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SegWit (Segregated Witness)

Also called: SegWit, Segregated Witness, SegWit v0, BIP 141, BIP141, native SegWit.

SegWit (Segregated Witness) is the 2017 soft fork that moved signatures into a separate witness, fixing malleability and introducing block weight.

SegWit, short for Segregated Witness, is a soft fork that separates signature data (the witness) from the rest of a transaction. It was specified mainly in BIP 141 by Eric Lombrozo, Johnson Lau and Pieter Wuille, and activated at block 481,824 on 24 August 2017.

What SegWit changed

Malleability fix. Signatures no longer affect the txid, ending third-party transaction malleability. That made chains of unconfirmed transactions safe, which the Lightning Network depends on.
Block weight. The 1 MB size limit became a limit of 4,000,000 weight units, with witness bytes counted at a quarter of the cost (block weight, vbytes).
New signature hashing. BIP 143 signs the input amount and avoids quadratic hashing (sighash).
Script versioning. Witness versions 1 to 16 were left for future upgrades; version 1 became Taproot.

SegWit output types and addresses

Native SegWit outputs are P2WPKH and P2WSH, with bech32 addresses starting bc1q (BIP 173). Nested SegWit wraps them in P2SH for older wallets (addresses starting with 3).

The SegWit BIPs

BIP	Covers
141	Consensus rules
143	Signature hashing
144	Peer-to-peer serialization
145	getblocktemplate changes
147	NULLDUMMY malleability fix
173	Bech32 addresses

Activation in 2017 went through BIP 9 miner signaling, the BIP 148 user-activated soft fork proposal and BIP 91, which locked SegWit in. More in the SegWit guide and BIP 141 vs BIP 341.

BIPs that define SegWit (Segregated Witness)

BIP 141 (SegWit): Segregated Witness (Consensus layer): Deployed. Defines Segregated Witness (SegWit), which moves signatures into a separate "witness" section of each transaction while remaining compatible with older nodes. This fixed transaction malleability,…
BIP 143 (SegWit signature hashing): Transaction Signature Verification for Version 0 Witness Program: Deployed. Changes how signatures are calculated for SegWit transactions so that verifying them no longer requires re-hashing the same data many times. It also makes the signature commit to the amount being…
BIP 144: Segregated Witness (Peer Services): Deployed. Specifies the network messages nodes use to pass SegWit transactions and blocks to one another, including the separate signature data. It lets upgraded nodes share full witness data while older…
BIP 145: getblocktemplate Updates for Segregated Witness: Deployed. Updates getblocktemplate, the interface miners and mining pools use to request a block to work on, so it supports SegWit. Without this, miners could not build blocks that include SegWit transactions…
BIP 147 (NULLDUMMY): Dealing with dummy stack element malleability: Deployed. Fixes a quirk in Bitcoin's multisignature opcode, which consumes an extra unused value that anyone could modify without invalidating the transaction. Requiring that value to be empty removes one…
BIP 173 (Bech32): Base32 address format for native v0-16 witness outputs: Deployed. Introduces Bech32, an address format for native SegWit outputs, recognizable by addresses starting with "bc1". It uses only lowercase letters and numbers and a strong checksum that catches typing…
BIP 91 (SegWit lock-in): Reduced threshold Segwit MASF: Deployed. Proposed a way to activate SegWit with support from a simple majority of mining power instead of the 95 percent originally required. Participating miners would reject blocks that did not signal for…
BIP 148 (UASF): Mandatory activation of segwit deployment: Deployed. Proposed a user-activated soft fork (UASF), where nodes would reject blocks that did not signal support for SegWit after a set date. It was a way for users, rather than miners alone, to push SegWit…

Related terms

Witness: The witness is the part of a SegWit transaction that holds signatures and scripts, kept out of the txid so it can't be changed to alter the transaction ID.
Block weight: Block weight is the SegWit-era measure of block size: non-witness bytes count 4 units and witness bytes 1, with a limit of 4,000,000 weight units.
Transaction malleability: Transaction malleability is the ability to change a transaction's ID without changing what it does, for example by re-encoding its signatures.
P2WPKH: P2WPKH (Pay to Witness Public Key Hash) is the native SegWit single-key output type. Its bc1q addresses are always 42 characters long.
Bech32: Bech32 is the checksummed base32 address format from BIP 173, used for native SegWit version 0 addresses that start with bc1q.