TERM_DEF // TRANSACTION / WITNESS
WITNESS
Witness. Per-input field introduced by SegWit holding signatures and witness scripts outside the legacy serialization.
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_WITNESS_IS
Witness — at a glance
TRANSACTION
Witness is part of the SERIALIZATION sub-family of transaction-level concepts. The signature(s) and other input-unlock data, segregated from the legacy transaction body into its own stack-of-stacks section. Per-input field introduced by SegWit holding signatures and witness scripts outside the legacy serialization.
Why it exists
DESIGN
Moving signatures out of scriptSig fixes malleability (txid no longer changes if a signature is re-encoded) AND lets the witness data carry a 4× weight discount, effectively expanding block capacity.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
For each input, the witness contains a list of stack items (the script's input data, not opcodes). For P2WPKH: 2 items — signature + pubkey. For P2WSH: M items + the redeemScript. For P2TR key-path: 1 item — the Schnorr signature.
1. Wallet selects UTXOs whose total value covers the spend amount + estimated fee (coin selection).
2. Wallet builds the transaction body: version, inputs (each with prev txid + vout + sequence), outputs (each with value + scriptPubKey), locktime.
3. Wallet computes the sighash for each input (which parts of the tx the signature commits to — controlled by the SIGHASH flag).
4. Wallet signs each input with the right private key. Witness/scriptSig is populated with the resulting signatures + pubkeys.
5. Tx is broadcast to peers. Mempool propagation: tens of seconds globally.
6. A miner includes it in a block. Confirmation count grows by 1 per block; after ~6 the tx is effectively final.
WORKED_EXAMPLE
Witness — SERIALIZATION
EXAMPLE
P2WPKH witness:
0x02 ← 2 items follow
0x47 ← 71 bytes
<71-byte ECDSA signature + sighash type>
0x21 ← 33 bytes
<33-byte compressed pubkey>
P2TR key-path witness:
0x01 ← 1 item
0x40 ← 64 bytes (or 65 with sighash byte)
<64-byte Schnorr signature>
KEY_PROPERTIES
ATOMIC
A transaction is either fully accepted into a block or fully rejected. There is no partial spend.
IMMUTABLE INPUTS
A UTXO can only ever be spent once. After that, it is permanently consumed.
NO BALANCES
IRREVERSIBLE
After ~6 confirmations (~1 hour), reversing the tx would require more proof-of-work than the honest network produces.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- address-reuse/">Address reuse degrades privacy — every reuse links more of your UTXOs together publicly. Modern wallets generate a fresh address per receive.
- Fee estimation matters: under-pay and your tx sits in the mempool for hours; over-pay and you tip the miner more than necessary. Use a fee estimator.
- "Change outputs" must go back to a fresh address you control. A missing output/">change output sends the difference to the miner as fee — a known footgun.
- RBF (Replace-By-Fee) lets you re-broadcast a tx with a higher fee. Useful for stuck txs but means a 0-confirmation tx is never truly final.
RELATED_CONCEPTS
Other terms from Transaction — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
Witness
Per-input field introduced by SegWit holding signatures and witness scripts outside the legacy serialization.
Transaction (Tx)
A signed payload spending one or more UTXOs and creating new ones; every state change in Bitcoin is a tx.
Raw Transaction
The hex-serialized bytes of a transaction, ready to broadcast or analyze.
Transaction ID (TXID)
wTXID (Witness TXID)
HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
Input
A reference to a previous output being spent, plus the data (scriptSig/witness) authorizing the spend.
Output
An (amount, scriptPubKey) pair created by a transaction; spendable later by a tx whose input references it.
UTXO (Unspent Transaction Output)
An output that hasn't been spent yet; your "balance" is the sum of UTXOs you can sign for.