International Association for Cryptologic Research

International Association
for Cryptologic Research


Ali El Kaafarani


Lossy CSI-FiSh: Efficient Signature Scheme with Tight Reduction to Decisional CSIDH-512 📺
Recently, Beullens, Kleinjung, and Vercauteren (Asiacrypt’19) provided the first practical isogeny-based digital signature, obtained from the Fiat-Shamir (FS) paradigm. They worked with the CSIDH-512 parameters and passed through a new record class group computation. However, as with all standard FS signatures, the security proof is highly non-tight and the concrete parameters are set under the heuristic that the only way to attack the scheme is by finding collisions for a hash function. In this paper, we propose an FS-style signature scheme, called Lossy CSI-FiSh, constructed using the CSIDH-512 parameters and with a security proof based on the “Lossy Keys” technique introduced by Kiltz, Lyubashevsky and Schaffner (Eurocrypt’18). Lossy CSI-FiSh is provably secure under the same assumption which underlies the security of the key exchange protocol CSIDH (Castryck et al. (Asiacrypt’18)) and is almost as efficient as CSI-FiSh. For instance, aiming for small signature size, our scheme is expected to take around $$approx 800$$  ms to sign/verify while producing signatures of size $$approx 280$$ bytes. This is only twice slower than CSI-FiSh while having similar signature size for the same parameter set. As an additional benefit, our scheme is by construction secure both in the classical and quantum random oracle model.
Attribute-Based Signatures for Unbounded Circuits in the ROM and Efficient Instantiations from Lattices
Ali El Kaafarani Shuichi Katsumata
Attribute-based signature (ABS), originally introduced by Maji et al. (CT-RSA’11), represents an essential mechanism to allow for fine-grained authentication. A user associated with an attribute x can sign w.r.t. a given public policy C only if his attribute satisfies C, i.e., $$C(x)=1$$ C(x)=1. So far, much effort on constructing bilinear map-based ABS schemes have been made, where the state-of-the-art scheme of Sakai et al. (PKC’16) supports the very wide class of unbounded circuits as policies. However, construction of ABS schemes without bilinear maps are less investigated, where it was not until recently that Tsabary (TCC’17) showed a lattice-based ABS scheme supporting bounded circuits as policies, at the cost of weakening the security requirement.In this work, we affirmatively close the gap between ABS schemes based on bilinear maps and lattices by constructing the first lattice-based ABS scheme for unbounded circuits in the random oracle model. We start our work by providing a generic construction of ABS schemes for unbounded-circuits in the rand om oracle model, which in turn implies that one-way functions are sufficient to construct ABS schemes. To prove security, we formalize and prove a generalization of the Forking Lemma, which we call “general multi-forking lemma with oracle access”, capturing the situation where the simulator is interacting with some algorithms he cannot rewind, and also covering many features of the recent lattice-based ZKPs. This, in fact, was a formalization lacking in many existing anonymous signatures from lattices so far (e.g., group signatures). Therefore, this formalization is believed to be of independent interest. Finally, we provide a concrete instantiation of our generic ABS construction from lattices by introducing a new $$\varSigma $$ Σ-protocol, that highly departs from the previously known techniques, for proving possession of a valid signature of the lattice-based signature scheme of Boyen (PKC’10).