TERM_DEF // UPGRADES_BIPS / BIP_147_NULLDUMMY_SOFT_FORK
BIP 147 (NULLDUMMY
SOFT FORK)
SOFT FORK)
fork/">BIP 147 (NULLDUMMY Soft Fork). Mandating the dummy element of OP_CHECKMULTISIG be empty.
This page sits in the Upgrades & BIPs section — The proposal process and the major upgrades that shaped Bitcoin. 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 Upgrades & BIPs section — The proposal process and the major upgrades that shaped Bitcoin. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_BIP_147_NULLDUMMY_SOFT_FORK_IS
BIP 147 (NULLDUMMY Soft Fork) — at a glance
UPGRADES
BIP 147 — "NULLDUMMY Soft Fork" is a proposal in the SegWit family, first published in 2017 with status final (soft fork). Mandating the dummy element of OP_CHECKMULTISIG be empty. Its technical mechanism: require the dummy item to be the empty byte vector (NULLDUMMY). enforced from SegWit activation; tightens malleability surface
Why it exists
DESIGN
BIP 147 (NULLDUMMY Soft Fork) exists to solve a specific problem: fix the OP_CHECKMULTISIG off-by-one dummy item from being any value. Without a written, numbered spec, every wallet and node implementer would interpret the requirement differently — and Bitcoin's value depends on every implementation agreeing exactly. BIP-147 is the single source of truth for this concern, so any new client built today can match the behaviour of every client built since 2017.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
Mechanically, BIP-147 require the dummy item to be the empty byte vector (NULLDUMMY). Because this is a soft fork, old software accepts new behaviour without modification — but new behaviour is rejected if old software produces it. Adoption today: enforced from SegWit activation; tightens malleability surface.
1. Author drafts BIP-147 against the BIP-2 template — abstract, motivation, specification, rationale.
2. The text + a reference implementation are posted on the bitcoin-dev list and as a PR to bitcoin/bips.
3. Reviewers tear it apart: ambiguities, security concerns, edge cases, interaction with prior BIPs.
4. Once stable, the BIP editor merges it; it gets a number (BIP-147) and the status "draft" or "proposed".
5. Implementations land in Bitcoin Core / wallets / other clients. For consensus changes: activation parameters chosen.
6. After deployment + adoption, BIP-147 is promoted to "final"; deprecated proposals get "replaced" or "withdrawn".
WORKED_EXAMPLE
BIP-147 — quick reference card
EXAMPLE
BIP number : 147
Title : NULLDUMMY Soft Fork
Area : SegWit
First published: 2017
Status : final
Fork class : soft
Motivation : fix the OP_CHECKMULTISIG off-by-one dummy item from being any value.
Mechanism : require the dummy item to be the empty byte vector (NULLDUMMY).
Where it shows up : enforced from SegWit activation; tightens malleability surface.
Read the spec : https://github.com/bitcoin/bips/blob/master/bip-0147.mediawiki
KEY_PROPERTIES
OPT-IN
No authority can force a BIP on anyone. Adoption depends entirely on whether the change is genuinely useful.
NUMBERED
Stable numeric reference — BIP-147 means the same thing across every wallet, miner, and node that has read this spec.
BIP-147 is a soft fork — it changes (or proposes to change) the rules every full node enforces.
DEPLOYED
Implementations of BIP-147 are in production. The behaviour is observable on mainnet today.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- BIP-147 is a soft fork — old clients accept new behaviour but don't enforce it. Validate against an upgraded node to be sure the rule is being checked.
- Don't confuse "draft" / "proposed" / "final" status — read the BIP header before relying on it in production.
- The BIP number is just an editorial counter — it doesn't imply correctness or stability. Always cross-reference with the latest bitcoin/bips repo.
RELATED_CONCEPTS
Other terms from Upgrades & BIPs — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
BIP 147 (NULLDUMMY Soft Fork)
Mandating the dummy element of OP_CHECKMULTISIG be empty.
Segregated Witness (SegWit, BIP 141)
The 2017 upgrade separating witness-data/">witness data, fixing tx malleability, and introducing weight units.
Taproot (BIP 341)
Tapscript (BIP 342)
Script-language updates accompanying Taproot — new opcodes, removed limits.
Schnorr (BIP 340)
The Schnorr signature spec adopted by Taproot.
BIP Process
The community workflow for proposing, discussing, and tracking changes to Bitcoin.
Soft Fork Activation
MASF (Miner-Activated Soft Fork)
Activation triggered by miner signaling in block versions.