Fabian Jahr explains **CISA** — Cross-Input Signature Aggregation — compressing every signature in a Bitcoin transaction into one.

The more inputs a transaction has, the more it saves. So the biggest winners are the transactions that need the most inputs: **CoinJoin and PayJoin**, expensive today precisely because each participant brings their own signature. CISA drops that penalty, making collaborative transactions cheaper without giving up their privacy. And it isn’t only a cypherpunk concern — anyone batching payments, exchanges most of all, pays the same per-signature cost.

CISA is an umbrella rather than a single proposal. Two BIPs, both authored by Fabian, are the building blocks: **BIP 458** (half-aggregation — non-interactive, roughly halves signature size) and **BIP 459** (full-aggregation — interactive, DahLIAS, one 64-byte signature for the whole transaction).

**Guest:** Fabian Jahr, Bitcoin Core contributor, author of BIP 458 and BIP 459 and of the Human Rights Foundation’s CISA research paper.

**Links**

- Optech — https://bitcoinops.org/en/topics/cross-input-signature-aggregation/

- HRF research paper — https://hrf.org/latest/cisa-research-paper/

- cisaresearch.org — https://cisaresearch.org/

*Recorded live at bitcoin++ Toronto 2026.*