 $(t_x, (m_{i,0}, r_{i,0}, p_{i,0}), \cdots, (m_{i,n_{i,x}}, r_{i,n_{i,x}}, p_{i,n_{i,x}})$ for all relevant $x$.  -   In the *tracing* phase, suppose user $j$ wants to check whether he has been in close contact with 