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Cryptanalysis of a round optimal lattice-based multisignature scheme

Kansal and Dutta recently proposed a multisignature scheme at AFRICACRYPT 2020. This is the first lattice-based multisignature scheme that generates a multisignature in only a single round of interaction and supports public key aggregation. In this letter, we provide a cryptanalysis of this multisignature scheme and demonstrate that the scheme does not satisfy unforgeability requirements. We present an attack strategy to demonstrate that if an adversary obtains a sufficient number of signatures from a signer, he/she can recover the private key of the signer in polynomial time. We also uncover the root cause of the attack and provide a possible solution for this attack to aid future designs of secure multisignature schemes.