Segregated Witness (SegWit) changes how Bitcoin transactions carry spending proofs such as signatures. For SegWit inputs, this information is placed in a witness structure that is excluded from the traditional transaction ID, or txid. The data is still transmitted and checked by validating nodes; it is not removed from Bitcoin’s verification process.
Stable identifiers and block commitments
Separating witness data addresses important forms of transaction malleability, where changing signing-related data could change an identifier without changing the intended payment. The protection depends on the transaction’s inputs and signing conditions; it should not be described as eliminating every possible transaction modification.
A separate witness transaction identifier, wtxid, includes witness data. Blocks commit to this data through a witness commitment associated with the coinbase transaction. SegWit was deployed as a soft fork.
Weight and practical effects
SegWit introduced block weight: non-witness data counts more heavily than witness data. This can reduce the virtual size of suitable transactions and improve block-space use. Actual fees still depend on transaction structure and the prevailing fee rate.
More stable transaction identifiers support protocols that build transactions on other unconfirmed transactions, including Lightning payment channels. SegWit does not make Bitcoin transactions private, fee-free, or automatically confirmed.