On memoryless provers and insincere verifiers

In this article, we introduce a Prover–Verifier model for analysing the computational complexity of a class of constraint satisfaction problems (CSPs) termed boolean binary constraint satisfaction problems (BBCSPs). BBCSPs represent an extremely general class of CSPs and find applications in a wide variety of domains including constraint programming, puzzle solving and program testing. The constraints in a BBCSP permit the combination of multiple theories as opposed to traditional constraint systems in which all constraints belong to the same theory. We establish that each instance of a BBCSP admits a coin-flipping Turing machine that halts in time polynomial in the size of the input. Furthermore, the algorithm is oblivious in that it never sees more than one constraint at a time. The prover, P, in the Prover–Verifier model is endowed with very limited powers. In particular, it has no memory and it can only pose restricted queries to the verifier. The verifier, on the other hand, is both omniscient in that it is cognisant of all the problem details and insincere in that it does not have to decide a priori on the intended proof. However, the verifier must stay consistent in its responses, i.e. it cannot rule out a certain possibility in one response to a query from the prover and then rule in the same possibility in response to a subsequent query. We note that the combination of the resources required by the prover and the type of certificate demanded of the verifier, determine the resources required by an algorithm. Inasmuch as our provers will be memoryless and our verifiers will be asked for extremely simple certificates, our work establishes the existence of a simple, randomised algorithm for BBCSPs. Our model itself serves as a basis for the design of zero-knowledge machine learning algorithms in that the prover ends up learning the proof desired by the verifier. Likewise, our work finds applications in the domain of certifying algorithm design, wherein the goal is to provide a proof of correctness of the algorithm on the input instance by providing an easily checkable certificate.

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