“…In the previous schemes, the secret sharing recovery algorithm is usually used to restore the mask values of exiting users, which improves the robustness of their scheme. Some methods [12,21] require a large number of mobile users to be online, and their masking formula is y u = x u + PRG(b u ) + ∑ v∈U:u<v (PRG(s u,v ) − ∑ v∈U:u>v (PRG(s u,v ) mod R. The x u represents the private data of each user and b u is a random number generated by the user who distributes shares of b u to the other users, and s u,v is the agreement key between user u and user v. Assume that there are no user drops out, then the cloud server will correctly compute the aggregation result ∑ n u=1 (y u ), and then the ∑ n u=1 (∑ v∈U:u<v PRG(s u,v ) − ∑ v∈U:u>v PRG(s u,v )) will be zero. Once a user drops out, a large number of users are needed to recover the shared values b u and s u,v by the secret reconstruction algorithm to make ∑ n u=1 (∑ v∈U:u<v PRG(s u,v ) − ∑ v∈U:u>v PRG(s u,v )) zero, so a large number of users need to be online to ensure that the scheme is executed correctly.…”