This week’s newsletter announces a new maintenance release of Bitcoin Core, describes continued discussion about new signature hashes, and links to a post about possible economic barriers to LN payments crossing different currencies. Descriptions of notable code changes in popular Bitcoin infrastructure projects are also provided.

## Action items

• Upgrade to Bitcoin Core 0.17.1: this new minor version released December 25th restores some previously-deprecated functionality to the listtransactions RPC and includes bug fixes and other improvements. Consider reading the release notes and upgrading.

## News

• Continued sighash discussion: as mentioned in the News section of Newsletter #25, developers on the Bitcoin-Dev mailing list discussed how signature hashes could be modified to give transactions access to new capabilities. Sighashes give spenders the ability to allow their transactions to be modified in specified ways after they are signed—for example, two people who open a payment channel together can use a particular type of sighash to allow either one of them to unilaterally attach additional transaction fees to a channel close transaction.

The most recent discussion in this thread of almost 70 posts has mostly involved edge cases related to new sighash flags, particularly a BIP118-like SIGHASH_NOINPUT_UNSAFE. As part of the discussion, protocol developer Johnson Lau described an optimization for Eltoo-based payment channels. Also discussed is whether the OP_CODESEPARATOR opcode should be disabled in a script update that supports MAST (e.g. via Taproot). That opcode is not in common use, but if you plan to use it in future Script versions, you should comment on the thread.

• Cross-chain LN as an options contract: pseudonymous LN contributor ZmnSCPxj started a thread on the Lightning-Dev mailing list pointing out that users could abuse payments that cross currencies to create almost free short-term options contracts by delaying payment settlement. A previous thread by Corné Plooy in May 2018 described the same thing.

For example, Mallory learns that Bob is willing to route payments from Bitcoin to Litecoin, so she sends a payment from one of her Bitcoin nodes through Bob to one of her Litecoin nodes. If this were a normal payment, she’d settle it immediately by releasing the preimage for the payment’s hashlock—but instead her node delays for 24 hours waiting for the exchange rate to change. If the exchange rate increases in Litecoin’s favor, Mallory settles the payment and receives litecoin today at yesterday’s exchange rate. If the exchange rate stays the same or increases in Bitcoin’s favor, Mallory causes the payment to fail and gets her bitcoin back. Since no fees are charged for failed payments, Mallory received an opportunity to temporarily lock-in the price of Litecoin for nothing but the cost of owning the bitcoins Mallory would’ve traded.

There currently aren’t any known cross-currency LN nodes, but the availability of this trick means that future such nodes could be abused for speculation rather than payment routing. If this turns out to be a real problem and if an acceptable solution isn’t found, it may be the case that payment channel networks for different currencies will be isolated from each other.

## Notable code changes

Notable code changes this week in Bitcoin Core, LND, C-lightning, and libsecp256k1.

• Bitcoin Core #14565 significantly improves the error handling for the importmulti RPC and will return a warnings field for each attempted import with an array of strings describing any problems with the that import (but only if there were any problems).

• Bitcoin Core #14811 updates the getblocktemplate RPC to require that the segwit flag be passed. This helps prevent miners from accidentally not using segwit, which reduces their fee income. See Newsletter #20 for a recent instance where this may have happened to a large mining pool.

• C-Lightning #2172 allows lightningd to be shutdown normally even if it’s operating as the primary process (PID 1), which can be useful in Docker containers. This is, for example, how the open source BTCPay server runs C-Lightning.

• C-Lightning #2188 adds notification subscription handlers that can be used by plugins, with initial support for notifications that the node has connected to a new peer or disconnected from an existing peer. The plugin documentation and sample plugin have been updated for these handlers.

• LND #2374 increases the maximum size of the gRPC messages the lncli tool will accept, raising it from 4 MB to 50 MB. This fixes a problem some nodes were encountering where the describegraph RPC was failing due to the network having grown so large that messages exceeded this limit. Developers using gRPC directly will need to increase their client-side maximum message size setting—descriptions of how to do this have already been added as comments to the PR for python and nodejs. Ultimately it’s expected that the network will grow large enough to exceed even this new maximum, so developers are planning to revamp the relevant RPCs to handle this situation.

• LND #2354 adds a new invoicestate field and deprecates the former settled field in RPCs that get information about invoices. The settled field was boolean but the new state field can support multiple values. Currently this is just “open” or “settled”, but additional future states are planned.