Table 3.

Requirements for contact tracing protocols.

Requirements	Properties	Details	How achieved in DIMY
Security	Minimise false	A user not being warned	Use of Bloom filter that provides guarantees against
	negatives.	despite being in close contact	false negatives during the matching process.
	(Completeness)	of an infected person.
	Minimise false	A user being warned	Use of Shamir secret sharing and Diffie–Hellman key
	positives.	without a valid close contact	exchange to mitigate false positives due to replay
	(Soundness)	with any infected person.	attacks. False positives are still possible with a low
			probability due to relay attacks and Bloom filter matching.
	Ensure system’s	Data maintained at the backend	Use of blockchain as the backend to provide integrity,
	integrity and	is trustworthy and the	availability, and trust.
	availability.	matching service accessible.
Privacy	Confidentiality	Only the health authorities	Health authorities are involved only in the authorisation
	of health status.	can learn about the status	stage. Use of bloom filters and smart contracts ensures no
	(infected or warned)	of an infected person.	one learns about close-contacts of an infected person.
	Privacy for meeting.	No entity can learn about	Use of Bloom filters to hide the time/date of contacts.
	/contact history.	the contact history of a user.	The back-end server cannot construct a social graph.
		No one can link the anonymous	Use of Ephemeral identifiers and
	Hide user’s	IDs with real identities. Health	storage of contact information in Bloom filters.
	identities.	authorities learn this when an
		infected or at-risk user contacts them.
	Location privacy.	An adversary cannot track	No location information is captured by the system.
		movement of a device.	Limited local device tracking is possible.
Operational	Minimise	Reducing the amount of contact	Use of space efficient Bloom filters for storage at the
	storage costs.	tracing data stored on mobile devices	client’s devices as well as the backend.
		as well as the backend.
	Minimise	Reducing bandwidth utilisation	Use of BLE advertisement messages reduces number of
	bandwidth usage.	directly helps in prolonging	messages exchanged between the devices. Uploads from
		the battery life of mobile devices.	client’s devices consist of short, fixed-size Bloom filters.
	Minimise	Computational cost directly affects	Contact matching and risk analysis process is only
	computational cost.	battery consumption for devices.	performed at the backend. The cryptographic operations
			such as DH key generation and exchange involves group
			exponentiation which are not as computation intensive.