Nonwitness

Nonwitness is a term used in Bitcoin and related protocols to describe the portion of a transaction that does not contain witness data introduced with SegWit. In SegWit transactions the transaction data is conceptually split into non-witness data and witness data. The non-witness data comprises the legacy fields: version, input count, inputs (each with the previous transaction ID, output index, scriptSig, and sequence), output count, outputs, and locktime. The witness data is stored separately and contains the signatures and scripts necessary to prove spending of SegWit inputs. Non-witness data contributes to the base size of a transaction, and in the SegWit weight calculation, base size is multiplied by four, while witness data adds to the total weight but not to the base size.

In practice, non-witness data serves as the traditional, pre-SegWit portion of a transaction, whereas witness data is the additional information used to authorize spending of SegWit outputs. This separation enables the adoption of SegWit features while preserving compatibility with older systems, and it underpins how transactions are counted toward block size and weight.

In Bitcoin’s Partially Signed Bitcoin Transaction (PSBT) standard, the term appears in fields such as non_witness_utxo. The non_witness_utxo field stores the complete previous transaction data for the input being signed, allowing validation of non-SegWit aspects without relying on witness data. Conversely, the witness_utxo field holds the value and scriptPubKey of the output being spent for SegWit inputs. The two fields facilitate offline signing and flexible workflows.

See also: SegWit, witness data, PSBT, BIP141, BIP174. Variants of spelling include non-witness, nonwitness, and related terminology.