Paper 2026/571

Playing Tag with Okamoto-Schnorr: Three-Move Pairing-Free Blind Signatures from DDH

Abstract

This paper presents the first blind signature scheme in a pairing-free group with the following properties: (1) the signing protocol consists of only three moves; (2) the proof of one-more unforgeability relies solely on the Decisional Diffie-Hellman (DDH) assumption in the Random Oracle Model (ROM); and (3) the construction makes only black-box use of the underlying group. This resolves a major open problem in the area, as all prior pairing-free blind signatures either additionally relied on the Algebraic Group Model (AGM) or required at least four moves. Moreover, a recent lower bound by Dietz et al. (ePrint, '26) shows that three moves are optimal for such constructions. Both the communication complexity and the signature size in our scheme consist of a constant number of group elements. Our construction in fact achieves strong one-more unforgeability (which was not known for any of the recent AGM-free constructions requiring four moves), and we also present a partially blind variant. Furthermore, blindness is statistical (in the ROM). Our approach is based on a new construction paradigm that combines a conventional (yet, by itself, not fully secure) blind signature scheme (specifically, the blind Okamoto-Schnorr scheme) with a carefully crafted algebraic MAC.