# Partially homomorphic addition - not all parties present

I often see homomorphic encryption as a solution to a problem where a server wants to know the sum of $n$ users' numbers but can not know each individual users' number plaintexts.

My question is: if $n=10$, and only $9$ of the ciphertexts are reported, can the server decrypt the sum of those $n-1$ ciphertexts?

Furthermore, do the $n$ users' keys depend on the server's 'master key'?

• With the most basic schemes, the server can in fact decrypt each share it receives. If you do some fancier things, like clever blinding and proxy-re-encryption and all that stuff, things may look differently. – SEJPM Mar 4 '16 at 20:56
• To really answer this question, we are going to have to know more details about how you would implement your system. – mikeazo Mar 4 '16 at 20:59
• Do you have any reference to homomorphic schemes in which the users have their own keys and the server has a master key? – Hilder Vitor Lima Pereira Mar 6 '16 at 2:57