  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.
