  The infection rate of COVID-19 and lack of an approved vaccine has forced
governments and health authorities to adopt lockdowns, increased testing, and
contact tracing to reduce the spread of the virus. Digital contact tracing has
become a supplement to the traditional manual contact tracing process. However,
although there have been a number of digital contact tracing apps proposed and
deployed, these have not been widely adopted owing to apprehensions surrounding
privacy and security. In this paper, we propose a blockchain-based
privacy-preserving contact tracing protocol, "Did I Meet You" (DIMY), that
provides full-lifecycle data privacy protection on the devices themselves as
well as on the back-end servers, to address most of the privacy concerns
associated with existing protocols. We have employed Bloom filters to provide
efficient privacy-preserving storage, and have used the Diffie-Hellman key
exchange for secret sharing among the participants. We show that DIMY provides
resilience against many well known attacks while introducing negligible
overheads. DIMY's footprint on the storage space of clients' devices and
back-end servers is also significantly lower than other similar state of the
art apps.
