CISA
Cross-input signature aggregation (CISA) was part of the original promise for adopting Schnorr signatures in Bitcoin, but it did not end up as a part of the Taproot soft fork and was never even formalized until now. With CISA, the inputs of a transaction share signature data instead of each carrying its own signature. That makes transactions with many inputs smaller and cheaper, and it makes CoinJoins cheaper per participant the larger they get. This talk will present the three BIPs that now specify a transaction-wide CISA proposal: BIP 458 for half-aggregation, which is non-interactive, BIP 459 for full-aggregation, which compresses all signatures into a constant size but requires an interactive signing session, and a new consensus BIP that adds both to Bitcoin with a new witness version. Each input chooses its own aggregation mode, and no signature can be aggregated without the consent of its signer.
