Paper 2023/1780 Original: eprint.iacr.org/2023/1780 ↗

Pairing-Free Blind Signatures from CDH Assumptions

Abstract

We present the first concurrently-secure blind signatures making black-box use of a pairing-free group for which unforgeability, in the random oracle model, can be proved {\em without} relying on the algebraic group model (AGM), thus resolving a long-standing open question. Prior pairing-free blind signatures without AGM proofs have only been proved secure for bounded concurrency, relied on computationally expensive non-black-box use of NIZKs, or had complexity growing with the number of signing sessions due to the use of boosting techniques. Our most efficient constructions rely on the chosen-target CDH assumption and can be seen as blind versions of signatures by Goh and Jarecki (EUROCRYPT '03) and Chevallier-Mames (CRYPTO '05). We also give a less efficient scheme with security based on (plain) CDH. The underlying signing protocols consist of four (in order to achieve regular unforgeability) or five moves (for strong unforgeability). All schemes are proved statistically blind in the random oracle model.