International Association
for Cryptologic Research

We revisit the construction of multiparty non-interactive key-exchange protocols with fine-grained security, which was recently studied in (Afshar et al., Eurocrypt 2023). Their work introduced a 4-party non-interactive key exchange with quadratic hardness, and proved it secure in Shoup's generic group model. This positive result was complemented with a proof that n-party non-interactive key exchange with superquadratic security cannot exist in Maurer's generic group model, for any n > 2. Because Shoup's model is stronger than Maurer's model, this leaves a gap between the positive and the negative result, and their work left as an open question the goal of closing this gap, and of obtaining fine-grained non-interactive key exchange without relying on idealized models. In this work, we make significant progress on both questions. We obtain two main results: - A 4-party non-interactive key exchange protocol, assuming the existence of exponentially secure injective pseudorandom generators, and the subexponential hardness of the computational Diffie-Hellman assumption. In addition, our scheme is conceptually simpler, and can be generalized to other settings (with more parties or from other assumptions). - Assuming the existence of non-uniformly secure injective pseudorandom generators with exponential hardness, we further show that our protocol is secure in Maurer's model, albeit with a smaller hardness gap (up to N^1.6), making progress on filling the gap between the positive and the negative result of (Afshar et al., Eurocrypt 2023). Somewhat intriguingly, proving the security of our scheme in Maurer's idealized model turns out to be significantly harder than proving its security in the standard model.

In 1974, Merkle showed that an ideal hash function (modeled as a random oracle) can be used between two parties to agree on a key that remains \emph{mildly} secure against adversaries whose running time is quadratic in those of honest parties. Shortly after, Diffie and Hellman improved this idea to a full-fledged key exchange protocol that is conjectured to be secure against super-polynomial adversaries. Both of these protocols have a crucial aspect in common: they are \emph{non-interactive}, as parties send their single message in parallel, and then they use their secret randomness and the public messages to derive the common key. Constructing $K$-NIKE protocols on well-founded assumptions turned out to be much challenging for $K>2$. For $K=3$ one can do this based on pairing-based assumptions, and for $K>3$ even stronger assumptions such as indistinguishability obfuscations have been used. In this work, we initiate a study of $K$-NIKE protocols in the \emph{fine-grained} setting, in which there is a \emph{polynomial} gap between the running time of the honest parties and that of the adversary. Our goal is to show the possibility, or impossibility, of basing such protocols on weaker assumptions than those of $K$-NIKE protocols for $K \geq 3$. Our contribution is threefold. 1. We show that random oracles can be used to obtain fine-grained $K$-NIKE protocols for \emph{every} constant $K$. In particular, we show how to generalize Merkle's two-party protocol to $K$ parties in such a way that the honest parties ask $n$ queries each, while the adversary needs $n^{K/(K-1)}$ queries to the random oracle to find the key. 2. We then improve the security by further using algebraic structure, while avoiding pairing. In particular, we show that there is a 4-party NIKE in Shoup's generic group model with a \emph{quadratic} gap between the number of queries by the honest parties vs. that of the adversary. 3. Finally, we show a limitation of using purely algebraic methods for obtaining $3$-NIKE. In particular, we show that any $n$-query $3$-NIKE protocol in Maurer's generic group model can be broken by a $O(n^2)$-query attacker. Maurer's GGM is more limited compared with Shoup's both for the parties and the adversary, as there are no explicit labels for the group elements. Despite being more limited, this model still captures the Diffie Hellman protocol. Prior to our work, it was open to break $3$-NIKE protocols in Maurer's model with \emph{any} polynomial number of queries. Our work leaves open to understand the optimality of our $K$-NIKE protocol in the random oracle model, which we conjecture to be optimal, and also to close the gap between our positive result in Shoup's model and the negative result in Maurer's model.

We put forth a template for constructing statistical ZAPs for NP. Our template compiles NIZKs for NP in the hidden bit model (which exist unconditionally) into statistical ZAPs using a new notion of interactive hidden-bit generator (IHBG), which adapts the notion of hidden-bit generator to the plain model by building upon the recent notion of statistically-hiding extractable commitments. We provide a construction of IHBG from the explicit hardness of the decision Diffie-Hellman assumption (where explicit refers to requiring an explicit upper bound on the advantage of any polynomial-time adversary against the assumption) and the existence of statistical ZAPs for a specific simple language, building upon the recent construction of dual-mode hidden-bit generator from (Libert et al., EUROCRYPT 2020). We provide two instantiations of the underlying simple ZAP: 1. Using the recent statistical ZAP for the Diffie-Hellman language of (Couteau and Hartmann, CRYPTO 2020), we obtain statistical ZAPs for NP assuming (the explicit hardness of) DDH in $G_1$ and kernel-DH in $G_2$ (a search assumption which is weaker than DDH), where $(G_1,G_2)$ are groups equipped with an asymmetric pairing. This improves over the recent work of (Lombardi et al., EUROCRYPT 2020) which achieved a relaxed variant of statistical ZAP for NP, under a stronger assumption. 2. Using the recent work of (Couteau et al., EUROCRYPT 2020), we obtain statistical ZAPs for NP assuming the explicit hardness of DDH, together with the assumption that no efficient adversary can break the key-dependent message one-wayness of ElGamal with respect to efficient functions over groups of size $2^\secpar$ with probability better than $\poly(\secpar)/2^{(c + o(1)) \cdot \secpar}$, denoted $2^{-c\secpar}$-\OWKDM, for a constant c = 1/2, in pairing-free groups. Note that the latter is a search discrete-log-style falsifiable assumption, incomparable to DDH (in particular, it is not known to imply public-key encryption).