http://tutacomm6oqwlbbpd246mljjxpupuxxqj2jg5mfdyegeaetdauigslad.onion/blog/first-search-encrypted-data
Here’s how it works: The data is indexed. The search index is encrypted and stored locally on the client. The search index is searched according to the user’s search queries. The code for Tutanota’s search is published as open source so anyone can check it.