Cross-input signature aggregation (CISA)
Also covering Half aggregation
Cross-input signature aggregation (CISA) is a proposal to reduce the number of signatures a transaction requires. In theory, every signature required to make a transaction valid could be combined into a single signature that covers the whole transaction.
For example, Alice controls two P2TR UTXOs. Normally, if she creates a transaction spending both UTXOs with a keypath spend, she’ll need to include one 16-vbyte signature in each output. However, any node could aggregate both public keys from the UTXOs and Alice could produce a single 16-vbyte MuSig-style scriptless multisigature that corresponded to the aggregate public key, proving that she controlled the private key for both of the original public keys.
Although inputs would still need to include a significant amount of other data, such as the 36-vbyte outpoint that uniquely identifies the UTXO being spent, CISA could provide a modest reduction in the size of transactions with multiple inputs. It could make the per-participant transaction fees for a coinjoin moderately cheaper than each participant creating a transaction on their own, which could lead to more people using coinjoin-style privacy.
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