He is just stating the definition of homomorphic encryption. There are lots of papers on the topic. The one mentioned in the article is a good starting point. "

He is just stating the definition of homomorphic encryption. There are lots of papers on the topic. The one mentioned in the article is a good starting point. "Homomorphic Encryption for Arithmetic of Approximate Numbers" https://eprint.iacr.org/2016/421.pdf https://eprint.iacr.org/2016/421.pdf