A smart contract on a blockchain cannot keep a secret because its data is replicated on all nodes in a network. To remedy this problem, it has been suggested to combine blockchains with trusted execution environments (TEEs), such as Intel SGX, for executing applications that demand privacy. Untrusted blockchain nodes cannot get access to the data and computations inside the TEE. This paper first explores some pitfalls that arise from the combination of TEEs with blockchains. Since TEEs are, in principle, stateless they are susceptible to rollback attacks, which should be prevented to maintain privacy for the application. However, in blockchains with non-final consensus protocols, such as the proof-of-work in Ethereum and others, the contract execution must handle rollbacks by design. This implies that TEEs for securing blockchain execution cannot be directly used for such blockchains; this approach works only when the consensus decisions are final. Second, this work introduces an architecture and a prototype for smart-contract execution within Intel SGX technology for Hyperledger Fabric, a prominent platform for enterprise blockchain applications. Our system resolves difficulties posed by the execute-order-validate architecture of Fabric and prevents rollback attacks on TEE-based execution as far as possible. For increasing security, our design encapsulates each application on the blockchain within its own enclave that shields it from the host system. An evaluation shows that the overhead moving execution into SGX is within 10%-20% for a sealed-bid auction application.

We present a new abstraction based on crusader agreement called Binding Crusader Agreement (BCA) for solving binary consensus in the asynchronous setting against an adaptive adversary. BCA has the validity, agreement, and termination properties of crusader agreement in addition to a new property called binding. Binding states that before the first non-faulty party terminates, there is a value v ∈ {0, 1} such that no non-faulty party can output the value v in any continuation of the execution. We believe that reasoning about binding explicitly, as a first order goal, greatly helps algorithm design, clarity, and analysis. Using our framework, we solve several versions of asynchronous binary agreement against an adaptive adversary in a simple and modular manner that either improves or matches the efficiency of state of the art solutions. We do this via new BCA protocols, given a strong common coin, and via new Graded BCA protocols given an ε-good common coin. For crash failures, we reduce the expected time to terminate and we provide termination bounds that are linear in the goodness of the common coin. For Byzantine failures, we improve the expected time to terminate in the computational setting with threshold signatures, and match the state of the art in the information theoretic setting, both with a strong common coin and with an ε-good common coin.

Intel Software Guard Extensions (SGX) provides a trusted execution environment (TEE) to run code and operate sensitive data. SGX provides runtime hardware protection where both code and data are protected even if other code components are malicious. However, recently many attacks targeting SGX have been identified and introduced that can thwart the hardware defence provided by SGX. In this paper we present a survey of all attacks specifically targeting Intel SGX that are known to the authors, to date. We categorized the attacks based on their implementation details into 7 different categories. We also look into the available defence mechanisms against identified attacks and categorize the available types of mitigations for each presented attack.