MuSig2 is an improved version of Musig1, a new signature scheme designed to enhance multi-signature transactions.

Original title: "What Is MuSig2?"

Written by: Che Kohler

Compile: MK, MarsBit

As a Bitcoin user, you use a digital signature and a specific message to prove that you are the sender, which is your legal order as the owner of the private key. These digital signatures are used to show that you know the private key associated with the address without exposing your key to the network.

Bitcoin has several signature schemes, all designed to perform different tasks. As the web matures and we use it in different ways, there is an urgent need to optimize how these signatures are created.

With the introduction of Taproot, Bitcoin continues to evolve, and developers can use these soft forks to build improved signature schemes that provide security, efficiency, and privacy for Satoshis. One of these recent developments is Musig1 and its improved version MuSig2, a new signature scheme designed to enhance multi-signature transactions.

What is a multi-signature transaction in Bitcoin?

When you send bitcoins from one wallet to another, you're usually using a single-signature transaction because that's all you need to transfer funds.

And multi-signature, commonly known as multi-signature, refers to the need for more than one key to authorize a Bitcoin transaction. Distributed signatures are often used to decentralize responsibility for bitcoin ownership, but are also used to communicate with second-layer solutions like the Lightning Network or the Liquid Network.

Bitcoin's oldest multi-signature trick, the "CHECKMULTISIG" OP-code, can be used to create these types of wallets/transactions, it requires less communication from the multi-signature transaction signers, but is less private than the MuSig1 multi-signature scheme Poor, the latter improves user privacy at the cost of adding an extra step to the signing process.

What is MuSig1?

MuSig1 is a multi-signature scheme that allows multiple parties to co-sign a single message or transaction, requiring a certain number of signatures to authorize the transaction. This enhances security and provides additional control over funds. MuSig uses less block space than traditional script-based multisig, but as a tradeoff, it requires more interaction between participants.

MuSig1, based on Schnorr signatures, is a major improvement over the traditional ECDSA-based multi-signature scheme used in Bitcoin. It allows key aggregation, meaning a group of signers can create a single joint public key and generate a single signature for a transaction.

This process not only simplifies multi-signature transactions, but also reduces transaction size, lowers transaction fees and improves privacy.

What is MuSig2?

MuSig2 is an upgraded version of MuSig1, providing better security, efficiency and privacy features. MuSig2, proposed by Blockstream researchers in November 2020, is a two-round multi-signature scheme, meaning it only requires two rounds of communication between signers to create a valid signature.

This improvement makes MuSig2 more practical and user-friendly, as it reduces the complexity of coordinating multiple signers.

What is the difference between MuSig1 and MuSig2?

The main difference between MuSig1 and MuSig2 is their communication rounds and security model:

Communication rounds

MuSig1 is a three-round multi-signature scheme that requires three rounds of communication steps to create a valid signature. In contrast, MuSig2 is a two-round scheme that makes it faster and easier for signers to coordinate their actions.

Security Model

MuSig1 relies on a random oracle model (ROM) for its security proofs, which assumes the existence of an ideal hash function. However, ROM is an idealized model and may not accurately represent real-world hash functions. On the other hand, MuSig2's security proof is based on the Algebraic Group Model (AGM), which provides a more realistic representation of cryptographic primitives and thus provides stronger security guarantees.

What will MuSig2 bring to Bitcoin?

The introduction of Bitcoin by MuSig2 will bring several benefits, including:

Improve efficiency

MuSig2's two-round communication model reduces the complexity of coordinating multi-signature transactions, making it faster and more convenient for users.

Improve Privacy

Like MuSig1, MuSig2 allows key aggregation, which means that multi-signature transactions look like regular single-signature transactions on the blockchain. This feature improves privacy by making it harder for third parties to identify multi-signature transactions.

Greater flexibility

MuSig2 supports more complex signature strategies such as threshold signatures and hierarchical key structures, providing users with greater control over their funds.

Better Security

MuSig2's security proof in AGM provides a stronger security guarantee than MuSig1's ROM-based proof, providing a stronger foundation for multi-signature transactions.

What are the application scenarios of MuSig2?

MuSig2 is particularly beneficial for application scenarios that require enhanced security, privacy, and efficiency. For example:

Shared Custody

MuSig2 allows multiple parties to securely manage shared funds, such as in trusts or joint bank accounts, by requiring a certain number of signatures to authorize transactions. This feature reduces the risk of a single point of failure and ensures that no single actor can unilaterally access funds.

Cold storage

MuSig2 can be used to create multi-signature cold storage solutions where you, as an individual, may wish to split your wallet access into multiple keys rather than a single key to access stored funds. This setup adds an extra layer of security because it reduces the possibility of unauthorized access due to key theft or loss.

Privacy-preserving wallet

Wallets that prioritize user privacy can implement MuSig2, creating multi-signature transactions that are indistinguishable from regular single-signature transactions. This feature helps users maintain their privacy on the blockchain without sacrificing the security and control provided by multi-signature transactions.

Layer 2 Protocol Improvements

MuSig2 can be used in second-layer protocols, such as the Lightning Network, to secure off-chain transactions and increase their efficiency. By aggregating signatures, MuSig2 reduces the on-chain footprint of layer-2 transactions, which reduces transaction fees for opening and closing channels, reduces blockchain bloat, and makes it harder for chain analysis companies to identify lightning transactions from standard transactions .

MuSig2 will also assist in optimizing the liquid network's anchoring mechanism, making it cheaper and easier for federation members to manage their bridges. Additionally, Liquid Network is also Taproot enabled, allowing users of L-BTC to use MuSig2 in production, so any innovations built on top of MuSig at a fundamental level can be replicated on Liquid Network and vice versa.

MuSig improvements are essential for Bitcoin.

MuSig2 is a promising development in the Bitcoin world, offering improved security, efficiency, and privacy features compared to its predecessor, MuSig1.

By simplifying multi-signature transactions and providing stronger security guarantees, MuSig2 has the potential to unlock new use cases and enhance existing ones, making Bitcoin more accessible and secure for users around the world.

As the technology matures and becomes widely available, we can expect MuSig2 to play an important role in shaping the future of Bitcoin and blockchain technology.

If you want to learn more about MuSig2 on Bitcoin, use this article as a starting point and don't take our word for it. Take the time to research other sources, you can start with the following resources:

• BIP 0327
• MuSig2: simple two-round Schnorr multi-signature
• Bitcoin Ops – MuSig
• MuSig2: simple two-round Schnorr multi-signature
• MuSig1 paper
• MuSig2 paper