On the Size of Pairing-Based Non-interactive Arguments

Non-interactive arguments enable a prover to convince a verifier that a statement is true. Recently there has been a lot of progress both in theory and practice on constructing highly efficient non-interactive arguments with small size and low verification complexity, so-called succinct non-interactive arguments (SNARGs) and succinct non-interactive arguments of knowledge (SNARKs). Many constructions of SNARGs rely on pairing-based cryptography. In these constructions a proof consists of a number of group elements and the verification consists of checking a number of pairing product equations. The question we address in this article is how efficient pairing-based SNARGs can be. Our first contribution is a pairing-based (preprocessing) SNARK for arithmetic circuit satisfiability, which is an NP-complete language. In our SNARK we work with asymmetric pairings for higher efficiency, a proof is only 3 group elements, and verification consists of checking a single pairing product equations using 3 pairings in total. Our SNARK is zero-knowledge and does not reveal anything about the witness the prover uses to make the proof. As our second contribution we answer an open question of Bitansky, Chiesa, Ishai, Ostrovsky and Paneth (TCC 2013) by showing that linear interactive proofs cannot have a linear decision procedure. It follows from this that SNARGs where the prover and verifier use generic asymmetric bilinear group operations cannot consist of a single group element. This gives the first lower bound for pairing-based SNARGs. It remains an intriguing open problem whether this lower bound can be extended to rule out 2 group element SNARGs, which would prove optimality of our 3 element construction

Impossibilities in Succinct Arguments: Black-box Extraction and More

The celebrated result by Gentry and Wichs established a theoretical barrier for succinct non-interactive arguments (SNARGs), showing that for (expressive enough) hard-on-average languages, we must assume non-falsifiable assumptions. We further investigate those barriers by showing new negative and positive results related to the proof size. 1. We start by formalizing a folklore lower bound for the proof size of black-box extractable arguments based on the hardness of the language. This separates knowledge-sound SNARGs (SNARKs) in the random oracle model (that can have black-box extraction) and those in the standard model. 2. We find a positive result in the non-adaptive setting. Under the existence of non-adaptively sound SNARGs (without extractability) and from standard assumptions, it is possible to build SNARKs with black-box extractability for a non-trivial subset of NP. 3. On the other hand, we show that (under some mild assumptions) all NP languages cannot have SNARKs with black-box extractability even in the non-adaptive setting. 4. The Gentry-Wichs result does not account for the preprocessing model, under which fall several efficient constructions. We show that also, in the preprocessing model, it is impossible to construct SNARGs that rely on falsifiable assumptions in a black-box way. Along the way, we identify a class of non-trivial languages, which we dub “trapdoor languages”, that bypass some of these impossibility results

Lattice-Based zk-SNARKs from Square Span Programs

Zero-knowledge SNARKs (zk-SNARKs) are non-interactive proof systems with short (i.e., independent of the size of the witness) and efficiently verifiable proofs. They elegantly resolve the juxtaposition of individual privacy and public trust, by providing an efficient way of demonstrating knowledge of secret information without actually revealing it. To this day, zk-SNARKs are widely deployed all over the planet and are used to keep alive a system worth billion of euros, namely the cryptocurrency Zcash. However, all current SNARKs implementations rely on so-called pre-quantum assumptions and, for this reason, are not expected to withstand cryptanalitic efforts over the next few decades. In this work, we introduce a new zk-SNARK that can be instantiated from lattice-based assumptions, and which is thus believed to be post-quantum secure. We provide a generalization in the spirit of Gennaro et al. (Eurocrypt'13) to the SNARK of Danezis et al. (Asiacrypt'14) that is based on Square Span Programs (SSP) and relies on weaker computational assumptions. We focus on designated-verifier proofs and propose a protocol in which a proof consists of just 5 LWE encodings. We provide a concrete choice of parameters, showing that our construction is practically instantiable

Lattice-based Zero-knowledge SNARGs for Arithmetic Circuits

Succinct non-interactive arguments (SNARGs) enable verifying NP computations with substantially lower complexity than that required for classical NP verification. In this work, we construct a zero-knowledge SNARG candidate that relies only on lattice-based assumptions which are claimed to hold even in the presence of quantum computers. Central to this new construction is the notion of linear-targeted malleability introduced by Bitansky et al. (TCC 2013) and the conjecture that variants of Regev encryption satisfy this property. Then, using the efficient characterization of NP languages as Square Arithmetic Programs we build the first quantum-resilient zk-SNARG for arithmetic circuits with a constant-size proof consisting of only 2 lattice-based ciphertexts. Our protocol is designated-verifier, achieves zero-knowledge and has shorter proofs and shorter CRS than the previous such schemes, e.g. Boneh et al. (Eurocrypt 2017)

On Black-Box Knowledge-Sound Commit-And-Prove SNARKs

Gentry and Wichs proved that adaptively sound SNARGs for hard languages need non-falsifiable assumptions. Lipmaa and Pavlyk claimed Gentry-Wichs is tight by constructing a non-adaptively sound zk-SNARG FANA for NP from falsifiable assumptions. We show that FANA is flawed. We define and construct a fully algebraic $F$-position-binding vector commitment scheme VCF. We construct a concretely efficient commit-and-prove zk-SNARK Punic, a version of FANA with an additional VCF commitment to the witness. Punic satisfies semi-adaptive black-box $G$-knowledge-soundness, a new natural knowledge-soundness notion for commit-and-prove SNARKs. We use a new proof technique to achieve global consistency using a functional somewhere-extractable commitment scheme to extract vector commitment\u27s local proofs

Separating Succinct Non-Interactive Arguments From All Falsifiable Assumptions

In this paper, we study succinct computationally sound proofs (arguments) for NP, whose communication complexity is polylogarithmic the instance and witness sizes. The seminal works of Kilian \u2792 and Micali \u2794 show that such arguments can be constructed under standard cryptographic hardness assumptions with four rounds of interaction, and that they be made non-interactive in the random-oracle model. The latter construction also gives us some evidence that succinct non interactive arguments (SNARGs) may exist in the standard model with a common reference string (CRS), by replacing the oracle with a sufficiently complicated hash function whose description goes in the CRS. However, we currently do not know of any construction of SNARGs with a formal proof of security under any simple cryptographic assumption. In this work, we give a broad black-box separation result, showing that black-box reductions cannot be used to prove the security of any SNARG construction based on any falsifiable cryptographic assumption. This includes essentially all common assumptions used in cryptography (one-way functions, trapdoor permutations, DDH, RSA, LWE etc.). More generally, we say that an assumption is falsifiable if it can be modeled as an interactive game between an adversary and an efficient challenger that can efficiently decide if the adversary won the game. This is similar, in spirit, to the notion of falsifiability of Naor \u2703, and captures the fact that we can efficiently check if an adversarial strategy breaks the assumption. Our separation result also extends to designated verifier SNARGs, where the verifier needs a trapdoor associated with the CRS to verify arguments, and slightly succinct SNARGs, whose size is only required to be sublinear in the statement and witness size

Non-Interactive Proofs: What Assumptions Are Sufficient?

A non-Interactive proof system allows a prover to convince a verifier that a statement is true by sending a single round of messages. In this thesis, we study under what assumptions can we build non-interactive proof systems with succinct verification and zero-knowledge. We obtain the following results. - Succinct Arguments: We construct the first non-interactive succinct arguments (SNARGs) for P from standard assumptions. Our construction is based on the polynomial hardness of Learning with Errors (LWE). - Zero-Knowledge: We build the first non-interactive zero-knowledge proof systems (NIZKs) for NP from sub-exponential Decisional Diffie-Hellman (DDH) assumption in the standard groups, without use of groups with pairings. To obtain our results, we build SNARGs for batch-NP from LWE and correlation intractable hash functions for TC^0 from sub-exponential DDH assumption, respectively, which may be of independent interest

Correlation Intractability and SNARGs from Sub-exponential DDH

We provide the first constructions of SNARGs for Batch-NP and P based solely on the sub-exponential Decisional Diffie Hellman (DDH) assumption. Our schemes achieve poly-logarithmic proof sizes. Central to our results and of independent interest is a new construction of correlation-intractable hash functions for ``small input\u27\u27 product relations verifiable in $\mathsf{TC}^0$, based on sub-exponential DDH

Non-Interactive Publicly-Verifiable Delegation of Committed Programs

In this work, we present the first construction of a fully non-interactive publicly-verifiable delegation scheme for committed programs. More specifically, we consider a setting where Alice is a trusted author who delegates to an untrusted worker the task of hosting a program $P$, represented as a Boolean circuit. Alice also commits to a succinct value based on $P$. Any arbitrary user/verifier without knowledge of $P$ should be convinced that they are receiving from the worker an actual computation of Alice\u27s program on a given input $x$. Before our work, the only object known to imply this challenging form of delegation was a SNARG/SNARK for $\mathcal{NP}$. This is because from the point of view of the user/verifier, the program $P$ is an unknown witness to the computation. However, constructing a SNARG for $\mathcal{NP}$ from standard assumptions remains a major open problem. In our work, we show how to achieve delegation in this challenging context assuming only the hardness of the Learning With Errors (LWE) assumption, bypassing the apparent need for a SNARG for $\mathcal{NP}$