Bare Multi-Signature
Some funds should not be movable by one person acting alone. This documents the direct way to require several signatures before an output can be spent, and is honest about what it costs in privacy.
Summary
- Why
- Funds sometimes need approval from several specific people before they can be spent, and there needs to be a direct, simple way to enforce that at the transaction level.
- What
- BRC-47 defines how to write a bare multi-signature locking script, using OP_CHECKMULTISIG directly, that requires a minimum number of signatures from a listed set of public keys before an output can be spent.
- How
- A developer writes a locking script listing the minimum number of required signatures, the public keys of all possible signers, and the total count, then unlocks it later by supplying an OP_0 followed by enough valid signatures to meet the minimum.
What this lets you do
- Lock an output so it needs M of N signers to spend it
- List public keys directly in the output script without hashing or wrapping
- Build a 2-of-3 or similar multi-party approval scheme
- Unlock funds by supplying OP_0 plus the required signatures
- Enforce shared control over funds without a separate coordination service
Written by claude-sonnet-5 from the specification text. Where the two differ, the original is correct.
Reference for an AI
Everything an assistant needs to answer questions about BRC-47 accurately, including what it depends on.
The specification
Abstract
This BRC standard outlines the implementation and use of bare multi-signature (multi-sig) transaction output scripts within the Bitcoin SV digital asset ecosystem. By employing OP_MULTISIG opcodes directly, this approach offers simplicity and ease of implementation while providing enhanced security and access control for transactions. However, it also highlights the trade-offs in terms of privacy for participants. The standard comprises a motivation section, detailing the benefits of bare multi-sig transactions; a specification section, explaining the structure of these transactions; an example section, demonstrating a 2-of-3 multi-signature locking and unlocking script; and a "how it works" section, delving into the fundamentals of bare multi-sig transactions and their role in the Bitcoin SV ecosystem.
Motivation
Bare multi-sig transactions offer a simple and straightforward method to enable multiple parties to authorize a transaction in a decentralized manner. This approach is particularly useful for securing funds, enhancing trust between parties, and enabling flexible access control. Although bare multi-sig transactions come with certain privacy trade-offs, their ease of implementation and direct use of low-level Bitcoin scripting constructs make them a valuable option in the Bitcoin SV ecosystem.
Specification
A bare multi-signature transaction output script adheres to the following structure:
<minimum_signatures> <pubkey1> <pubkey2> ... <pubkeyn> <maximum_signatures> OP_CHECKMULTISIG
minimum_signatures: The minimum number of signatures required to unlock the funds (also known as the "M" value).pubkey1, pubkey2, ..., pubkeyn: The public keys involved in the multi-signature scheme.maximum_signatures: The maximum number of public keys (also known as the "N" value).OP_CHECKMULTISIG: The Bitcoin opcode that validates the provided signatures against the specified public keys.
Example
The following example demonstrates a 2-of-3 multi-signature locking script:
Locking Script:
2 <pubkey1> <pubkey2> <pubkey3> 3 OP_CHECKMULTISIG
To spend the funds locked by this script, an unlocking script containing the required signatures must be provided:
OP_0 <signature1> <signature2>
In this example, the OP_0 opcode is required due to a known bug in the original implementation of the OP_CHECKMULTISIG opcode, which results in an extra item being consumed from the stack.
How it Works
Bare multi-sig transactions work by requiring a minimum number of signatures from a given set of public keys to unlock the funds. When the locking script is executed, the OP_CHECKMULTISIG opcode verifies if the provided signatures correspond to the specified public keys and meet the minimum signature requirement. If the validation succeeds, the funds are unlocked and can be spent in a new transaction.
While bare multi-sig transactions offer simplicity in implementation, they also expose the public keys and the multi-signature scheme directly in the transaction output script, which may reveal information about the participants and their relationships. Despite these privacy concerns, bare multi-sig remains an important tool for creating secure and flexible transactions within the Bitcoin SV digital asset ecosystem.