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MuSig2: Simple Two-Round Schnorr Multisignatures

This raises the question of which scheme to use in a given application. The table above demonstrates that there is no reason to prefer MuSig1 over MuSig2. In practice, we expect that most applications will choose MuSig2 over MuSig-DN because simplicity is a dominant factor for adoption. This is especially true when creating interoperable implementations since all signers must agree on using the same protocol. Moreover, support for non-interactive signing increases usability significantly.

On the other hand, if open signing sessions need to be stored on a persistent medium, the statelessness property of MuSig-DN is beneficial. To demonstrate the risk with MuSig2 in that scenario, imagine we perform the following sequence of events:

Start a MuSig2 signing session. Save the session to a hard drive. Perform a hard drive backup. Finish the signing session. Restore the backup. Complete the session again.

The result is that we create two signatures with the same nonce, which can be used to steal our secret key. Therefore, implementers of MuSig2 must be careful and make sure that the above scenario can not occur. In contrast, MuSig-DN is robust against this attack.

The challenge of constructing two-round multi-signatures

Constructing a simple Schnorr multisignature scheme that needs only two rounds and is still secure under concurrent sessions (i.e., if some signer is involved in multiple signing sessions simultaneously) was an unsolved research problem. All previous attempts (including an early version of the MuSig1 paper) suffered from a subtle attack discovered by Drijvers et al. in which an attacker opens many sessions with a victim signer and is able to obtain a signature on a message that the victim did not intend to sign.

Let us quickly look at what makes MuSig2 secure under concurrent sessions. Where in MuSig1, each signer i creates a single nonce, in MuSig2, each signer creates two nonces R_i,1, R_i,2, sends them to the other s...

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