Paper 2019/552

Continuous Space-Bounded Non-Malleable Codes from Stronger Proofs-of-Space

Binyi Chen, Yilei Chen, Kristina Hostáková, and Pratyay Mukherjee

Abstract

Non-malleable codes are encoding schemes that provide protections against various classes of tampering attacks. Recently Faust et al. (CRYPTO 2017) initiated the study of space- bounded non-malleable codes that provide such protections against tampering within small- space devices. They put forward a construction based on any non-interactive proof-of-space (NIPoS). However, the scheme only protects against an a priori bounded number of tampering attacks. We construct non-malleable codes that are resilient to an unbounded polynomial number of space-bounded tamperings. Towards that we introduce a stronger variant of NIPoS called proof-extractable NIPoS (PExt-NIPoS), and propose two approaches of constructing such a primitive. Using a new proof strategy we show that the generic encoding scheme of Faust et al. achieves unbounded tamper-resilience when instantiated with a PExt-NIPoS. We show two methods to construct PExt-NIPoS: 1. The first method uses a special family of “memory-hard” graphs, called challenge-hard graphs (CHG), a notion we introduce here. We instantiate such family of graphs based on an extension of stack of localized expanders (first used by Ren and Devadas in the context of proof-of-space). In addition, we show that the graph construction used as a building block for the proof-of-space by Dziembowski et al. (CRYPTO 2015) satisfies challenge-hardness as well. These two CHG-instantiations lead to continuous space-bounded NMC with different features in the random oracle model. 2. Our second instantiation relies on a new measurable property, called uniqueness of NIPoS. We show that standard extractability can be upgraded to proof-extractability if the NIPoS also has uniqueness. We propose a simple heuristic construction of NIPoS, that achieves (partial) uniqueness, based on a candidate memory-hard function in the standard model and a publicly verifiable computation with small-space verification. Instantiating the encoding scheme of Faust et al. with this NIPoS, we obtain a continuous space-bounded NMC that supports the “most practical” parameters, complementing the provably secure but “relatively impractical” CHG-based constructions. Additionally, we revisit the construction of Faust et al. and observe that due to the lack of uniqueness of their NIPoS, the resulting encoding schemes yield “highly impractical” parameters in the continuous setting. We conclude the paper with a comparative study of all our non-malleable code constructions with an estimation of concrete parameters.