Secure and Efficient Decentralized Bitcoin Mixing Scheme using Trusted Execution Environment

Mixing schemes have been applied by Bitcoin users to break their payment links in the blockchain to enhance privacy. However, most mixing schemes cannot provide secure mixing service without compromising efficiency since they are relying on complex cryptographic techniques or interactive protocols. To provide secure yet efficient mixing service, researchers introduce Intel SGX enclave, which provides Trusted Execution Environment (TEE) with confidentiality and integrity guarantees to execute mixing operations. Unfortunately, users will lose their mixing funds if a malicious service provider compromises the confidentiality guarantee of his/her enclave. Moreover, the scheme cannot scale to a large number of users in a single mixing round, that is, limited scalability. In this paper, we present a novel decentralized mixing scheme with multiple enclaves run by different service providers, which uses Shamir secret sharing scheme and additive homomorphic property of keys in Elliptic Curve Cryptography to tolerate a subset of enclaves to be compromised. Moreover, our scheme also provides stronger scalability so it achieves anonymity sets by orders of magnitude higher than the existing TEE-based mixing scheme. The experiment shows our scheme can provide stronger security and anonymity guarantees without compromising efficiency which outperforms existing mixing schemes.