International Association
for Cryptologic Research

This work resolves the open problem of whether verifiable delay functions (VDFs) can be constructed in the random oracle model. A VDF is a cryptographic primitive that requires a long time to compute (even with parallelization), but produces a unique output that is efficiently and publicly verifiable. We prove that VDFs do not exist in the random oracle model. This also rules out black-box constructions of VDFs from other cryptographic primitives, such as one-way functions, one-way permutations and collision-resistant hash functions. Prior to our work, Mahmoody, Smith and Wu (ICALP 2020) prove that \emph{perfectly unique} VDFs (a much stronger form of VDFs) do not exist in the random oracle model; on the other hand, Ephraim, Freitag, Komargodski, and Pass (Eurocrypt 2020) construct VDFs in the random oracle model assuming the hardness of repeated squaring. Our result is optimal -- we bridge the current gap between previously known impossibility results and existing constructions. We initiate the study of \emph{proof of work functions}, a new cryptographic primitive that shares similarities with both VDFs and proof of works. We show that a stronger form of it does not exist in the random oracle model, leaving open the fascinating possibility of a random-oracle-based construction.

We analyze the post-quantum security of succinct interactive arguments constructed from interactive oracle proofs (IOPs) and vector commitment schemes. Specifically, we prove that an interactive variant of the *BCS transformation* is secure in the standard model against quantum adversaries when the vector commitment scheme is collapse binding. Prior work established the post-quantum security of Kilian's succinct interactive argument, a special case of the BCS transformation for one-message IOPs (i.e., PCPs). That analysis is inherently limited to one message because the reduction, like all prior quantum rewinding reductions, aims to extract classical information (a PCP string) from the quantum argument adversary. Our reduction overcomes this limitation by instead extracting a *quantum algorithm* that implements an IOP adversary; representing such an adversary classically may in general require exponential complexity. Along the way we define *collapse position binding*, which we propose as the ``correct'' definition of collapse binding for vector commitment schemes, eliminating shortcomings of prior definitions. As an application of our results, we obtain post-quantum secure succinct arguments, in the standard model (no oracles), with the *best asymptotic complexity known*.

A relativized succinct argument in the random oracle model (ROM) is a succinct argument in the ROM that can prove/verify the correctness of computations that involve queries to the random oracle. We prove that relativized succinct arguments in the ROM do not exist. The impossibility holds even if the succinct argument is interactive, and even if soundness is computational (rather than statistical). This impossibility puts on a formal footing the commonly-held belief that succinct arguments in the ROM require non-relativizing techniques. Moreover, our results stand in sharp contrast with other oracle models, for which a recent line of work has constructed relativized succinct non-interactive arguments (SNARGs). Indeed, relativized SNARGs are a powerful primitive that, e.g., can be used to obtain constructions of IVC (incrementally-verifiable computation) and PCD (proof-carrying data) based on falsifiable cryptographic assumptions. Our results rule out this approach for IVC and PCD in the ROM.

Sigma protocols are elegant cryptographic proofs that have become a cornerstone of modern cryptography. A notable example is Schnorr's protocol, a zero-knowledge proof-of-knowledge of a discrete logarithm. Despite extensive research, the security of Schnorr's protocol in the standard model is not fully understood. In this paper we study \emph{Kilian's protocol}, an influential public-coin interactive protocol that, while not a sigma protocol, shares striking similarities with sigma protocols. The first example of a succinct argument, Kilian's protocol is proved secure via \emph{rewinding}, the same idea used to prove sigma protocols secure. In this paper we show how, similar to Schnorr's protocol, a precise understanding of the security of Kilian's protocol remains elusive. We contribute new insights via upper bounds and lower bounds. \begin{itemize} \item \emph{Upper bounds.} We establish the tightest known bounds on the security of Kilian's protocol in the standard model, via strict-time reductions and via expected-time reductions. Prior analyses are strict-time reductions that incur large overheads or assume restrictive properties of the PCP underlying Kilian's protocol. \item \emph{Lower bounds.} We prove that significantly improving on the bounds that we establish for Kilian's protocol would imply improving the security analysis of Schnorr's protocol beyond the current state-of-the-art (an open problem). This partly explains the difficulties in obtaining tight bounds for Kilian's protocol. \end{itemize}

Proof-carrying data (PCD) is a powerful cryptographic primitive that allows mutually distrustful parties to perform distributed computation in an efficiently verifiable manner. Real-world deployments of PCD have sparked keen interest within the applied community and industry. Known constructions of PCD are obtained by recursively-composing SNARKs or related primitives. Unfortunately, known security analyses incur expensive blowups, which practitioners have disregarded as the analyses would lead to setting parameters that are prohibitively expensive. In this work we study the concrete security of recursive composition, with the goal of better understanding how to reasonably set parameters for certain PCD constructions of practical interest. Our main result is that PCD obtained from SNARKs with \emph{straightline knowledge soundness} has essentially the same security as the underlying SNARK (i.e., recursive composition incurs essentially no security loss). We describe how straightline knowledge soundness is achieved by SNARKs in several oracle models, which results in a highly efficient security analysis of PCD that makes black-box use of the SNARK's oracle (there is no need to instantiated the oracle to carry out the security reduction). As a notable application, our work offers an idealized model that provides new, albeit heuristic, insights for the concrete security of \emph{recursive STARKs} used in blockchain systems. Our work could be viewed as partial evidence justifying the parameter choices for recursive STARKs made by practitioners.