Segregated Witness (Consensus layer)

Summary

A soft fork that moves signature data into a new structure called the witness, committed to the block separately from the transaction merkle tree. The starting observation is that a transaction's effects are determined by which outputs it consumes and creates; signatures are needed only to validate state, not to determine it. Taking signatures out of the transaction hash structurally eliminates transaction malleability — a third party rewriting how a transaction was signed no longer changes its identifier. The BIP argues this is superior to the canonical-signature approach of BIP62, and explicitly notes that it allows chains of unconfirmed transactions without counterparty risk, an important property for off-chain protocols such as the Lightning Network. The witness commitment is nested into the existing merkle root through the coinbase transaction so the change can deploy as a soft fork, and a weight measure that discounts witness data raises effective block capacity.

Why it matters

Activated in August 2017, it is one of the largest protocol changes in Bitcoin's history. Removing malleability was a precondition for the Lightning Network, and the weight measure raised effective capacity. It works as a set together with BIP143 (signature digest), BIP144 (peer messages), and BIP173 (bech32 addresses).

Related BIPs

• BIP 9 — Version bits with timeout and delay
• BIP 143 — Transaction Signature Verification for Version 0 Witness Program
• BIP 144 — Segregated Witness (Peer Services)
• BIP 173 — Base32 address format for native v0-16 witness outputs
• BIP 341 — Taproot: SegWit version 1 spending rules

Related articles

• Transactions Deep Dive
• Forks & Chain Divergence
• Layer 2 & Scaling