International Association for Cryptologic Research

International Association
for Cryptologic Research


Leo de Castro


Functional Commitments for All Functions, with Transparent Setup and from SIS
Leo de Castro Chris Peikert
A *functional commitment* scheme enables a user to concisely commit to a function from a specified family, then later concisely and verifiably reveal values of the function at desired inputs. Useful special cases, which have seen applications across cryptography, include vector commitments and polynomial commitments. To date, functional commitments have been constructed (under falsifiable assumptions) only for functions that are essentially *linear*, with one recent exception that works for arbitrarily complex functions. However, that scheme operates in a strong and non-standard model, requiring an online, trusted authority to generate special keys for any opened function inputs. In this work, we give the first functional commitment scheme for nonlinear functions---indeed, for *all functions* of any bounded complexity---under a standard setup and a falsifiable assumption. Specifically, the setup is ``transparent,'' requiring only public randomness (and not any trusted entity), and the assumption is the hardness of the standard Short Integer Solution~(SIS) lattice problem. Our construction also has other attractive features, including: *stateless updates* via generic composability; excellent *asymptotic efficiency* for the verifier, and also for the committer in important special cases like vector and polynomial commitments, via preprocessing; and *post-quantum security*, since it is based on SIS.
Lightweight, Maliciously Secure Verifiable Function Secret Sharing 📺
Leo de Castro Anitgoni Polychroniadou
In this work, we present a lightweight construction of verifiable two-party function secret sharing (FSS) for point functions and multi-point functions. Our verifiability method is lightweight in two ways. Firstly, it is concretely efficient, making use of only symmetric key operations and no public key or MPC techniques are involved. Our performance is comparable with the state-of-the-art non-verifiable DPF constructions, and we outperform all prior DPF verification techniques in both computation and communication complexity, which we demonstrate with an implementation of our scheme. Secondly, our verification procedure is essentially unconstrained. It will verify that distributed point function (DPF) shares correspond to some point function irrespective of the output group size, the structure of the DPF output, or the set of points on which the DPF must be evaluated. This is in stark contrast with prior works, which depended on at least one and often all three of these constraints. In addition, our construction is the first DPF verification protocol that can verify general DPFs while remaining secure even if one server is malicious. Prior work on maliciously secure DPF verification could only verify DPFs where the non-zero output is binary and the output space is a large field. As an additional feature, our verification procedure can be batched so that verifying a polynomial number of DPF shares requires the exact same amount of communication as verifying one pair of DPF shares. We combine this packed DPF verification with a novel method for packing DPFs into shares of a multi-point function where the evaluation time, verification time, and verification communication are independent of the number of non-zero points in the function. An immediate corollary of our results are two-server protocols for PIR and PSI that remain secure when any one of the three parties is malicious (either the client or one of the servers).
Asymptotically Quasi-Optimal Cryptography 📺
The question of minimizing the {\em computational overhead} of cryptography was put forward by the work of Ishai, Kushilevitz, Ostrovsky and Sahai (STOC 2008). The main conclusion was that, under plausible assumptions, most cryptographic primitives can be realized with {\em constant} computational overhead. However, this ignores an additive term that may depend polynomially on the (concrete) computational security parameter $\lambda$. In this work, we study the question of obtaining optimal efficiency, up to polylogarithmic factors, for {\em all} choices of $n$ and $\lambda$, where $n$ is the size of the given task. In particular, when $n=\lambda$, we would like the computational cost to be only $\tilde O(\lambda)$. We refer to this goal as {\em asymptotically quasi-optimal} (AQO) cryptography. We start by realizing the first AQO semi-honest batch oblivious linear evaluation (BOLE) protocol. Our protocol applies to OLE over small fields and relies on the near-exponential security of the ring learning with errors (RLWE) assumption. Building on the above and on known constructions of AQO PCPs, we design the first AQO zero-knowledge (ZK) argument system for Boolean circuit satisfiability. Our construction combines a new AQO ZK-PCP construction that respects the AQO property of the underlying PCP along with a technique for converting statistical secrecy into soundness via OLE reversal. Finally, combining the above results, we get AQO secure computation protocols for Boolean circuits with security against malicious parties under RLWE.