New Circular Security Counterexamples from Decision Linear and Learning with Errors.

We investigate new constructions of n-circular counterexamples with a focus on the case of $$n=2$$. We have a particular interest in what qualities a cryptosystem must have to be able to separate such circular security from IND-CPA or IND-CCA security. To start, we ask whether there is something special about the asymmetry in bilinear groups that is inherent in the works of [1, 18] or whether it is actually the bilinearity that matters. As a further question, we explore whether such counterexamples are derivable from other assumptions such as the Learning with Errors LWE problem. If it were difficult to find such counterexamples, this might bolster our confidence in using 2-circular encryption as a method of bootstrapping Fully Homomorphic Encryption systems that are based on lattice assumptions. The results of this paper broadly expand the class of assumptions under which we can build 2-circular counterexamples. We first show for any constant $$k \\ge 2$$ how to build counterexamples from a bilinear group under the decision k-linear assumption. Recall that the decision k-linear assumption becomes progressively weaker as k becomes larger. This means that we can instantiate counterexamples from symmetric bilinear groups and shows that asymmetric groups do not have any inherently special property needed for this problem. We then show how to create 2-circular counterexamples from the Learning with Errors problem. This extends the reach of these systems beyond bilinear groups and obfuscation.