This week’s newsletter includes information about a new language to describe output scripts, an update on Bitcoin Core’s support for partially signed Bitcoin transactions, and news on several other notable Bitcoin Core merges.

Action items

  • Bitcoin Core 0.16.2RC2 released for testing in preparation for a maintenance release that will provide bugfixes and backports. Community testing is highly appreciated. Note, there was no RC1 due to a metadata problem being detected during the release process.

Dashboard items

  • Transaction fees are lower than they were this time last week. Anyone who can wait 12 or more blocks for confirmation can reasonably pay the default minimum feerate. It’s a good time to consolidate UTXOs.

  • The number of native segwit outputs had been increasing steadily over time, but dropped by about 400,000 (80%) this week, possibly due to UTXO consolidation by an exchange. The average number of new native segwit outputs created per hour remains relatively constant, indicating no obvious decrease in adoption.


  • Bitcoin Optech publicly announced: we received great coverage in Bitcoin Magazine, Coindesk, and several other publications. This wouldn’t have been possible without the support of our founding sponsors and member companies. Thank you!

  • First Optech workshop held in San Francisco: as previously announced, we held our first workshop in San Francisco last week. There were 14 engineers from Bay Area companies and open source projects in attendence, and we had great discussions about coin selection, replace-by-fee, and child-pays-for-parent. Thanks to Square for hosting and Coinbase for helping with organization.

    If you work at a member company and have any requests or suggestions for future Optech events (be that location, venue, dates, format, topics, or anything else), please contact us. We’re here to help our member companies!

  • Coin selection RPC unlikely: In Bitcoin Core’s weekly meeting, Andrew Chow raised the possibility of creating an RPC that would allow users to pass in information about a transaction they wanted to create, including a list of available inputs, and receive back a list of which inputs would be selected by the Bitcoin Core wallet’s coin selection algorithm.

    Meeting participants were mostly opposed to providing this feature, suggesting that it would be better if it was a library and that Bitcoin Core’s recent and continuing work towards encapsulating its coin selection code would simplify development of a third-party library later. A particular opposition to the idea was that it might reduce the pace of development for direct users of the Bitcoin Core wallet; as Gregory Maxwell said, “Pressure to maintain a stable interface to [coin selection] would be harmful to the project. […] I don’t want to hear ‘we can’t implement privacy feature X because it’ll break [the coin selection] interface’.”

  • First use of output script descriptors: Pieter Wuille has opened PR #13697 to Bitcoin Core that implements his output script descriptors language for describing which output scripts (scriptPubKeys) a wallet should monitor for. This particular PR only applies to the recently-added scantxoutset RPC but Wuille’s ultimate goal is to use this new language elsewhere in the API and “to remove the need for importing scripts and keys entirely, and instead make the wallet just be a list of these descriptors plus associated metadata.”

  • BIP174 Partially Signed Bitcoin Transaction (PSBT) support merged: this provides a standardized format that multiple wallets can use to communicate information about transactions that need to be signed, so that hot wallets can get signatures from cold wallets or hardware wallets, multisig transactions can be signed by multiple wallets, and multiple wallets can collaboratively create multiparty transactions such as CoinJoins. Several RPCs are added with this merge: walletprocesspsbt, walletcreatefundedpsbt, decodepsbt, combinepsbt, finalizepsbt, createpsbt, and convertpsbt. For a full description, see PR #13557.

Notable Bitcoin Core merges

Not including those previously discussed in the News section.

  • Bitcoin Core #9662: New wallets can now be created with private keys disabled. This is primarily meant for users who want to exclusively use their wallet in conjunction with another program or hardware wallet that stores private keys. This could also be useful to companies that want to use Bitcoin Core features (like coin selection) by creating a wallet, importing their addresses (but not private keys), and then performing whatever actions they desire, such as using the fundrawtransaction RPC.

  • Bitcoin Core #12196: New scantxoutset RPC method that allows searching the set of spendable bitcoins (UTXOs) for those matching an address, public key, private key, or HD keypath. The main expected use for this is “funds sweeping” where transactions matching an old wallet are found and transferred to a new wallet. Although this RPC will almost certainly be included in Bitcoin Core 0.17, it will likely be marked as experimental so that its API can be freely changed in subsequent releases. This API is likely to be updated to support output script descriptors, which is planned to happen before 0.17.

  • Bitcoin Core #13604: Bitcoin-Qt is now built by default in addition to bitcoind on 32-bit ARM systems, and should be distributed by default with the other binaries for that system from for future releases. Bitcoin-Qt with 64-bit ARM is not yet supported by default.

  • Bitcoin Core #13298: The node now sends all announcements (invs) for new transactions to all of its incoming peers at the same time, after a random delay. Previously, Satoshi Nakamoto added a feature to Bitcoin (the software) that waited for a different random delay for each peer before sending an announcement so that a transaction would propagate around the network somewhat unpredictably, preventing spy nodes from being able to assume that the first peer they received a transaction from was likely the peer that created it.

    However, later investigators realized that someone operating multiple spy nodes could make multiple connections to each node to increase their chances of being the first to receive a given transaction, allowing the spy to again guess which node create the transaction. This merge improves the situation by preventing a spy making multiple connections from receiving any more information than a spy with one connection. Outgoing connections (which are selected by the node itself using certain rules) continue to use the old behavior so that transactions continue to propagate unpredictably.

    This change might increase transaction propagation delay slightly, although developers commenting on the PR think the effect will be minimal. It may also cause bandwidth usage to be less evenly distributed over time. However, it could (in theory) end up reducing the number of incoming connections to upgraded nodes, if spy nodes no longer find making multiple connections to be useful, reducing overall wasted bandwidth.

  • Bitcoin Core #13652: The abandontransaction RPC has been fixed to abandon all descendant transactions, not just children.

Coming attractions

Next week’s newsletter will feature a field report from Anthony Towns, a developer at Xapo, about how they consolidated around 4 million UTXOs to prepare for potential future fee increases.

We love getting contributions to the newsletter from member companies. If you’d like to share your experiences in implementing better Bitcoin technology, please contact us!