Instance-Dependent Commitment and Its Non-Malleability

In this paper, we define a new security property called “instance-non-malleability” for the Instance-dependent commitment (IDC). Our definition can be consistent with the definition of non-malleability for zero-knowledge proofs, which was not the case for previous definitions of non-malleability for commitments. Our definition of instance-non-malleable instance-dependent commitment requires the non-malleability of the instances as well as the committed messages. We also present a DDHbased IDC scheme, which satisfies previous definitions of non-malleability but not our definition of instance-nonmalleable IDC, to show that instance-non-malleability is a stronger notion. Finally, we modify our DDH-based construction to satisfy our definition of instance-non-malleable IDC. The security of our construction is proved in the random oracle model.