Bitcoin Optech Newsletter #98

This week’s newsletter relays a request for comments on a proposed change to the BIP341 taproot transaction digest and briefly summarizes discussion about a new and more concise protocol for atomic swaps. Also included are our regular sections describing changes to services and client software, new releases and release candidates, and notable changes to popular Bitcoin infrastructure software.

Action items

  • Evaluate proposed changes to BIP341 taproot transaction digest: as described in last week’s newsletter, there has been a request for taproot signatures to make an additional commitment to the scriptPubKeys of all the UTXOs being spent in a transaction. Anthony Towns has suggested how BIP341 might be updated for this change and Pieter Wuille has asked whether anyone has any objections. If you have any questions or concerns about the proposed transaction digest revision, we suggest either replying to the mailing list or contacting the BIP341 authors directly.

News

  • Two-transaction cross chain atomic swap or same-chain coinswap: Ruben Somsen posted to the Bitcoin-Dev mailing list and created a video describing a procedure for a trustless exchange of coins using only two transactions, named Succinct Atomic Swaps (SAS). The previous standard protocol uses four transactions. Somsen’s SAS protocol leaves each party holding coins that they can spend at any time—but which they may need to spend on short notice if their counterparty attempts theft (similar to how LN channels need to be monitored). Additionally the keys necessary to spend the coins won’t be contained in the user’s BIP32 HD wallet, so additional backups may be required.

    Advantages of the protocol are that it requires less block space than existing protocols, it saves on transaction fees (both by using less block space and potentially by requiring less urgency for its settlement transactions), it only requires consensus-enforced timelocks on one of the chains in a cross-chain swap, and it doesn’t depend on any new security assumptions or Bitcoin consensus changes. If taproot is adopted, swaps can be made even more privately and efficiently.

    Commenting on the protocol, Lloyd Fournier noted the “elegance” of a simplified version of the protocol that uses three transactions. Dmitry Petukhov posted about a specification he’d written for the protocol in the TLA+ formal specification language, helping to test the correctness of the protocol.

Changes to services and client software

In this monthly feature, we highlight interesting updates to Bitcoin wallets and services.

  • Lightning-based messenger application Juggernaut launches: In a blog post announcing the first release of Juggernaut, John Cantrell describes how the messaging and wallet features are built using keysend payments.

  • Lightning Loop using multipath payments: The latest upgrade from Lightning Labs now uses multipath payments to convert onchain funds into funds within LN channels.

  • Blockstream Satellite 2.0 supports initial block download: Blockstream outlines version 2.0 upgrades to their satellite service which include expanded Asia-Pacific coverage, additional bandwidth, and an updated protocol that enables a full node to complete an initial sync using only the satellite feed.

  • Breez wallet enables spontaneous payments: Version 0.9 of Breez wallet adds the ability to send spontaneous payments to Lightning nodes that support keysend.

  • Copay enables CPFP for incoming transactions: Version 9.3.0 adds the ability for the user to speed up an incoming transaction using child-pays-for-parent. The feature is only enabled after the wallet observes the transaction remaining unconfirmed for four hours.

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.

  • Bitcoin Core 0.20.0rc2 is the newest release candidate for the next major version of Bitcoin Core. It contains several bug fixes and improvements since the first release candidate.

  • LND 0.10.1-beta.rc1 is the first release candidate for the next maintenance release of LND.

Notable code and documentation changes

Notable changes this week in Bitcoin Core, C-Lightning, Eclair, LND, Rust-Lightning, libsecp256k1, Bitcoin Improvement Proposals (BIPs), and Lightning BOLTs.

Note: the commits to Bitcoin Core mentioned below apply to its master development branch and so those changes will likely not be released until version 0.21, about six months after the release of the upcoming version 0.20.

  • Bitcoin Core #18877 is the first step towards support for serving compact block filters on the P2P network, as specified in BIP157. Nodes that enable the compact block filter index with the -blockfilterindex configuration parameter can now respond to getcfcheckpt requests with a cfcheckpt compact block filters checkpoint response. The getcfheaders and getcfilters messages are not yet supported, and the node won’t advertise support for BIP157 with NODE_COMPACT_FILTERS in its version message.

    The feature is disabled by default and can be enabled with the -peerblockfilters configuration parameter.

  • Bitcoin Core #18894 fixes a UI bug that affected people who simultaneously used multi-wallet mode in the GUI and manual coin control. The bug was described as a known issue in the Bitcoin Core 0.18 release notes. This is included in the second release candidate for Bitcoin Core 0.20 linked in the preceding section.

  • Bitcoin Core #18808 causes Bitcoin Core to ignore any P2P protocol getdata requests that specify an unknown type of data. The new logic will also ignore requests for types of data that aren’t expected to be sent over the current connection, such as requests for transactions on block-relay-only connections.

  • C-Lightning #3614 adds a new notification type named coin_movements that is triggered by finalized ledger updates. Clients subscribed to these notifications will receive updates on both definitively resolved HTLCs and confirmed bitcoin transactions, allowing them to construct a canonical ledger for coin movements through their C-Lightning node. See its documentation for details.

  • Eclair #1395 updates the route pathfinding used by Eclair to factor in channel balances and to use Yen’s algorithm. The PR description says that “the new algorithm consistently finds more routes and cheaper ones. The route prefixes are more diverse, which is good as well (especially for MPP). […] and the new [code] is consistently 25% faster on my machine (when looking for 3 routes).”