### Bitcoin Optech Newsletter #198

This week’s newsletter summarizes a post about implementing MuSig2, relays the responsible disclosure of a security issue affecting some older LN implementations, discusses a proposal for measuring support for consensus changes through transaction signaling, and examines the effect of rate limiting on more bandwidth efficient LN gossiping. Also included are our regular sections summarizing new software releases and release candidates plus notable changes to popular Bitcoin infrastructure projects.

## News

• MuSig2 implementation notes: Olaoluwa Osuntokun replied to the draft BIP for MuSig2 mentioned in Newsletter #195 with notes from the implementations he and others have worked on for btcd and LND:

• Interaction with BIP86: keys created by a BIP32 HD wallet implementing BIP86 follow the BIP341 recommendation for creating keypath-only keys by tweaking the key by a hash of itself. This helps prevent the key from being used in a multisignature which could allow one participant to secretly include a scriptpath spending option they control, giving them the ability to steal all the funds. However, if the multisignature participants deliberately want to include a scriptpath spending option, they need to share the un-tweaked versions of their keys with each other.

Osuntokun recommends that BIP86 implementations return both the original key (internal key) and the tweaked key (output key) so that the calling function can use whichever one is appropriate for its context.

• Interaction with scriptpath spends: keys meant to be used with scriptpath spends have a related problem: in order to use the scriptpath, the spender must know the internal key. Again, this suggests that implementations return the internal key so that it’s available to be used in other code that needs it.

• Shortcut for final signer: Osuntokun also sought clarification on a section in the BIP which describes how the final signer (and only the final signer) can use deterministic randomness or a lower-quality source of randomness for generating their signature nonce. Brandon Black replied to describe the situation that had motivated the section—they had a signer that would have a difficult time securely managing a regular MuSig2 signing session but which they were instead able to always use as the final signer.

• Measuring user support for consensus changes: Keagan McClelland posted to the Bitcoin-Dev mailing list a proposal similar to previous proposals to have transactions signal whether or not they supported a particular effort to change the consensus rules. In the thread, several related sentiment measurement ideas were also discussed, but all appeared to have problems, such as technical challenges, significantly reducing user privacy, favoring certain parts of the Bitcoin economy over others, or penalizing early voters over those who waited to participate in consensus formation.

As on previous occasions where this topic has been discussed, it did not appear that any of the suggested methods would produce a result that would be sufficiently respected by most of the discussion participants when it came to informing their decisions about changing Bitcoin’s consensus rules.

• LN anchor outputs security issue: Bastien Teinturier posted to the Lightning-Dev mailing list the announcement of a security issue he had previously responsibly disclosed to LN implementation maintainers. The issue affected older versions of Core Lightning (with experimental features enabled) and LND. Anyone still using the versions mentioned in Teinturier’s post are strongly encouraged to upgrade.

Prior to the implementation of anchor outputs, revoked HTLC transactions only contained a single output, so many implementations only tried to claim that single output. The new design of anchor outputs for LN allows combining multiple revoked HTLC outputs into a single transaction, but this is only safe if implementations claim all of the relevant outputs in the transaction. Any funds which have not been claimed by the time the HTLC timelock expires may be stolen by the party who broadcast the revoked HTLC. Teinturier’s implementation of anchor outputs for Eclair allowed him to test the other LN implementations and discover the vulnerability.

As with a previous attack related to anchor outputs (see Newsletter #115), the problem appears to be related to adding support for signing with SIGHASH_SINGLE|SIGHASH_ANYONECANPAY while still retaining legacy support for signing with SIGHASH_ALL.

• LN gossip rate limiting: Alex Myers posted to the Lightning-Dev mailing list about his research into using minisketch-based set reconciliation to reduce the amount of bandwidth used by nodes for learning about updates to the LN channel graph. His method assumes all peers have almost all of the same information about almost all of the same public channels. One peer can then generate a minisketch from its complete graph of the public network and send that to all of its peers, who can use the minisketch to find any updates to the network since its last reconciliation. This is different than the proposed use of minisketch for the Bitcoin P2P network via the erlay protocol where only updates (new unconfirmed transactions) from the last few seconds are sent.

One challenge of reconciling over all public channels is that it requires all LN nodes keep the same information. Any filtering that produces a persistent difference in the view of the channel graph between nodes will result in either a bandwidth overhead or a failure of the protocol. Matt Corallo suggested that this could be addressed by applying the erlay model to LN—if only new information was synced, there wouldn’t be a concern about persistent differences, although large variations in filtering rules could still result in wasted bandwidth or reconciliation failure. Myers was concerned about the amount of state tracking required by only sending updates—a Bitcoin Core node maintains a separate state for each of its peers in order to avoid resending updates previously sent to the node. The alternative of reconciling over all channels eliminates the need for per-peer state, greatly simplifying the implementation of gossip management.

The discussion about the tradeoffs implicit in each of these approaches was ongoing as this summary was being written.

## 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.

• BTCPay Server 1.5.1 is a new release of this popular self-hosted payment processing software which includes a new main-page dashboard, a new transfer processors feature, and the ability to allow pull payments and refunds to be automatically approved.

• BDK 0.18.0 is a new release of this wallet library. It includes a critical security fix from one of its dependencies, the rust-miniscript library. It also includes several improvements and minor bug fixes.

## Notable code and documentation changes

Notable changes this week in Bitcoin Core, Core Lightning, Eclair, LDK, LND, libsecp256k1, Hardware Wallet Interface (HWI), Rust Bitcoin, BTCPay Server, BDK, Bitcoin Improvement Proposals (BIPs), and Lightning BOLTs.

• Bitcoin Core #18554 prevents the same Bitcoin Core wallet file from being used on multiple fully independent block chains by default. When Bitcoin Core scans a new block for transactions affecting one of its wallets, it records the hash of that block’s header in the wallet. This PR checks whether the most recent recorded scan block is descended from the same genesis block as the currently used block chain. If it isn’t, an error is returned unless the new -walletcrosschain configuration option is set. This prevents a wallet intended for use with one network (e.g. mainnet) from being used with another network (e.g. testnet), reducing the risk of accidental money loss or privacy loss. This only affects users of Bitcoin Core’s internal wallet; other Bitcoin wallet software is unaffected.

• Bitcoin Core #24322 is part of a larger effort to extract out a consensus engine by creating a library to use Bitcoin Core’s consensus code as-is, then incrementally pruning modules to make the library more minimal. Namely, this PR introduces a libbitcoinkernel library delineating all of the source files the bitcoin-chainstate executable (introduced in Bitcoin Core #24304) needs to link against. The list includes files that may not seem logically related to consensus, illustrating the current dependencies of Bitcoin Core’s consensus engine. Future work will modularize consensus from the rest of the codebase, removing these files from libbitcoinkernel’s source list.

• Bitcoin Core #21726 adds the ability to maintain a coinstats index even on pruned nodes. Coinstats includes the MuHash digest of the UTXO state at each block, which allows validating assumeUTXO states. Previously this was only guaranteed to be available on archival full nodes—those that stored every block on the block chain. This merged PR also makes the information available to pruned full nodes (those that delete blocks some time after validating them) when the -coinstatsindex configuration option is enabled.

• BDK #557 adds the Oldest First coin selection algorithm. There are now four coin selection algorithms: Branch and Bound (BnB), Single Random Draw (SRD), Oldest First, and Largest First. By default BDK will use BnB with SRD as a fallback if no solution was found by BnB.

• LDK #1425 adds support for large channels (“wumbo channels”), which are channels which support high value payments.

• LND #6064 adds new bitcoind.config and bitcoind.rpccookie configuration options to specify non-default paths for the configuration and RPC cookie files.

• LND #6361 updates the signrpc method to be able to create signatures using the MuSig2 algorithm. See the documentation added in this merged PR for details. Note that support for MuSig2 is experimental and may change, especially if there are major changes to the proposed BIP for MuSig2 (see Newsletter #195).

• BOLTs #981 removes from the specification the ability for queries and results about the LN network graph to be compressed. It is believed the compression wasn’t being used and dropping support reduces the complexity and number of dependencies for LN implementations.