PISA Prototype Combines Payjoin and CISA to Cut Bitcoin Transaction Sizes

Developers Fabian Jahr and Dan Gould demonstrated PISA at the Plan ₿ event in Lugano, showcasing a prototype that merges Payjoin with Cross-Input Signature Aggregation (CISA). The demonstration showed a two-input transaction saving 16 virtual bytes (vB) by employing a single 64-byte aggregate signature across inputs. Estimates show CISA proposals could lower transaction sizes and fees by 7% to 26%, with each extra input beyond the first saving about 64 weight units. The project remains experimental on Bitcoin's regtest environment as underlying proposals BIP 77, 458, 459, and 460 remain in draft status.