  With the popularity of Blockchain comes grave security-related concerns.
Achieving privacy and traceability simultaneously remains an open question.
Efforts have been made to address the issues, while they may subject to
specific scenarios. This paper studies how to provide a more general solution
for this open question. Concretely, we propose Onionchain, featuring a suite of
protocols, offering both traceability and privacy. As the term implies, our
Onionchain is inspired by Onion routing. We investigate the principles of Onion
routing carefully and integrate its mechanism together with Blockchain
technology. We advocate the Blockchain community to adopt Onionchain with the
regards of privacy and traceability. To this end, a case-study of Onionchain,
which runs in the context of Vehicular Ad Hoc Networks (VANETs), is proposed,
providing the community a guideline to follow. Systematic security analysis and
extensive experiments are also conducted to validate our secure and
cost-effective Onionchain.
