In the DP$-3$T protocol for COVID$-19$ contact tracing, the server accu$-$

mulates the EphIDs created by the smartphones of those users that have

reported to be COVID$-19$ positive. Explain:

$($a$)$ Why the protocol is privacy$-$preserving, that is$,$ why the server cannot

nd out who where the contacts of a user X who reported positive.

$(1\mathrm{.}5$ points$)$

$($b$)$ Why the protocol is useful, that is$,$ why a user Y who was a close

contact of an infected X can learnt it$.(1\mathrm{.}5$ points$)$