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 ?
[3] And the LN ecosystem does have an interest to fix zero-confs security, if "turbo-channels"-like become normalized for mobile nodes