CommiTEE : An Efficient and Secure Commit-Chain Protocol using TEEs.

Permissionless blockchain systems such as Bitcoin or Ethereum are slow and expensive, since transactions are processed in a distributed network by a large set of parties. To improve on these shortcomings, a prominent approach is given by so-called 2nd-layer protocols. In these protocols parties process transactions off-chain directly between each other, thereby drastically reducing the costly and slow interaction with the blockchain. In particular, in the optimistic case, when parties behave honestly, no interaction with the blockchain is needed. One of the most popular off-chain solutions are Plasma protocols (often also called commit-chains). These protocols are orchestrated by a so-called operator that maintains the system and processes transactions between parties. Importantly, the operator is trustless, i.e., even if it is malicious users of the system are guaranteed to not lose funds. To achieve this guarantee, Plasma protocols are highly complex and rely on involved and expensive dispute resolution processes. This has significantly slowed down development and deployment of these systems. In this work we propose COMMITEE - a simple and efficient Plasma system leveraging the power of trusted execution environments (TEE). Besides its simplicity, our protocol requires minimal interaction with the blockchain, thereby drastically reducing costs and improving efficiency. An additional benefit of our solution is that it allows for switching between operators, in case the main operator goes offline due to system failure, or behaving maliciously. We implemented and evaluated our system over Ethereum and show that it is at least 2 times (and in some cases more than 16 times) cheaper in terms of communication complexity when compared to existing Plasma implementations. Moreover, for protocols using zero-knowledge proofs (like NOCUST-ZKP), COMMITEE decreases the on-chain gas cost by a factor approximate to 19 compared to prior solution.