currently I am working on some secure accounting application. I have few requirements:
- Application should store items which can be searched by name.
- Every item has price and tax.
- Every invoice contains arbitrary amount of different items.
- Every invoice has gross and net sum of items
My idea is to use RSA or ElGamal schema for multiplication and Paillier for addition. Calculating net sum is pretty easy I can just use Paillier for doing it. Problem is with gross sum. In my solution for each item I am doing multiplication: price*tax using RSA and here I am stucked. I have only one solution in my mind: adding another trusted proxy to my system which will receive result of each multiplication, decrypt it and encrypt using Paillier scheme and than send it back to my server (which now can sum gross values of each item). That solution requires a lot of communication between server and proxy and client has to give his private keys to the proxy. Do you have any idea how can I improve it? Thanks in advance!