BA-07 · LESSON 01

SegWit architecture

KEY DEFINITIONS

Terms used in this lesson

SegWit: Segregated Witness: a Bitcoin upgrade that separates authorization data from the transaction identifier and introduces versioned witness programs.
Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
Signature malleability: The possibility that related signature representations can authorize the same message unless a scheme and its rules enforce a canonical form.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.
Witness transaction ID: An identifier that commits to a transaction including its witness data.
Coinbase transaction: The special first transaction in a Bitcoin block, which claims permitted subsidy and fees and has no ordinary prior-output input.
P2WPKH: Pay to Witness Public Key Hash: a native SegWit output for one public-key hash.
P2WSH: Pay to Witness Script Hash: a native SegWit output that commits to a witness script.

SegWit separates witness data, fixes third-party malleability, changes weight accounting, and enables safer second-layer commitments.

35 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 · segwit architectureSegWit separates stable transaction identity from authorization dataSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
BASE TRANSACTIONVersion, inputs, outputs, locktime
The stripped serialization excludes marker, flag, and witness stacks. Its double SHA-256 hash is the txid used by later outpoints.
02
WITNESSSignature and public key or script
Authorization data is serialized per input after the outputs. It remains part of the transaction and block, not an off-chain attachment.
03
IDENTIFIERSTxid excludes, wtxid includes witness
Changing only a witness cannot change the outpoint identifier, which fixes third-party txid malleability for SegWit inputs.
04
BLOCK COMMITMENTCoinbase commits witness root
The coinbase witness commitment binds transaction wtxids into the ordinary block Merkle root and proof of work.
05
WEIGHT AND UPGRADEFour times base bytes plus witness bytes
Witness bytes count one weight unit while stripped bytes count four. Versioned witness programs provide a compatible path for future spending rules.

Rule shown: A SegWit transaction keeps spendable output references in the stripped transaction, moves signatures and scripts into witness fields, and commits those witnesses through the wtxid and the block's witness commitment.

STAGE 1 OF 5BASE TRANSACTION: Version, inputs, outputs, locktime

The stripped serialization excludes marker, flag, and witness stacks. Its double SHA-256 hash is the txid used by later outpoints.

Governing relationship: A SegWit transaction keeps spendable output references in the stripped transaction, moves signatures and scripts into witness fields, and commits those witnesses through the wtxid and the block's witness commitment. Complete path: 1. BASE TRANSACTION, Version, inputs, outputs, locktime. The stripped serialization excludes marker, flag, and witness stacks. Its double SHA-256 hash is the txid used by later outpoints. 2. WITNESS, Signature and public key or script. Authorization data is serialized per input after the outputs. It remains part of the transaction and block, not an off-chain attachment. 3. IDENTIFIERS, Txid excludes, wtxid includes witness. Changing only a witness cannot change the outpoint identifier, which fixes third-party txid malleability for SegWit inputs. 4. BLOCK COMMITMENT, Coinbase commits witness root. The coinbase witness commitment binds transaction wtxids into the ordinary block Merkle root and proof of work. 5. WEIGHT AND UPGRADE, Four times base bytes plus witness bytes. Witness bytes count one weight unit while stripped bytes count four. Versioned witness programs provide a compatible path for future spending rules.

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

BEFORE

A valid SegWit transaction has txid T and wtxid W. Consider a modified copy with one changed signature byte that still parses.

MECHANISM

An output script contains a version and program. Version 0 defines P2WPKH and P2WSH semantics from program length.

AFTER

Excluding witness from txid fixes a dependency identifier problem; it does not make witness optional or unvalidated.

Text equivalent: Starting from A valid SegWit transaction has txid T and wtxid W. Consider a modified copy with one changed signature byte that still parses. The mechanism begins when An output script contains a version and program. Version 0 defines P2WPKH and P2WSH semantics from program length. The resulting lesson is Excluding witness from txid fixes a dependency identifier problem; it does not make witness optional or unvalidated.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 6

Commit a witness program

An output script contains a version and program. Version 0 defines P2WPKH and P2WSH semantics from program length.

Complete path: 1. Commit a witness program: An output script contains a version and program. Version 0 defines P2WPKH and P2WSH semantics from program length. 2. Move authorization to witness: A native SegWit input leaves scriptSig empty and provides signatures, keys, scripts, or other satisfaction data in per-input witness stacks. 3. Separate identifiers: txid hashes the serialization without witness. wtxid hashes the serialization with marker, flag, and witness, so changing witness affects wtxid but not txid. 4. Commit witnesses in the block: The coinbase contains a witness commitment derived from transaction wtxids, ensuring witness data remains part of validated block history. 5. Apply weight accounting: Non-witness bytes count four weight units and witness bytes one, with a maximum block weight. This discounts witness data without removing its validation cost. 6. Build safer dependencies: Because a third party cannot change a SegWit input's txid by altering its witness, protocols can refer to an unconfirmed parent txid with fewer malleability risks.

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

ACCEPTED PATH

Non-witness and witness serializations independently reproduce txid and wtxid, block coinbase commitment binds witness data, and weight prices both byte regions.

CHANGED CONDITION

Witness mutation preserves txid but changes wtxid and signature validity; output mutation changes both identifiers and the signed transaction commitment.

Important distinction: SegWit removed signatures from the blockchain. Witness signatures are transmitted, validated, stored subject to node configuration, and committed in blocks. They are separated from legacy txid calculation, not removed from consensus.

Text equivalent: The accepted path is enforced by Non-witness and witness serializations independently reproduce txid and wtxid, block coinbase commitment binds witness data, and weight prices both byte regions. If the required condition changes, Witness mutation preserves txid but changes wtxid and signature validity; output mutation changes both identifiers and the signed transaction commitment.

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Learn the idea

Segregated Witness, or SegWit, moved input authorization data into a separate witness structure, introduced versioned witness programs, changed block resource accounting, and committed witness data through the block's coinbase. It fixed third-party txid malleability for SegWit inputs because their witness bytes are excluded from txid while remaining committed by wtxid and the block.

GUIDED EXPLANATION1/6 ideas inspected

Commit a witness program

An output script contains a version and program. Version 0 defines P2WPKH and P2WSH semantics from program length.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“SegWit removed signatures from the blockchain.”

Witness signatures are transmitted, validated, stored subject to node configuration, and committed in blocks. They are separated from legacy txid calculation, not removed from consensus.

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