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Segregated Witness

BIP 141. Eric Lombrozo, Johnson Lau and Pieter Wuille.

Move witness data outside the input script. The transaction id that contracts depend on no longer covers the witness. A new weight limit counts witness bytes less than base bytes.

A reading of the public document. Not a copy of it, and not a claim about a later network that reused the name.

A contract that keys off a transaction id should say whether it means txid or wtxid.
Evidence
Primary paper
Re-measured
No
Assumptions
3
Records linked
3

Code

Source, repository, and what is actually hosted

  • BIP 141https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki
  • bitcoin/bipsRepository taken from the source URL. 97 other studies in this library cite the same repository. This is not an audit.
  • Full upstream textMirrored 2026-10-10 under PD. The desk study is not part of that licence.
  • Add to a reading list

01 Claim ledger

What the paper is allowed to say

Each row is a sentence already in the study. The status is the same on every row: a model claim, not a live measurement.

  1. Claim 01 · Paper model

    The defect

    A transaction's signature data sat in the part of the transaction that is hashed into the id and counted fully toward the block limit, which made upgrades and capacity both awkward.

  2. Claim 02 · Paper model

    The rule

    Move witness data outside the input script. The transaction id that contracts depend on no longer covers the witness. A new weight limit counts witness bytes less than base bytes.

  3. Claim 03 · Paper model

    How it is put together

    The witness is a separate structure. The txid excludes it. The wtxid includes it. Weight, not raw bytes, is the block constraint.

  4. Claim 04 · Paper model

    Where the claim stops

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

02 Three cuts

Observation, model, falsifier

A desk does not stop at the summary. Each claim is cut three ways, using only this study's own assumptions and checks. Nothing here is a new figure.

  1. 01 The defect

    Observation

    What the study says

    A transaction's signature data sat in the part of the transaction that is hashed into the id and counted fully toward the block limit, which made upgrades and capacity both awkward.

    Model

    What has to hold

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

    Falsifier

    What would retire it

    Does the identifier include the witness?

  2. 02 The rule

    Observation

    What the study says

    Move witness data outside the input script. The transaction id that contracts depend on no longer covers the witness. A new weight limit counts witness bytes less than base bytes.

    Model

    What has to hold

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

    Falsifier

    What would retire it

    Does the identifier include the witness?

  3. 03 How it is put together

    Observation

    What the study says

    The witness is a separate structure. The txid excludes it. The wtxid includes it. Weight, not raw bytes, is the block constraint.

    Model

    What has to hold

    Old nodes treat these outputs as anyone-can-spend unless they upgrade. That was the activation argument.

    Falsifier

    What would retire it

    Does the identifier include the witness?

  4. 04 Where the claim stops

    Observation

    What the study says

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

    Model

    What has to hold

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

    Falsifier

    What would retire it

    What does a non-upgraded node think a segwit output is?

03 Sequence

One action, as an operating tape

  1. 01Spend a segwit output by providing a witness.
  2. 02Hash the base transaction for the txid.
  3. 03Count witness bytes with the discount the BIP specifies.
  4. 04Does the identifier include the witness?

04 Load-bearing

The argument, and where a pitch drops it

  1. Why it is still on the desk

    The cut

    A contract that keys off a transaction id should say whether it means txid or wtxid.

    Why it carries weight

    If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.

    Where it is dropped

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

  2. After the text

    The cut

    Taproot later nested a new witness version. Segwit is the split that made that possible.

    Why it carries weight

    If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.

    Where it is dropped

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

05 Register

What has to be true

  • Model · Not re-measured

    Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.

  • Cryptography · Not re-measured

    The discount is a policy. It is not a moral claim about signatures.

  • Model · Not re-measured

    Old nodes treat these outputs as anyone-can-spend unless they upgrade. That was the activation argument.

06 Divergence

What happened after the paper

Taproot later nested a new witness version. Segwit is the split that made that possible.

A later client, parameter set, or reward formula is a different object. Cite this paper for the mechanism. Cite a primary release for the network. This desk has not re-run the proof.

07 Pre-mortem

What to check before you use the idea

  4. 0 of 3 marked on this browser. A mark is a reading note, not a pass, a rating, or a recommendation.

08 Anatomy

The paper, in the order a builder needs

The problem it names

A transaction's signature data sat in the part of the transaction that is hashed into the id and counted fully toward the block limit, which made upgrades and capacity both awkward.

What the design proposes

  • The witness is a separate structure.
  • The txid excludes it. The wtxid includes it.
  • Weight, not raw bytes, is the block constraint.

How the mechanism is specified

  • Spend a segwit output by providing a witness.
  • Hash the base transaction for the txid.
  • Count witness bytes with the discount the BIP specifies.

What this page does not treat as proven

  • Segwit is not the Lightning protocol. It is the transaction format Lightning wanted.
  • The discount is a policy. It is not a moral claim about signatures.
  • Old nodes treat these outputs as anyone-can-spend unless they upgrade. That was the activation argument.

Why the desk still reads it

A contract that keys off a transaction id should say whether it means txid or wtxid.

09 Lexicon

Terms, opened into the record

Witness
The signatures and related data, stored apart from the base transaction.
Weight
The block accounting that discounts witness bytes.

10 Repository

Every linked record on this page

Underlined words open a page that already exists: a concept, a protocol profile, a failure record, or another paper. If a word is not underlined, this desk does not have a record for it.

Concepts

Protocols

Upstream

The file, in full

Licence recorded on the mirror: PD. Canonical file: https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki. Numbers and web addresses in the file open a study or the address itself.

Loading the mirrored file.

This is Blockchain Lab's reading of a public design paper. It is not the paper, not a copy of it, and not an offer of tokens, equity, custody or a partnership. Later network behaviour can diverge from the text. Nothing here is investment, legal or technical advice.

Research status: Design paper. Last reviewed: 1 October 2026. This is a reading of a public paper, not investment, legal or security advice.