Alternatives to Non-Malleability: Definitions, Constructions and Applications

We explore whether non-malleability is necessary for the applications typically used to mo-tivate it, and propose two alternatives. The first we call weak non-malleability (wnm) and showthat it suffices to achieve secure contract bidding (the application for which non-malleability wasinitially introduced), despite being strictly weaker than non-malleability. The second we calltag-based non-malleability (tnm), and show that it suffices to construct an efficient universally-composable secure message transmission (SMT) protocol, for which the only previous solu-tion was based on a public key encryption functionality whose security is equivalent to non-malleability. We also demonstrate constructions for wnm and tnm encryption schemes that aresimpler than known constructions of non-malleable encryption schemes.