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Patient location sharing is an important part of modern smart healthcare and mobile medical services. Blockchain has many attractive properties and is suitable for managing patient locations in telecare medical information systems (TMIS). Recently, Ji et al. proposed a blockchain-based multilevel privacy-preserving location sharing (BMPLS) scheme for TMIS. In this paper, we show that Ji et al.’s BMPLS scheme does not achieve confidentiality and multilevel privacy-preserving. An adversary outside the system can use an ordinary personal computer to completely break the system within a dozen hours and obtain the location of any patient at any time. The adversary inside the system can use an ordinary personal computer to obtain the location of the designated patient within tens of seconds. Using salting technology, we propose an improved BMPLS scheme to fix our attacks. We also optimized the BMLS scheme to make it correct and executable. The security analysis shows that the improved BMPLS scheme achieves decentralization, untamperability, confidentiality, multilevel privacy-preserving, retrievability, and verifiability. The simulation shows that the improved BMPLS scheme is practical, the computational overhead of the location record phase is within 10 ms, and the computational overheads of the location sharing and location extraction phases are both within 30 ms.

Analysis and Improvement of Blockchain-Based Multilevel Privacy-Preserving Location Sharing Scheme for Telecare Medical Information Systems