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Bitcoin Optech Newsletter #104
This week’s newsletter summarizes a discussion about mining incentives related to HTLCs and links to an announcement about a proposed service to store and relay presigned transactions. Also included are our regular sections with recently transcribed talks and conversations, new releases and release candidates, and notable changes to popular Bitcoin infrastructure projects.
None this week.
● Discussion of HTLC mining incentives: Itay Tsabary, Matan Yechieli, Ittay Eyal posted to the Bitcoin-Dev mailing list with a paper they’ve written. They claim that rational miners should want to run a modified Bitcoin node that allows a user to bribe it with a transaction that’s timelocked and can’t be confirmed until some point in the future. As a result of this bribe, the miner won’t confirm any transaction that could be mined immediately but which conflicts with the bribe (as long as the bribe pays a sufficiently higher feerate than any transactions it blocks).
If this theory is accurate, it affects Hash TimeLock Contracts (HTLCs) which can be settled immediately by one party (Alice) using a preimage, or which can be settled later by a second party (Bob) after a timeout. According to the authors, if Bob sees Alice use the preimage to spend the HTLC, Bob should send his conflicting timeout settlement transaction to miners with a sufficiently higher feerate than Alice’s preimage settlement—even though miners can’t immediately include Bob’s transaction—bribing them to ignore Alice’s transaction in favor of waiting to confirm his alternative transaction. This allows Bob to steal the money Alice should receive. HTLCs are currently used in LN, atomic swaps, and several other contract protocols.
The authors of the paper propose a solution they call Mutually Assured Destruction HTLCs (MAD-HTLCs). These require Bob to provide a hashlocked bond that makes him reveal his own preimage when sending his timeout settlement. If both Alice and Bob reveal their respective preimages, miners will be able to claim both the payment/refund amount and the bond collateral—destroying the incentive for Bob to attempt to steal by bribing miners.
The downsides to this approach are that it requires Bob to use more collateral—raising the cost of using HTLCs—and it uses more block chain space when MAD-HTLCs are settled onchain compared to traditional HTLCs—also raising costs. It was also claimed in the mailing list discussion that MAD-HTLCs might have their own problems with theft when the bonded user’s counterparty is a miner (e.g. Alice is a miner).
ZmnSCPxj noted that a mechanism already exists to allow Alice to discourage Bob from attempting his theft: Alice can enact a scorched earth policy where she spends all of her legitimate funds to fees in order to prevent Bob from receiving them. In theory, this should prevent Bob from even trying the theft. Another paper by Majid Khabbazian, Tejaswi Nadahalli, and Roger Wattenhofer was mentioned in the discussion, which showed (among other things) that nearly all miners would need to switch to the proposed bribe-accepting software before the attack would become particularly effective under normal conditions.
In the short term, and probably the medium term, this attack does not appear to pose any significant danger to HTLCs whose receivers resolve their preimages well before their timelocks expire. In the long term, this is an incentive compatibility concern that protocol developers may want to keep in mind.
● Proposed service for storing, relaying, and broadcasting presigned transactions: Jacob Swambo sent a request for comments to the Bitcoin-Dev mailing list about creating software and a protocol for allowing applications to store presigned transactions with third-party services for later broadcast. The software could also perhaps determine when to broadcast the transactions based on certain conditions being met. This could be useful for software such as vaults and watchtowers. Swambo asks anyone interested in the idea, especially those interested in using it with their protocol, to contact him.
Recently transcribed talks and conversations
Bitcoin Transcripts is the home for transcripts of technical Bitcoin presentations and discussions. In this monthly feature, we highlight a selection of the transcripts from the previous month.
● Magical Bitcoin: Alekos Filini presented at LA BitDevs on Magical Bitcoin, a set of tools and libraries under development for onchain wallet developers. He explained that coin selection logic and transaction signing logic currently have to be rewritten multiple times across multiple projects—a problem Magical Bitcoin aims to address with modular, extensible, and peer-reviewed components. A longer term ambition is to provide a platform for building native wallets and integrating them with existing projects. Filini demoed the current functionality, which includes a playground with a Policy to Miniscript compiler and some rudimentary visualizations. It is written in Rust and leverages the rust-miniscript library and the open source Esplora block explorer. (transcript, video)
● Watchtowers and BOLT13: Sergi Delgado appeared on Potzblitz to discuss the latest state of watchtower development and a proposed watchtower protocol specification. He explored the various tradeoffs when designing a watchtower and the interplay between privacy requirements, accessibility, storage, and fees charged. Delgado is working on the watchtower implementation The Eye of Satoshi at Talaia Labs, which is aiming to be compliant with the proposed BOLT13 (see Newsletter #75). Delgado also highlighted how watchtowers are becoming increasingly critical in multiple settings such as Bitcoin vault designs, statechains, and atomic swaps in addition to LN. (transcript, video, slides)
● Coinswap: Aviv Milner presented on coinswap at the Wasabi Research Club. He explained the property of covertness and how Chris Belcher’s coinswap proposal (see Newsletter #100) provides covertness in a manner that other privacy schemes such as coinjoin fail to do. Milner also went through the motivation for routing coinswaps, namely to address the concern of unwittingly entering into a coinswap with an adversary such as a chain surveillance company. (transcript, video)
● Schnorr signatures and multisignatures: BIP340 co-authors Tim Ruffing, Pieter Wuille, and Jonas Nick held a discussion at London Bitcoin Devs on the specification of schnorr signatures. This covered earlier ideas for implementing schnorr signatures in Bitcoin and what the co-authors thought the community should be concerned about when considering a possible future soft fork deployment. Wuille noted that he is most concerned about how schnorr is adopted and what is built using it. The following day, Tim Ruffing presented on the challenges of multisignature and threshold signature schemes using schnorr signatures (see also Newsletter #68). He’s been working with collaborators on a speculative MuSig signature scheme that only requires two rounds of interaction rather than three, without relying on zero knowledge proofs, which would greatly simplify some threshold and multisignature signing workflows. (Meeting transcript, meeting video, presentation transcript, presentation video, presentation slides)
● Sydney meetup discussion: A number of Bitcoin and LN developers joined this Sydney meetup to discuss various topics. Ruben Somsen gave a short presentation on Succinct Atomic Swaps (SAS) (see Newsletter #98) and how it compares to Chris Belcher’s coinswap proposal. Another topic was Bitcoin Core’s reliance on DNS seeds to find initial peers, whether it’s a potential attack vector against newly started nodes, and how work by Matt Corallo and Antoine Riard on allowing alternative transports could help mitigate some risks. Finally, Lloyd Fournier discussed how experimenting with his toy Rust implementation of secp256k1 (secp256kfun) led to a small fix in the ECDSA signature code in the actual secp256k1 library. The transcript was anonymized to encourage open discussion. (transcript)
Releases and release candidates
New releases and release candidates for popular Bitcoin infrastructure projects. Please consider upgrading to new releases or helping to test release candidates.
● Hardware Wallet Interface (HWI) 1.1.2: this release includes support for handling PSBTs with serialized previous segwit transactions, which is now required or suggested for several hardware wallets in response to the fee overpayment attack.
● LND 0.10.2-beta.rc4: this release candidate for an LND maintenance release is now available for testing. It includes several bug fixes, including an important fix related to the creation of backups.
● LND 0.10.3-beta.rc1: this release candidate, separate from the 0.10.2 RC, includes a package refactoring in addition to the bug fixes provided in 0.10.2. For details, see a mailing list post by LND developer Olaoluwa Osuntokun.
Notable code and documentation changes
Notable changes this week in Bitcoin Core, C-Lightning, Eclair, LND, Rust-Lightning, libsecp256k1, Hardware Wallet Interface, Bitcoin Improvement Proposals (BIPs), and Lightning BOLTs.
● Bitcoin Core #19305 adds release notes for the upcoming codebase transition from the C++11 standard to C++17. The plan is to add C++17 compatibility in the 0.21 release (expected end of 2020) and break C++11 compatibility later in 0.22 (expected mid-2021). Although this change won’t affect most users, those planning to build the 0.22 release on older systems may want to check their toolchain’s standard support against the C++ compiler support matrix.
● Bitcoin Core #11413 updates the
walletcreatefundedpsbtRPCs to allow manually specifying the feerate to use in the newly created or updated transaction. By default, these commands still use either an automatic feerate computed by Bitcoin Core’s built-in transaction fee estimate or the fallback feerate if there’s not enough data for estimation. For details on how to use the updated RPCs, see the proposed release note. Despite not affecting any major system, this PR was open for almost three years—the second longest of any PR that has been merged into Bitcoin Core so far. We thank the author, Kalle Alm, and everyone else involved for their persistence.
● Eclair #1466 decreases the length of time that an Eclair node will wait before closing a channel when a peer becomes unresponsive while an HTLC is pending. To prevent payments from becoming stuck forever, each HTLC includes a timelock. If the timelock expires, then the sending party can reclaim the funds in the HTLC. To prevent the sending party from pulling back funds that an intermediate party has already paid to the next hop, the intermediate party must settle the pending HTLC onchain if the sending party becomes unresponsive. This PR increases Eclair’s safety window—it will now broadcast the onchain transaction 24 blocks before the HTLC timeout, instead of 6 blocks before the timeout.
● LND #4018 adds the ability to delay forwarding a payment (HTLC), giving an external process the ability to review it and decide whether to cancel it or continue forwarding it. This is similar to hold invoices but it applies to payments being routed to a node’s peers rather than received by the node itself. Several use cases are described in the PR—for example, one idea is to hold an HTLC, detect that its next hop is offline (e.g. a mobile node), send a notification to that node which will restart it, and then continue relaying the HTLC.
● LND #4106 adds a
getrecoveryinfoRPC that returns information about the progress of restoring from a backup.
● BIPs #933 adds the BIP339 specification for transaction relay announcement using Witness Transaction Identifiers (wtxids). Currently, nodes announce new unconfirmed transactions to their peers using the transaction’s txid, which is a hash over the legacy fields of the transaction. The new fields used in segwit transactions are not included in the txid, which was necessary to eliminate third-party and counterparty transaction malleability. However, during his June 2016 review of segwit, Peter Todd noticed that announcing transactions by their txid could allow nodes to modify the segwit fields before relaying a transaction. This couldn’t be used to steal money directly, but it did allow a relay node to mutate a valid transaction into an invalid or unacceptable transaction without changing its txid.
Before Todd’s discovery, this was a problem: Bitcoin Core would track the txids of invalid transactions it had recently seen and refuse to download them again. That meant a malicious node could prevent any of its peers from downloading a valid segwit transaction from any of their other peers by sending them a mutated version of that transaction. Arbitrary transaction censorship, even just at the relay level, is bad by itself—but it has especially severe consequences for time-sensitive protocols such as LN.
At the time of Todd’s analysis, segwit was nearing release, so a quick fix was implemented that prevents Bitcoin Core from caching the rejection status of segwit transactions that fail for the type of witness-related errors that malicious nodes can inject. This eliminated the issue, but it means that Bitcoin Core uses more bandwidth than necessary when it encounters segwit transactions that are invalid because of accidental mistakes. It may also complicate the development of new relay protocols such as package relay.
The long-term solution to the problem is that transactions should be announced using a hash that commits to the entire transaction—both the legacy fields and the new segwit fields. That’s exactly what wtxids do, and they’re already needed in the protocol to construct the witness merkle root in each block’s coinbase transaction. This new BIP proposes updating the P2P protocol’s
invmessage that announces new transactions, and the
getdatamessage that requests a transaction, to allow them both to use wtxids. That will allow nodes to skip re-downloading a transaction if it has the same wtxid as a transaction that was previously found to be invalid or unacceptable.
The BIP339 proposal also adds the additional feature negotiation between newly started nodes described in Newsletter #87. See also the proposed implementation for Bitcoin Core.
● BIPs #923 adds the BIP78 specification for the version of the payjoin protocol originally implemented in BTCPay (see Newsletter #94). Payjoin allows a spender and a receiver to both add UTXOs to a transaction. This decreases the reliability of the assumption made by third-party block chain surveillance companies that any set of UTXOs spent in the same transaction were all received by the same person. BIP78 is based on the BIP79 specification of the Bustapay payjoin variant (see Newsletter #13) but contains several notable differences, including different extensions to BIP21
bitcoin:URIs, usage of PSBTs, and some additional requirements designed to enhance privacy. Several programs already support this protocol and several more are currently working on adding support. The PR for this BIP received significant discussion and may provide useful reference material for anyone interested in the subject.
● BIPs #550 revises the BIP8 alternative to BIP9 versionbits-based soft fork deployment. BIP8 previously allowed a soft fork to be activated by miner signaling using the same parameters as would be used for BIP9, but it required that the soft fork activate at the end of the signaling period even if miners were still not signaling readiness to follow the new consensus rules. This could be used to override miners who were obstructing activation of a fork, but it could also lead to diverging block chains between nodes that enforced the fork’s new consensus rules and those that didn’t.
The main change to BIP8 from its previous version is that it may now be used initially without requiring mandatory lock-in. Implementations may choose to commit to lock-in after their initial deployment of a BIP8-activated soft fork. BIP8 mandates signaling for a period after the mandatory lock-in height, which will trigger even deployments without mandatory lock-in to begin enforcing the new rules at the same time as the mandatory lock-in nodes; in the best case, this allows the entire economy to synchronously begin enforcing the new rules.