -0700
Message-ID: <CAGpPWDbpg=sLN9rNze+mYaWP=0_Q64neh-Hj1V-=vU6NdEr__Q@mail.gmail.com>
To: Antoine Riard <antoine.riard@gmail.com>, 
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Proposal: Full-RBF in Bitcoin Core 24.0
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Russel O'Connor recently opined
<https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-June/019061.html>
that RBF should be standard treatment of all transactions, rather than as a
transaction opt-in/out. I agree with that. Any configuration in a
transaction that has not been committed into a block yet simply can't be
relied upon. Miners also have a clear incentive to ignore RBF rules and
mine anything that passes consensus. At best opting out of RBF is a weak
defense, and at worst it's simply a false sense of security that is likely
to actively lead to theft events.

Do we as a community want to support 0-conf payments in any way at this
point? It seems rather silly to make software design decisions to
accommodate 0-conf payments when there are better mechanisms for fast
payments (ie lightning).

One question I have is: how does software generally inform the user about
0-conf payment detection? Does software generally tell the user something
along the lines of "This payment has not been finalized yet. All recipients
should wait until the transaction has at least 1 confirmation, and most
recipients should wait for 6 confirmations" ? I think unless we pressure
software to be very explicit about what counts as finality, users will
simply continue to do what they've always done. Rolling out this policy
change over the course of a year or two seems fine, no need to rush. But I
suppose it would depend on how often 0-conf is used in the bitcoin
ecosystem at this point, which I don't have any data on.

On Tue, Jun 15, 2021 at 10:00 AM Antoine Riard via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi,
>
> I'm writing to propose deprecation of opt-in RBF in favor of full-RBF as
> the Bitcoin Core's default replacement policy in version 24.0. As a
> reminder, the next release is 22.0, aimed for August 1st, assuming
> agreement is reached, this policy change would enter into deployment phase
> a year from now.
>
> Even if this replacement policy has been deemed as highly controversial a
> few years ago, ongoing and anticipated changes in the Bitcoin ecosystem are
> motivating this proposal.
>
> # RBF opt-out as a DoS Vector against Multi-Party Funded Transactions
>
> As explained in "On Mempool Funny Games against Multi-Party Funded
> Transactions'', 2nd issue [0], an attacker can easily DoS a multi-party
> funded transactions by propagating an RBF opt-out double-spend of its
> contributed input before the honest transaction is broadcasted by the
> protocol orchester. DoSes are qualified in the sense of either an attacker
> wasting timevalue of victim's inputs or forcing exhaustion of the
> fee-bumping  reserve.
>
> This affects a series of Bitcoin protocols such as Coinjoin, onchain DLCs
> and dual-funded LN channels. As those protocols are still in the early
> phase of deployment, it doesn't seem to have been executed in the wild for
> now.  That said, considering that dual-funded are more efficient from a
> liquidity standpoint, we can expect them to be widely relied on, once
> Lightning enters in a more mature phase. At that point, it should become
> economically rational for liquidity service providers to launch those DoS
> attacks against their competitors to hijack user traffic.
>
> Beyond that, presence of those DoSes will complicate the design and
> deployment of multi-party Bitcoin protocols such as payment
> pools/multi-party channels. Note, Lightning Pool isn't affected as there is
> a preliminary stage where batch participants are locked-in their funds
> within an account witnessScript shared with the orchestrer.
>
> Of course, even assuming full-rbf, propagation of the multi-party funded
> transactions can still be interfered with by an attacker, simply
> broadcasting a double-spend with a feerate equivalent to the honest
> transaction. However, it tightens the attack scenario to a scorched earth
> approach, where the attacker has to commit equivalent fee-bumping reserve
> to maintain the pinning and might lose the "competing" fees to miners.
>
> # RBF opt-out as a Mempools Partitions Vector
>
> A longer-term issue is the risk of mempools malicious partitions, where an
> attacker exploits network topology or divergence in mempools policies to
> partition network mempools in different subsets. From then a wide range of
> attacks can be envisioned such as package pinning [1], artificial
> congestion to provoke LN channels closure or manipulation of
> fee-estimator's feerate (the Core's one wouldn't be affected as it relies
> on block confirmation, though other fee estimators designs deployed across
> the ecosystem are likely going to be affected).
>
> Traditionally, mempools partitions have been gauged as a spontaneous
> outcome of a distributed systems like Bitcoin p2p network and I'm not aware
> it has been studied in-depth for adversarial purposes. Though, deployment
> of second-layer
> protocols, heavily relying on sanity of a local mempool for fee-estimation
> and robust propagation of their time-sensitive transactions might lead to
> reconsider this position. Acknowledging this, RBF opt-out is a low-cost
> partitioning tool, of which the existence nullifies most of potential
> progresses to mitigate malicious partitioning.
>
>
> To resume, opt-in RBF doesn't suit well deployment of robust second-layers
> protocol, even if those issues are still early and deserve more research.
> At the same time, I believe a meaningful subset of the ecosystem  are still
> relying
> on 0-confs transactions, even if their security is relying on far weaker
> assumptions (opt-in RBF rule is a policy rule, not a consensus one) [2] A
> rapid change of Core's mempool rules would be harming their quality of
> services and should be
> weighed carefully. On the other hand, it would be great to nudge them
> towards more secure handling of their 0-confs flows [3]
>
> Let's examine what could be deployed ecosystem-wise as enhancements to the
> 0-confs security model.
>
> # Proactive security models : Double-spend Monitoring/Receiver-side
> Fee-Topping with Package Relay
>
> From an attacker viewpoint, opt-in RBF isn't a big blocker to successful
> double-spends. Any motivated attacker can modify Core to mass-connect to a
> wide portion of the network, announce txA to this subset, announce txA' to
> the
> merchant. TxA' propagation will be encumbered by the privacy-preserving
> inventory timers (`OUTBOUND_INVENTORY_BROADCAST_INTERVAL`), of which an
> attacker has no care to respect.
>
> To detect a successful double-spend attempt, a Bitcoin service should run
> few full-nodes with well-spread connection graphs and unlinkable between
> them, to avoid being identified then maliciously partitioned from the rest
> of the network.
>
> I believe this tactic is already deployed by few Bitcoin services, and
> even one can throw flame at it because it over consumes network resources
> (bandwidth, connection slots, ...), it does procure a security advantage to
> the ones doing it.
>
> One further improvement on top of this protection could be to react after
> the double-spend detection by attaching a CPFP to the merchant transaction,
> with a higher package feerate than the double-spend. Expected deployment of
> package-relay as a p2p mechanism/mempool policy in Bitcoin Core should
> enable it to do so.
>
> # Reactive security models : EconomicReputation-based Compensations
>
> Another approach could be to react after the fact if a double-spend has
> been qualified. If the sender is already known to the service provider, the
> service account can be slashed.  If the sender is a low-trusted
> counterparty to the merchant, "side-trust" models could be relied on. For
> e.g a LN pubkey with a stacked reputation from your autopilot, LSATs, stake
> certificates, a HTLC-as-a-fidelity-bond, ... The space is quite wide there
> but I foresee those trust-minimized, decentralized solutions being adopted
> by the LN ecosystem to patch the risks when you enter in a channel/HTLC
> operation with an anonymous counterparty.
>
> What other cool new tools could be considered to enhance 0-confs security ?
>
> To conclude, let's avoid replaying the contentious threads of a few years
> ago. What this new thread highlights is the fact that a transaction
> relay/mempool acceptance policy might be beneficial to some class of
> already-deployed
> Bitcoin applications while being detrimental to newer ones. How do we
> preserve the current interests of 0-confs users while enabling upcoming
> interests of fancy L2s to flourish is a good conversation to have. I think.
>
> If there is ecosystem agreement on switching to full-RBF, but 0.24 sounds
> too early, let's defer it to 0.25 or 0.26. I don't think Core has a
> consistent deprecation process w.r.t to policy rules heavily relied-on by
> Bitcoin users, if we do so let sets a precedent satisfying as many folks as
> we can.
>
> Cheers,
> Antoine
>
> [0]
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-May/003033.html
>
> [1] See scenario 3 :
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-June/002758.html
>
> [2] https://github.com/bitcoin/bitcoin/pull/10823#issuecomment-466485121
>
> [3] And the LN ecosystem does have an interest to fix zero-confs security,
> if "turbo-channels"-like become normalized for mobile nodes
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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<div dir=3D"ltr">Russel O&#39;Connor <a href=3D"https://lists.linuxfoundati=
on.org/pipermail/bitcoin-dev/2021-June/019061.html">recently opined</a> tha=
t RBF should=C2=A0be standard treatment of all transactions, rather than as=
 a transaction opt-in/out. I agree with that. Any configuration in a transa=
ction that has not been committed into a block yet simply can&#39;t be reli=
ed upon. Miners also have a clear incentive to ignore RBF rules and mine an=
ything that passes consensus. At best opting out of RBF is a weak defense, =
and at worst it&#39;s simply a false sense of security that is likely to ac=
tively=C2=A0lead to theft events.=C2=A0<br><div><br></div><div>Do we as a c=
ommunity want to support 0-conf payments in any way at this point? It seems=
 rather silly=C2=A0to make software design decisions to accommodate=C2=A00-=
conf payments when there are better mechanisms for fast payments (ie lightn=
ing).=C2=A0</div><div><br></div><div>One question I have is: how does softw=
are generally inform the user about 0-conf payment detection? Does software=
 generally tell the user something along the lines of &quot;This payment ha=
s not been finalized yet. All recipients should wait until the transaction =
has at least 1 confirmation, and most recipients should wait for 6 confirma=
tions&quot; ? I think unless we pressure software to be very explicit about=
 what counts as finality, users will simply continue to do what they&#39;ve=
 always done. Rolling out this policy change over the course of a year or t=
wo seems fine, no need to rush. But I suppose it would depend on how often =
0-conf is used in the bitcoin ecosystem at this point, which I don&#39;t ha=
ve any data on.=C2=A0</div></div><br><div class=3D"gmail_quote"><div dir=3D=
"ltr" class=3D"gmail_attr">On Tue, Jun 15, 2021 at 10:00 AM Antoine Riard v=
ia bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org"=
>bitcoin-dev@lists.linuxfoundation.org</a>&gt; wrote:<br></div><blockquote =
class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px sol=
id rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr"><div>Hi,<br><br>I=
9;m writing to propose deprecation of opt-in RBF in favor of full-RBF as th=
e Bitcoin Core&#39;s default replacement policy in version 24.0. As a remin=
der, the next release is 22.0, aimed for August 1st, assuming agreement is =
reached, this policy change would enter into deployment phase a year from n=
ow. <br><br>Even if this replacement policy has been deemed as highly contr=
oversial a few years ago, ongoing and anticipated changes in the Bitcoin ec=
osystem are motivating this proposal.<br><br># RBF opt-out as a DoS Vector =
against Multi-Party Funded Transactions<br><br>As explained in &quot;On Mem=
pool Funny Games against Multi-Party Funded Transactions&#39;&#39;, 2nd iss=
ue [0], an attacker can easily DoS a multi-party funded transactions by pro=
pagating an RBF opt-out double-spend of its contributed input before the ho=
nest transaction is broadcasted by the protocol orchester. DoSes are qualif=
ied in the sense of either an attacker wasting timevalue of victim&#39;s in=
puts or forcing exhaustion of the fee-bumping =C2=A0reserve.<br><br>This af=
fects a series of Bitcoin protocols such as Coinjoin, onchain DLCs and dual=
-funded LN channels. As those protocols are still in the early phase of dep=
loyment, it doesn&#39;t seem to have been executed in the wild for now.=C2=
=A0 That said, considering that dual-funded are more efficient from a liqui=
dity standpoint, we can expect them to be widely relied on, once Lightning =
enters in a more mature phase. At that point, it should become economically=
 rational for liquidity service providers to launch those DoS attacks again=
st their competitors to hijack user traffic.<br><br>Beyond that, presence o=
f those DoSes will complicate the design and deployment of multi-party Bitc=
oin protocols such as payment pools/multi-party channels. Note, Lightning P=
ool isn&#39;t affected as there is a preliminary stage where batch particip=
ants are locked-in their funds within an account witnessScript shared with =
the orchestrer.<br><br>Of course, even assuming full-rbf, propagation of th=
e multi-party funded transactions can still be interfered with by an attack=
er, simply broadcasting a double-spend with a feerate equivalent to the hon=
est transaction. However, it tightens the attack scenario to a scorched ear=
th approach, where the attacker has to commit equivalent fee-bumping reserv=
e to maintain the pinning and might lose the &quot;competing&quot; fees to =
miners.<br><br># RBF opt-out as a Mempools Partitions Vector<br><br>A longe=
r-term issue is the risk of mempools malicious partitions, where an attacke=
r exploits network topology or divergence in mempools policies to partition=
 network mempools in different subsets. From then a wide range of attacks c=
an be envisioned such as package pinning [1], artificial congestion to prov=
oke LN channels closure or manipulation of fee-estimator&#39;s feerate (the=
 Core&#39;s one wouldn&#39;t be affected as it relies on block confirmation=
, though other fee estimators designs deployed across the ecosystem are lik=
ely going to be affected).<br><br>Traditionally, mempools partitions have b=
een gauged as a spontaneous outcome of a distributed systems like Bitcoin p=
2p network and I&#39;m not aware it has been studied in-depth for adversari=
al purposes. Though, deployment of second-layer<br>protocols, heavily relyi=
ng on sanity of a local mempool for fee-estimation and robust propagation o=
f their time-sensitive transactions might lead to reconsider this position.=
 Acknowledging this, RBF opt-out is a low-cost partitioning tool, of which =
the existence nullifies most of potential progresses to mitigate malicious =
partitioning.<br><br><br>To resume, opt-in RBF doesn&#39;t suit well deploy=
ment of robust second-layers protocol, even if those issues are still early=
 and deserve more research. At the same time, I believe a meaningful subset=
 of the ecosystem =C2=A0are still relying<br>on 0-confs transactions, even =
if their security is relying on far weaker assumptions (opt-in RBF rule is =
a policy rule, not a consensus one) [2] A rapid change of Core&#39;s mempoo=
l rules would be harming their quality of services and should be<br>weighed=
 carefully. On the other hand, it would be great to nudge them towards more=
 secure handling of their 0-confs flows [3]<br><br>Let&#39;s examine what c=
ould be deployed ecosystem-wise as enhancements to the 0-confs security mod=
el.<br><br># Proactive security models : Double-spend Monitoring/Receiver-s=
ide Fee-Topping with Package Relay<br><br>From an attacker viewpoint, opt-i=
n RBF isn&#39;t a big blocker to successful double-spends. Any motivated at=
tacker can modify Core to mass-connect to a wide portion of the network, an=
nounce txA to this subset, announce txA&#39; to the<br>merchant. TxA&#39; p=
ropagation will be encumbered by the privacy-preserving inventory timers (`=
OUTBOUND_INVENTORY_BROADCAST_INTERVAL`), of which an attacker has no care t=
o respect.<br><br>To detect a successful double-spend attempt, a Bitcoin se=
rvice should run few full-nodes with well-spread connection graphs and unli=
nkable between them, to avoid being identified then maliciously partitioned=
 from the rest of the network.<br><br>I believe this tactic is already depl=
oyed by few Bitcoin services, and even one can throw flame at it because it=
 over consumes network resources (bandwidth, connection slots, ...), it doe=
s procure a security advantage to the ones doing it.<br><br>One further imp=
rovement on top of this protection could be to react after the double-spend=
 detection by attaching a CPFP to the merchant transaction, with a higher p=
ackage feerate than the double-spend. Expected deployment of package-relay =
as a p2p mechanism/mempool policy in Bitcoin Core should enable it to do so=
.<br><br># Reactive security models : EconomicReputation-based Compensation=
s<br><br>Another approach could be to react after the fact if a double-spen=
d has been qualified. If the sender is already known to the service provide=
r, the service account can be slashed.=C2=A0 If the sender is a low-trusted=
 counterparty to the merchant, &quot;side-trust&quot; models could be relie=
d on. For e.g a LN pubkey with a stacked reputation from your autopilot, LS=
ATs, stake certificates, a HTLC-as-a-fidelity-bond, ... The space is quite =
wide there but I foresee those trust-minimized, decentralized solutions bei=
ng adopted by the LN ecosystem to patch the risks when you enter in a chann=
el/HTLC operation with an anonymous counterparty. <br><br></div><div>What o=
ther cool new tools could be considered to enhance 0-confs security ?<br></=
div><div><br>To conclude, let&#39;s avoid replaying the contentious threads=
 of a few years ago. What this new thread highlights is the fact that a tra=
nsaction relay/mempool acceptance policy might be beneficial to some class =
of already-deployed <br>Bitcoin applications while being detrimental to new=
er ones. How do we preserve the current interests of 0-confs users while en=
abling upcoming interests of fancy L2s to flourish is a good conversation t=
o have. I think.<br><br>If there is ecosystem agreement on switching to ful=
l-RBF, but 0.24 sounds too early, let&#39;s defer it to 0.25 or 0.26. I don=
&#39;t think Core has a consistent deprecation process w.r.t to policy rule=
s heavily relied-on by Bitcoin users, if we do so let sets a precedent sati=
sfying as many folks as we can.<br><br>Cheers,<br>Antoine<br><br>[0] <a hre=
f=3D"https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-May/003=
033.html" target=3D"_blank">https://lists.linuxfoundation.org/pipermail/lig=
htning-dev/2021-May/003033.html</a><br><br>[1] See scenario 3 : <a href=3D"=
https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-June/002758.=
html" target=3D"_blank">https://lists.linuxfoundation.org/pipermail/lightni=
ng-dev/2020-June/002758.html</a><br><br>[2] <a href=3D"https://github.com/b=
itcoin/bitcoin/pull/10823#issuecomment-466485121" target=3D"_blank">https:/=
/github.com/bitcoin/bitcoin/pull/10823#issuecomment-466485121</a><br><br></=
div>[3] And the LN ecosystem does have an interest to fix zero-confs securi=
ty, if &quot;turbo-channels&quot;-like become normalized for mobile nodes<b=
r></div>
_______________________________________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">=
bitcoin-dev@lists.linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.org/mail=
man/listinfo/bitcoin-dev</a><br>
</blockquote></div>

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