Witness

The witness is the segregated portion of a bitcoin transaction that carries signatures and other unlocking data, introduced by Segregated Witness in August 2017. Moving this data outside the traditional transaction structure fixed txid malleability and priced witness bytes at a 75 percent discount in the block weight formula.

Why it matters

Separating the witness had consequences well beyond tidiness. Stable transaction identifiers made chains of pre-signed transactions safe, which the Lightning Network requires. The discount raised effective block capacity without a disruptive hard fork. And witness versioning created a clean upgrade path for new script systems: Taproot, activated in 2021, is simply witness version 1, deployed without disturbing anything that came before.

Because old nodes skip witness data entirely, SegWit shipped as a backward-compatible soft fork, one of the more consequential engineering compromises in bitcoin's history.

How it works

Each input in a SegWit transaction has an associated witness stack holding its signatures and script data. The legacy txid is computed without witnesses, while a second identifier, the wtxid, covers everything, and blocks commit to all witness data through a Merkle root placed in the coinbase transaction. Upgraded nodes verify the full package; pre-SegWit nodes see valid transactions and remain none the wiser.