Blind Multisignatures for Anonymous Tokens with Decentralized Issuance

We propose the first constructions of anonymous tokens with decentralized issuance. Namely, we consider a dynamic set of signers/issuers; a user can obtain a token from any subset of the signers, which is publicly verifiable and unlinkable to the issuance process. To realize this new primitive we formalize the notion of blind multi-signatures (BMS), which allow a user to interact with multiple signers to obtain a (compact) signature; even if all the signers collude they are unable to link a signature to an interaction with any of them. We then present two BMS constructions, one based on BLS signatures and a second based on discrete logarithms without pairings. We prove security of both our constructions in the Algebraic Group Model. We also provide a proof-of-concept implementation and show that it has low-cost verification, which is the most critical operation in blockchain applications.