PISA demo shows how Payjoin and CISA could shrink Bitcoin transactions

Privacy on Bitcoin has always come with a surcharge. A new prototype called PISA is trying to lower the bill. Developers demonstrated PISA on October 23, 2026, during the Plan ₿ Week and Forum events in Lugano, Switzerland. The project combines Payjoin with Cross-Input Signature Aggregation, better known as CISA, to make collaborative Bitcoin transactions smaller and cheaper. What PISA actually does Start with the two ingredients. Payjoin, defined in BIP77 and BIP78, is a method for collaborative transactions with built-in privacy features. Collaborative transactions pull in inputs from more than one source. That makes them harder for outside observers to read, which is the privacy benefit. The catch is size. Every input in a Bitcoin transaction normally carries its own signature, and every signature takes up space. CISA attacks that problem directly. It lets multiple signatures across different inputs be aggregated together, rather than each one sitting in the transaction separately. PISA stitches these two ideas into a single working prototype. The goal is to keep Payjoin’s privacy while using CISA to trim the transaction’s footprint. The numbers and the people behind them The sa...