Witness data

The witness is the part of a SegWit transaction holding signatures and unlocking data. It sits outside the txid and is discounted. Learn how it works.

What's in it

For each input, the witness holds a list of items: the data needed to satisfy that input's lock.

Where it sits

In serialized transactions, the witness comes after the outputs and before the locktime. See Anatomy of a raw transaction.

Committed separately

Because the txid excludes the witness, blocks need another way to commit to it. The The coinbase transaction includes a witness commitment: an OP_RETURN data outputs output holding the merkle root of every transaction's wtxid (the hash that includes witness data).

Discounted

Witness bytes count as 1 weight unit instead of 4. See Transaction size, weight and vbytes.

Reading a witness on MyBitcoinExplorer

Open any SegWit transaction and look at the raw data: after the outputs comes a witness stack for each input. For a typical bc1q spend it holds two items: a signature (about 71 bytes) and a public key (33 bytes).

Why witness data could be discounted

Witness data is needed to validate a transaction once, but it doesn't change the UTXO set nodes must keep. Discounting it rewards transactions that put less long-term burden on every node.