Segregated Witness (SegWit) is a protocol upgrade, activated on Bitcoin as a backwards-compatible soft fork, that moves the witness data — the digital signatures authorising spends — out of the main transaction body into a separate structure. By segregating signatures, it fixes transaction malleability, since the transaction identifier no longer depends on mutable signature data, and effectively increases block capacity through a block-weight accounting scheme. SegWit also created the foundation for second-layer protocols such as the Lightning Network and enabled later script upgrades.
Overview
- Before SegWit, a transaction’s signatures were part of the data hashed to produce its transaction identifier. Because signatures could be altered without changing their validity, the identifier was malleable, complicating any protocol that referenced unconfirmed transactions — including payment channels.
- SegWit separates the witness data so that the transaction identifier is computed only from the non-witness portion. This makes identifiers stable, eliminating malleability and unblocking second-layer designs. The witness is still committed to the block, via a dedicated commitment, so security is preserved.
- SegWit also introduced block weight, a new accounting unit that discounts witness data. This raised the effective number of transactions a block can hold without a contentious hard fork to the nominal block-size limit, making it a soft fork that older nodes still accept.
Mechanisms
- Witness segregation — Signature and script-witness data are placed in a separate transaction field, removed from the legacy transaction serialisation used for the identifier.
- Malleability fix — Because identifiers no longer depend on mutable signatures, transactions become referentially stable, a prerequisite for reliable payment channels.
- Block weight accounting — A weight metric counts witness bytes at a discount, increasing effective capacity while keeping legacy nodes compatible and contrasting with a simple Block Size increase.
- Witness commitment — The block commits to witness data through an additional structure linked via the Merkle Tree, preserving integrity and verifiability.
- Soft-fork activation — Deployment occurred as a soft fork through the network’s Consensus Mechanism, so non-upgraded nodes continue to validate blocks.
- Script extensibility — Versioned witness programs created a clean path for future upgrades, later used by subsequent improvements to Bitcoin scripting.
Applications
- Layer-two payments — SegWit’s malleability fix made the Lightning Network and other channel-based systems practical, enabling fast, low-cost off-chain payments.
- Capacity improvement — The block-weight scheme raised throughput, advancing Blockchain Scalability without a hard fork.
- Fee efficiency — SegWit transactions receive a discount on witness data, lowering fees for users who adopt the format.
- Foundation for upgrades — Versioned witness programs provided the mechanism for later script enhancements.
- Improved transaction handling — Stable identifiers simplify wallets, exchanges, and any system that tracks unconfirmed Transactions.