The Layered Games Framework

We establish rigorous foundations to the use of modular, layered design for building complex distributed systems. Layering is key to the design of the Internet and other distributed systems, hence such solid, theoretical foundations are essential, especially when considering adversarial settings, such as for security and cryptographic protocols. We define the basic concepts for modular, layered design: protocols, systems, configurations, executions, and models, and three relations: indistinguishability (between two systems), satisfaction (of a model by a system), and realization (by protocol, of one model over another model). We prove several basic properties, including the layering lemma and the indistinguishability lemma. The indistinguishability lemma shows that if two systems ΓL, ΓR are indistinguishable, and ΓL satisfies some model M, then ΓR also satisfies M. The layering lemma shows that given protocols {πi}i=1, if every protocol πi realizes modelMi over modelMi−1, then the composite protocol π1||...||u realizes model Mu over M0. This allows specification, design and analysis of each layer independently, and combining the results to ensure properties of the complete system. Our framework is based on games, following many works in applied cryptography. This differs from existing frameworks allowing compositions of cryptographic protocols, based on simulatability of ideal functionality. Game-based models are more general and flexible than ideal functionality specifications, supporting different adversarial models and avoiding over-specification, which is essential for practical distributed systems and