It utilises K-anonymity, making it difficult if not impossible for this to become a massive data leak. The email gets hashed on your device, and the start of t

It utilises K-anonymity, making it difficult if not impossible for this to become a massive data leak. The email gets hashed on your device, and the start of that hash is sent off to the server. The server returns a list of all the hashes that might match. The client then checks that list for complete matches.