Concurrently Composable Security with Shielded Super-Polynomial Simulators

We propose a new framework for concurrently composable security that relaxes the security notion of UC security. As in previous frameworks, our notion is based on the idea of providing the simulator with super-polynomial resources. However, in our new framework simulators are only given restricted access to the results computed in super-polynomial time. This is done by modeling the super-polynomial resource as a stateful oracle that may directly interact with a functionality without the simulator seeing the communication. We call these oracles “shielded oracles”. Our notion is fully compatible with the UC framework, i. e., protocols proven secure in the UC framework remain secure in our framework. Furthermore, our notion lies strictly between SPS and Angel-based security, while being closed under protocol composition. Shielding away super-polynomial resources allows us to apply new proof techniques where we can replace super-polynomial entities by indistinguishable polynomially bounded entities. This allows us to construct secure protocols in the plain model using weaker primitives than in previous composable frameworks involving simulators with super-poly resources. In particular, we only use non-adaptive-CCA-secure commitments as a building block in our constructions. As a feasibility result, we present a constant-round general MPC protocol in the plain model based on standard assumptions that is secure in our framework.