On Round-Optimal Blind Signatures from NIWI Arguments of Knowledge

252 / BBB

2026-09-24

Nicolas Adelmann

15:45

A blind-signature scheme is an interactive cryptographic two-party protocol that allows
a user to obtain a signature on a message without revealing the message to the signer or
enabling the signer to link the resulting signature to the corresponding issuing interaction.
Blind-signature schemes play an important role in privacy-enhancing technologies, such
as electronic cash. Recent work by Ciampi, Della Monica, and Visconti presents a GUC-
secure, round-optimal blind-signature scheme based on one-way functions in the global
non-programmable random-oracle (NPRO) model, using signature schemes, extractable
commitments and non-interactive witness-indistinguishable (NIWI) proof systems. How-
ever, in every issuing interaction, the signer must generate and transmit an additional NIWI
proof.
We modify this construction by moving the signer-side NIWI proof into the verification
key so that it is generated only once, during key generation. We prove that the resulting
construction is correct, blind, and one-more unforgeable in the game-based model. We then
modify the protocol to satisfy a stronger unforgeability notion for blind signatures that
is closely inspired by the UC security definition for blind signatures. We prove that the
second construction satisfies this stronger notion. Finally, we show that our first modified
construction does not, in general, satisfy this stronger unforgeability notion.