Blockstream Announces New Multisignature Scheme For Bitcoin’s Taproot Update

On November 4, Blockstream released details about their new multisignature scheme, titled MuSig2, which will introduce a new key-aggregation feature (a regular Schnorr signature) that can be processed by Bitcoin once the Taproot update goes live. MuSig2 is said to reduce transaction fees and increase privacy compared to other methods.

According to the announcement, the new multisignature scheme requires only two communication rounds in order to complete the signing process. Blockstream, along with cryptographer Yannick Seurin from the French National Cybersecurity Agency (ANSSI), published a preprint of MuSig2 security, thoroughly explaining its new features. The scheme is said to be undergoing a peer review at the moment, and is expected to be presented to the community in January, 2021, at the Real World Crypto 2021 conference.

In their statement, Blockstream also discussed some of the benefits and upgrades that MuSig2 will provide compared to the first version of the scheme. The company claimed that MuSig2 allows for non-interactive signing, where users can create a signature with only two rounds of communication instead of having to visit the signing device safety deposit twice in order to do so.

Moreover, with the help of additional computation, the new version will allow users to go through the signing process for the same time as they would in other CHECKMULTISIG-based wallets, while retaining the simplicity and efficiency of the product.

Blockstream made a comparison between MuSig1 and MuSig2, saying that most applications will probably choose the latter as simplicity is a dominant factor for adoption. They also claimed that support for non-interactive signing increases usability significantly, with the only potential drawback being that open signing sessions need to be stored on a persistent medium.

In such cases, when a user starts a MuSig2 signing session, saves the session to a hard drive, and then performs a hard drive backup, they can finish the signing session and restore the backup while completing the session for a second time. Thus, they can create two signatures with the same nonce, which can be used to steal their secret key.

The announcement concluded:

“If the Bitcoin community chooses to adopt the Taproot softfork, MuSig2 is applicable to a range of Blockstream’s products, such as Blockstream Green and c-lightning, as well as to the Liquid peg mechanism. Indeed, the Liquid Network may activate Taproot before Bitcoin does, letting us use MuSig2 in production even earlier!”