|Full title||Segregated Witness (Consensus layer)|
|BIP number||BIP 141|
|Purpose||Prevent unintended transaction malleability|
|Signalling bit||Bit 1 "segwit"|
|Supermajority||95% of last 2016 block period|
(abbreviated as SegWit
) is an implemented protocol upgrade intended to provide protection from transaction malleability
and increase block capacity
. SegWit defines a new structure called a witness
that is committed to blocks separately from the transaction merkle tree. This structure contains data required to check transaction validity but is not required to determine transaction effects. In particular, signatures and redeem scripts are moved into this new structure, which does not count towards the traditional 1 MB block size limit
. Instead, a new weight
parameter is defined, and blocks are allowed to have at most 4 million weight units (WU). A byte in the original 1 MB zone of the block weighs 4 WU, but a byte in a witness structure only weighs 1 WU, allowing blocks that are technically larger than 1 MB without a hardforking change.
After the successful activations of OP_CLTV and OP_CSV, SegWit was the last protocol change needed to make the Lightning Network safe to deploy on the Bitcoin network.
Because the witness structure contains Script versioning, it is also possible to make changes to or introduce new opcodes to SegWit scripts that would have originally required a hardfork to function without SegWit.
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