Batched Splicing Considered Risky

Offchain transactions are characterized by a single input signed by all parties involved, requiring unanimous consensus for any changes. This mechanism inherently protects against double-spending, as all signatories must agree to any transaction, essentially making unauthorized transactions impossible without full cooperation. In contrast, 0-conf transactions can be approved by a single participant, making them more vulnerable to double-spending attacks since a transaction could be altered or not include funds promised to a receiver after the initial agreement.

A significant concern arises with splice-in designs, which aim to blend the characteristics of both offchain and 0-conf transactions. These designs typically involve one input that operates on an n-of-n consensus basis (similar to offchain transactions) but also include additional inputs that may only require a single signature from one of the parties involved (mirroring 0-conf transactions). This structure presents a security risk by potentially allowing an individual participant to execute a double-spend attack on the splice transaction. By combining elements of both transaction types, splice designs inadvertently inherit the vulnerabilities of 0-conf transactions while compromising the security assurances provided by fully offchain methods. This blend creates a scenario where the theoretical safety and consensus benefits of offchain transactions are diluted by the introduction of 0-conf's susceptibility to unilateral actions by individual participants.