I am currently developing a service that calculates statistics (currently only sum/average) on homomorphically encrypted user data, and then gives the results to a third party. Because encryption is computationally expensive, and user devices are routers or small SoCs, I want to blind the data (e.g. add a random big number to the data value), use a blind for multiple data values, and encrypt only the blind.
Now the question is, how big does this blind have to be compared to the actual data so that it can be considered secure? For example, if I have temperature values between -20°C and +40°C, how big would I have to choose the blinds? If I take extremely big blinds, I will run into the issue of overflows, which will probably be hard to deal with, since I am summing up encrypted numbers.