BA-05 · LESSON 12

Witnesses, wtxids, and block commitments

KEY DEFINITIONS

Terms used in this lesson

Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
Witness transaction ID: An identifier that commits to a transaction including its witness data.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.
Coinbase transaction: The special first transaction in a Bitcoin block, which claims permitted subsidy and fees and has no ordinary prior-output input.
SegWit: Segregated Witness: a Bitcoin upgrade that separates authorization data from the transaction identifier and introduces versioned witness programs.
Signature malleability: The possibility that related signature representations can authorize the same message unless a scheme and its rules enforce a canonical form.
HASH256: Bitcoin's name for applying SHA-256 twice to the same byte sequence.
OP_RETURN: A Bitcoin Script operation commonly used to create an unspendable output carrying a small amount of data.
Outpoint: A transaction ID plus output number that identifies one exact prior output.

Trace witness serialization, txid versus wtxid, the coinbase witness reserved value, and the witness commitment that binds SegWit data into a block.

65 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · witness commitmentWitness bytes change wtxid and the block witness commitmentSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
BASE TXVersion, inputs, outputs
The stripped serialization excludes marker, flag, and witness stacks.
02
WITNESSSignature stack A
Each input has a CompactSize-prefixed witness stack.
03
IDENTIFIERSTxid A, wtxid A
A witness-only mutation leaves txid stable and changes wtxid.
04
WITNESS ROOTRoot A
Ordered wtxids, with a zero coinbase leaf, form the witness Merkle root.
05
COINBASE COMMITHASH256(root A || reserved)
The commitment output binds witness data into the ordinary block Merkle root.

Rule shown: SegWit separates outpoint-stable txid bytes from complete witness commitment through wtxid and the coinbase.

STAGE 1 OF 5BASE TX: Version, inputs, outputs

The stripped serialization excludes marker, flag, and witness stacks.

Governing relationship: SegWit separates outpoint-stable txid bytes from complete witness commitment through wtxid and the coinbase. Complete path: 1. BASE TX, Version, inputs, outputs. The stripped serialization excludes marker, flag, and witness stacks. 2. WITNESS, Signature stack A. Each input has a CompactSize-prefixed witness stack. 3. IDENTIFIERS, Txid A, wtxid A. A witness-only mutation leaves txid stable and changes wtxid. 4. WITNESS ROOT, Root A. Ordered wtxids, with a zero coinbase leaf, form the witness Merkle root. 5. COINBASE COMMIT, HASH256(root A || reserved). The commitment output binds witness data into the ordinary block Merkle root.

1. State transitionWhat exists, what changes, and what results

BEFORE

A SegWit transaction has one input and one witness signature.

MECHANISM

After version, marker 00 and nonzero flag select extended serialization. The following input and output vectors keep their ordinary format.

AFTER

SegWit fixes third-party transaction-identifier malleability by separating the identifier used for outpoints from the commitment that binds witness data.

Text equivalent: Starting from A SegWit transaction has one input and one witness signature. The mechanism begins when After version, marker 00 and nonzero flag select extended serialization. The following input and output vectors keep their ordinary format. The resulting lesson is SegWit fixes third-party transaction-identifier malleability by separating the identifier used for outpoints from the commitment that binds witness data.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Detect SegWit serialization

After version, marker 00 and nonzero flag select extended serialization. The following input and output vectors keep their ordinary format.

Complete path: 1. Detect SegWit serialization: After version, marker 00 and nonzero flag select extended serialization. The following input and output vectors keep their ordinary format. 2. Decode one witness per input: Each input receives a CompactSize item count. Every witness item has its own CompactSize byte length followed by exact bytes. Empty witness stacks are still represented for inputs without witness data. 3. Compute txid and wtxid separately: Reconstruct stripped serialization without marker, flag, or witness for txid. Hash the full extended serialization for wtxid. Both use HASH256 and user-facing reverse-byte display. 4. Build the witness Merkle root: Use each transaction wtxid in order, except the coinbase leaf is 32 zero bytes. Duplicate the final hash at odd tree levels as specified and hash ordered pairs. 5. Verify the coinbase commitment: The coinbase input witness contains a 32-byte reserved value. HASH256 of witness root concatenated with that reserved value must match the aa21a9ed commitment payload in a coinbase OP_RETURN output.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

A node independently reconstructs both identifiers, every witness program result, the witness root, and the coinbase commitment from exact transaction bytes.

CHANGED CONDITION

A miner cannot alter a witness while retaining the old block witness commitment, even though the legacy txid remains unchanged.

Important distinction: Witness data is outside the blockchain and is not committed by proof of work. Witness bytes are transmitted and stored with SegWit blocks. Their wtxids feed a witness commitment inside the coinbase, which is committed by the ordinary block Merkle root and header.

Text equivalent: The accepted path is enforced by A node independently reconstructs both identifiers, every witness program result, the witness root, and the coinbase commitment from exact transaction bytes. If the required condition changes, A miner cannot alter a witness while retaining the old block witness commitment, even though the legacy txid remains unchanged.

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Learn the idea

SegWit serialization inserts marker 00 and flag 01 after version, keeps the same inputs and outputs, then appends one witness stack for every input before locktime. The txid excludes marker, flag, and witnesses. The wtxid includes them. A block commits all transaction witness identifiers through a witness Merkle root and a commitment in the coinbase output.

GUIDED EXPLANATION1/5 ideas inspected

Detect SegWit serialization

After version, marker 00 and nonzero flag select extended serialization. The following input and output vectors keep their ordinary format.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Witness data is outside the blockchain and is not committed by proof of work.”

Witness bytes are transmitted and stored with SegWit blocks. Their wtxids feed a witness commitment inside the coinbase, which is committed by the ordinary block Merkle root and header.

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