rich1.0npub1vjzmc45k8dgujppapp2ue20h3l9apnsntgv4c0ukncvv549q64gsz4x8ddhttps://nostr.ae/npub1vjzmc45k8dgujppapp2ue20h3l9apnsntgv4c0ukncvv549q64gsz4x8ddnjumphttps://nostr.ae📅 Original date posted:2022-11-02 📝 Original message:Hi list, Reading Suhas's post on mempool policy consistency rules, and the grounded suggestion that as protocol developers we should work on special policy rules to support each reasonable use case on the network rather to arbiter between class of use-cases in the design of an unified set of rules, reminded me there is another solution to solve multi-party funding pinning rather than wide deployment of fullrbf. This was communicated to me a while back, and it was originally dismissed because of the privacy trade-offs (and potential slight fees overhead cost). However, if widely adopted, they might sound acceptable to contracting protocol developers and operators. ## The Problem: Pinning Contracting Protocols Funding Flows with Opt-out Double-Spend As originally laid out [0], multi-party collaborative flows (coinjoin/dual-funding/swaps/splicing/etc), where every participant contributes at least one input, are suffering from a low-cost and high-success DoS vector with asymmetric damages. E.g with lightning interactive transaction construction protocols limits of 252 inputs, 1 single input can bleed the timevalue of the remaining 251 inputs, or engage in a MEV attack where the fee-bumping entity is lured to inflate feerate beyond the current blockspace demand. The attack can be hidden and a posteriori assigning blame consistently stays an open question in the lack of a consensus mechanism between participants on the network mempools states. The issue lies in the fact that participants joining inputs together don't have control, or even view, of the replacement signaling of any potential double-spend of the other participants inputs. Indeed the opt-in fullrbf signaling is enforced based on the nSequence field, and this one is fully malleable by the UTXO spender. There is no current mechanism to require replacement signaling provable to a third-party only on the knowledge of the UTXO spents. # The Solution: Opt-in Full-Replace-by-Fee Spent-nVersion Signaling A new policy is specified in a new way a transaction can signal that it is replaceable. 1. A confirmed transaction is considered to have opted in to allowing replacement of any of its spends (or their descendants), if the last bit of the nVersion field is set. Rational: The future replacement status of any UTXO spend can be determined by inspecting the nVersion, therefore protecting the collaborative participants of a multi-party flows that the target transaction should propagate to the miners, if the fee/feerate offered are the best ones without opt-out based pinning. It can be required the UTXOs to have few confirmations in case of shallow reorgs to increase DoS protection. ## Solution trade-offs On the validation-side, there is one engineering issue, as I think there is no access to the spent nversion fields by the mempool logic. This would presume we add some new cache of all the confirmed UTXOs, so ~50M * 4bytes, 300 MB of additional state for policy-enforcing full-nodes. I don't know if there is another strong drawback, even the reorg logic the replaceable spends shouldn't be evicted if the confirmed ancestor is back to the mempool, as mempool validity shouldn't be reevaluated before a replacement candidate shows up. A fee penalty could be requested for nVersion-signaling transactions to compensate for the additional state stored by full-node operators (even if obviously they're not the ones earning the fees). For the contracting protocols wallets, as you don't know in advance which coins are going to be used for a collaborative flow, you're better off to mark all your coins nVersion fields opting fullrbf. Otherwise, you will have to go through an on-chain fee cost to change the replacement status of the spends of your coins. However, this policy bookmarking comes as a protocol fingerprint leak for an observer of the transaction logs. If all the second-layers used by default, this is constituting a single anonymity set, though it might still be the privacy gains we're harvesting from Taproot output usage in the optimistic case (e.g in Lightning no commitment + HTLC transactions broadcast). For the zeroconf operators, assuming they have access to the UTXO set, they can inspect the receiving transactions ancestors nVersion fields, and sort those transactions in the wider set of the replaceable ones, as they're currently doing for BIP125 opt-in ones. Long-term, the annoying privacy issue and the assumption that any wallet will be a Lightning one could lead to the majority of wallets signaling RBF for their spends. Therefore making those wallets incompatible with zeroconf services, slowly economically outlawing them. From my perspective, though it might be a simplification, it sounds an alternative full rbf way forward, where rather than having miners deciding on the policy enforcement, we let the users decide with their coins. However, this new policy enforcement efficiency is still dependent on the existence of relay paths and support at the endpoints that matter, the miner mempools. So in fine we might have to realize incentive alignment with hashrate is what matters in terms of transaction-relay rules ? Credit to Greg Maxwell for this idea. Cheers, Antoine [0] https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-May/003033.html -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20221101/7190a0df/attachment-0001.html>