This week’s newsletter describes a proposal that taproot signatures make an additional commitment to spent scriptPubKeys and includes our regular sections with the summary of a Bitcoin Core PR Review Club meeting, a list of releases and release candidates, and descriptions of notable changes to popular Bitcoin infrastructure software.

None this week.

## News

• Request for an additional taproot signature commitment: an idea previously discussed1 among Bitcoin experts would be allowing hardware wallets to automatically sign any transaction as long as it provably increases the user’s balance. This could make it easy for the hardware wallet to automatically participate in coinjoin transactions or LN payment routing.

However, several years ago, Greg Sanders described an attack that could be used to trick a hardware wallet into thinking that its balance was increasing when it was actually decreasing. An attacker would create an unsigned transaction spending two of the hardware wallet’s inputs to two outputs—one output paying the wallet slightly more than the larger of the two inputs and the other output paying the attacker. The attacker would ask for a signature on the first input (without disclosing that the second input belonged to the wallet); the wallet would sign that input after verifying that the output paying back into the wallet was larger than the input. Then the attacker would ask for a signature for the second input—pretending that this was for a completely different transaction—causing the wallet to also sign the second input after again verifying that there was an output larger than the input. Finally, the attacker would put both signatures into a final transaction and broadcast it, stealing money from the wallet. Sanders’s description of the problem also described a potential solution, but it requires that wallets know the scriptPubKeys for each previous output being spent in the transaction.

Last week, Andrew Kozlik posted to the Bitcoin-Dev mailing list to request that taproot signatures directly commit to the scriptPubKey of every input’s previous output. This commitment is already made indirectly by committing to the outpoints of all the transaction’s inputs,2 but making it directly in the transaction digest would allow a Partially Signed Bitcoin Transaction (PSBT) to trustlessly provide signers with a copy of all scriptPubKeys being spent in a transaction. The scriptPubKeys in the PSBT wouldn’t require any trust because, if any of the scriptPubKeys were missing or modified, the signer’s commitment to the scriptPubKeys would be invalid, making the transaction invalid. This would allow hardware wallets to use the solution described by Sanders in 2017 without needing to trust an external program to provide correct copies of the scriptPubKeys being spent.

## Bitcoin Core PR Review Club

In this monthly section, we summarize a recent Bitcoin Core PR Review Club meeting, highlighting some of the important questions and answers. Click on a question below to see a summary of the answer from the meeting.

Flush undo files after last block write is a PR (#17994) by Karl-Johan Alm that changes the way that undo files are flushed to disk. Undo files are created when the UTXO set is updated to reflect the transactions in a new block; if the block is later removed during a reorg, the undo file contains the changes that will restore the UTXO set to its previous state, undoing the effects of originally processing the block.

Most of the discussion was around the basic concepts and the details of the bug fix:

• Can the existing bug result in data loss (and potentially consensus failure)?

No. The bug can result in using disk space unnecessarily, but cannot result in data loss.

• Is it possible to create the undo data for a block without having all the preceding blocks?

No. Creating the undo data for a block requires having the UTXO set at the point that the block was connected. For that, we need to have validated and connected all preceding blocks.

• Do we ever modify block or undo data after it has been written to disk?

In general, no. Block and undo data are write-only. Once they’ve been written to disk, they are not modified. A pruning node may delete block and undo data after it’s been buried by enough new blocks.

• Is undo data written to disk in the same order as the block data?

No. Block data is written in the order that the blocks were received (which may not be in height order, since we fetch blocks in parallel during initial sync), and undo data is written in the order that the blocks were connected. However, undo data always appears in the corresponding rev* file to the blk* file that the block appears in.

• How is space allocated in the block and undo files? Why?

Space is pre-allocated in the files in chunks of 16MB for block files and 1MB for undo files. This is to reduce filesystem fragmentation.

• What does this PR change?

In this PR, the undo file is finalized and flushed to disk when the highest block in the file has been connected, instead of when the corresponding block file is flushed.

## 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.

• Eclair 0.4 is a new major version release that upgrades Eclair’s major dependencies, adds support for the latest version of Bitcoin Core, and deprecates Eclair Node GUI (instead users are encouraged to run Phoenix or Eclair Mobile).

• C-Lightning 0.8.2.1 is a new maintenance release that fixes an incompatibility in large channels (“wumbo channels”) between C-Lightning and Eclair. See the linked release notes for details.

• Bitcoin Core 0.20.0rc1 is a release candidate for the next major version of Bitcoin Core.

## 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 #16224 displays all of the GUI’s translated error messages in both the locale’s language and in English. This can help users find help and quickly describe the problem to developers. Additionally, the English version of the error message is now what is written to the debug log, again to make it easier for developers to comprehend the problem and provide assistance.

• C-Lightning #3659 has channeld create the penalty transaction, as it has enough information about previous commitment transactions. With this change, C-Lightning now supports watchtowers, such as Talaia’s The Eye of Satoshi watchtower. Possible future extensions are to include the HTLC transactions for watchtower settlement and to generate penalty transactions for commitments created before the watchtower plugin was activated.

• Rust-Lightning #539 adds support for option_static_remotekey channels and requires connecting peers to support this feature. In the case of data loss on your end, this feature allows your channel counterparty to pay an untweaked key agreed upon during the initial channel open, which will close the channel and allow your wallet to spend your funds. See Newsletter #67 for more details on this feature.

• LND #4139 extends the gettransactions and listsweeps RPCs to allow passing start and end block heights to only retrieve transactions confirmed within that block range. A value of -1 for the end height may be used to also list unconfirmed transactions.

## Footnotes

1. “You keep the wallet connected to the computer, and the connected computer runs a watch-only lightning wallet, and then communicates with the hardware wallet for signatures on transactions that strictly increase the balance of your channel.” —Excerpt from the rough transcript of a talk about hardware wallets by Stephan Snigirev; transcript by Bryan Bishop

2. In the existing BIP341 specification of taproot, each input commits to the outpoints of every input included in the transaction. Outpoints are the txid and output index (vout) of the output being spent. Txids are a hash of most parts of the transaction containing that output. So a commitment to an outpoint is a commitment to a txid which is a commitment to the previous output (including the output’s scriptPubKey).