Improved Concurrent-Secure Blind Schnorr Signatures

Since the work of Chaum in ’82, the problem of designing secure blind signature protocols for existing signature schemes has been of great interest. In particular, when considering Schnorr signatures, nowadays used in Bitcoin, designing corresponding efficient and secure blind signature schemes is very challenging in light of the ROS attack (Eurocrypt’21), which affected all previous efficient constructions. Currently, the main positive result about concurrent-secure blind Schnorr signatures is the one of Fuchsbauer and Wolf (Eurocrypt’24). Their construction is quite demanding, indeed it requires trusted parameters, non-interactive zero-knowledge arguments and CPA-secure public-key encryption. Moreover, it is proven secure under a game-based definition only, is limited to computational blindness and is vulnerable to harvest now “link” later quantum attacks. Nicely, their construction is also a predicate blind signature (PBS) scheme, allowing signers to have some partial control on the content of the blindly signed message. In this work, we show neat improvements to the state-of-the-art presenting a new construction for concurrent-secure blind Schnorr signatures that relies on milder/reduced cryptographic assumptions, enjoys statistical blindness, replaces the problematic trusted setup with a non-programmable random oracle (NPRO), and satisfies also a one-sided simulation-based property providing deniability in PBSs.