This week’s newsletter links to a proof-of-concept implementation for proving in zero-knowledge that an output is part of the UTXO set, describes one new and two previous proposals for allowing offline LN payments, and summarizes research about DNS seeding for non-IP network addresses. Also included are our regular sections describing changes to clients and services, announcing new releases and release candidates, and summarizing notable changes to popular Bitcoin infrastructure software.

News

  • ● Proving UTXO set inclusion in zero knowledge: Johan Halseth posted to Delving Bitcoin to announce a proof-of-concept tool that allows someone to prove that they control one of the outputs in the current UTXO set without revealing which output. The eventual goal is to allow the co-owners of an LN funding output to prove they control a channel without revealing any specific information about their onchain transactions. That proof can be attached to next-generation channel announcement messages that are used to build decentralized routing information for LN.

    The method used differs from the aut-ct method described in Newsletter #303, and some of the discussion focused on clarifying the differences. Additional research is needed, with Halseth describing several open problems.

  • ● LN offline payments: Andy Schroder posted to Delving Bitcoin to sketch a communication process an LN wallet could use to generate tokens that could be provided to an internet-connected wallet in order to pay it. For example, Alice’s wallet would normally be connected to an always-online LN node she controls or that is controlled by a Lightning service provider (LSP). While online, Alice will pregenerate authentication tokens.

    Later, when Alice’s node is offline and she wants to pay Bob, she gives Bob an authentication token that allows him to connect to her always-online node or LSP and withdraw an amount indicated by Alice. She can provide the authentication token to Bob using NFC or another widely available data transfer protocol that doesn’t require Alice to access the internet, keeping the protocol simple and making it easy to implement on devices with limited computing resources (such as smart cards).

    Developer ZmnSCPxj mentioned an alternative approach he had previously described and Bastien Teinurier referenced a method for node remote control he designed for this type of situation (see Newsletter #271).

  • ● DNS seeding for non-IP addresses: developer Virtu posted to Delving Bitcoin a survey of the availability of seed nodes on anonymity networks and discussed methods for allowing new nodes that exclusively use those networks to learn about peers through DNS seeders.

    As background, a Bitcoin node or P2P client needs to learn the network addresses of peers that it can download data from. Newly installed software, or software that has been offline for a long time, may not know the network address of any active peers. Normally, Bitcoin Core nodes solve this by querying a DNS seed that returns the IPv4 or IPv6 addresses of several peers that are likely to be available. If DNS seeding fails, or if it’s unavailable (such as for anonymity networks that don’t use IPv4 or IPv6 addresses), Bitcoin Core includes the network addresses of peers that were available when the software release was made; those peers are used as seed nodes, where the node requests additional peer addresses from the seed node and uses them as potential peers. DNS seeds are preferred over seed nodes as their information is usually more current and the global DNS caching infrastructure can prevent a DNS seed from learning the network address of each querying node.

    Virtu examined the seed nodes listed in the last four major releases of Bitcoin Core and found that a satisfactory number of them were still available, indicating that users of anonymity networks should be able to find peers. They and other discussion participants also examined the possibility of modifying Bitcoin Core to allow it to use DNS seeding for anonymity networks via either DNS NULL records or encoding alternative network addresses into pseudo-IPv6 addresses.

Changes to services and client software

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

  • ● Strike adds BOLT12 support: Strike announced support for BOLT12 offers, including using offers with BIP353 DNS payment instructions.

  • ● BitBox02 adds silent payment support: BitBox02 announced support for silent payments and an implementation of payment requests.

  • ● The Mempool Open Source Project v3.0.0 released: The v3.0.0 release includes new CPFP fee calculations, additional RBF features including fullrbf support, P2PK support, and new mempool and blockchain analysis features, among other changes.

  • ● ZEUS v0.9.0 released: The v0.9.0 post outlines additional LSP features, watch-only wallets, hardware signing device support, support for transaction batching including channel open transactions, and other features.

  • ● Live Wallet adds consolidation support: The Live Wallet application analyzes the cost to spend a set of UTXOs at different feerates including determining when outputs would be uneconomical to spend. The 0.7.0 release includes features to simulate consolidation transactions and generate consolidation PSBTs.

  • ● Bisq adds Lightning support: Bisq v2.1.0 adds the ability for users to settle trades using the Lightning Network.

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.

  • ● HWI 3.1.0 is a release of the next version of this package providing a common interface to multiple different hardware signing devices. This release adds support for the Trezor Safe 5 and makes several other improvements and bug fixes.

  • ● Core Lightning 24.08.1 is a maintenance release that fixes crashes and other bugs discovered in the recent 24.08 release.

  • ● BDK 1.0.0-beta.4 is a release candidate for this library for building wallets and other Bitcoin-enabled applications. The original bdk Rust crate has been renamed to bdk_wallet and lower layer modules have been extracted into their own crates, including bdk_chain, bdk_electrum, bdk_esplora, and bdk_bitcoind_rpc. The bdk_wallet crate “is the first version to offer a stable 1.0.0 API.”

  • ● Bitcoin Core 28.0rc2 is a release candidate for the next major version of the predominant full node implementation. A testing guide is available.

Notable code and documentation changes

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

Note: the commits to Bitcoin Core mentioned below apply to its master development branch, so those changes will likely not be released until about six months after the release of the upcoming version 28.

  • ● Bitcoin Core #28358 drops the dbcache limit because the previous 16 GB limit was no longer sufficient to complete an Initial Block Download (IBD) without flushing the UTXO set from RAM to disk, which can provide about a 25% speed up. It was decided to remove the limit rather than raise it because there was no optimal value that would be future-proof and to give users complete flexibility.

  • ● Bitcoin Core #30286 optimizes the candidate search algorithm used in cluster linearizations, based on the framework laid out in Section 2 of this Delving Bitcoin post, but with some modifications. These optimizations minimize iterations to improve linearization performance, but may increase startup and per-iteration costs. This is part of the cluster mempool project. See Newsletter #315.

  • ● Bitcoin Core #30807 changes the signaling of an assumeUTXO node that is syncing the background chain from NODE_NETWORK to NODE_NETWORK_LIMITED so that peer nodes don’t request blocks older than about a week from it. This fixes a bug where a peer would request a historical block and get no response, causing it to disconnect from the assumeUTXO node.

  • ● LND #8981 refactors the paymentDescriptor type to only use it within the lnwallet package. This is to later replace paymentDescriptor with a new structure called LogUpdate to simplify how updates are logged and handled, as part of a series of PRs implementing dynamic commitments, a type of channel commitment upgrade.

  • ● LDK #3140 adds support for paying static BOLT12 invoices to send async payments as an always online sender as defined in BOLTs #1149, but without including the invoice request in the payment onion message. Sending as an often offline sender or receiving async payments is not yet possible, so the flow cannot yet be tested end-to-end.

  • ● LDK #3163 updates the offers message flow by introducing a reply_path in BOLT12 invoices. This allows the payer to send the error message back to the payee in case of an invoice error.

  • ● LDK #3010 adds functionality for a node to retry sending an invoice request to an offer reply path if it hasn’t yet received the corresponding invoice. Previously, if an invoice request message on a single reply path offer failed due to network disconnection, it wasn’t retried.

  • ● BDK #1581 introduces changes to the coin selection algorithm by allowing for a customizable fallback algorithm in the BranchAndBoundCoinSelection strategy. The signature of the coin_select method is updated to allow a random number generator to be passed directly to the coin selection algorithm. This PR also includes additional refactorings, internal fallback handling, and simplification of error handling.

  • ● BDK #1561 removes the bdk_hwi crate from the project, to simplify dependencies and CI. The bdk_hwi crate contained HWISigner, which has now been moved to the rust_hwi project.

Want more?

For more discussion about the topics mentioned in this newsletter, join us for the weekly Bitcoin Optech Recap on Twitter Spaces at 14:30 UTC on September 24. The discussion is also recorded and will be available from our podcasts page.