PISA demo shows how Payjoin and CISA could shrink Bitcoin transactions

A prototype unveiled at Plan ₿ Week in Lugano pairs Payjoin privacy with cross-input signature aggregation to cut fees on collaborative payments 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. 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.

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