Pretty much ANY online service that provides sync has decided not to use private keys. The same goes also with tasklist apps. They are either only web-based (and obviously readable at the server) or use the sync without encryption to provide web access. http://timegt.com is using private key for encrypting sync data but it came with the cost of not having any web access to your tasks!

The same with dropbox etc, private key encryption renders any server-side magic like web access very complex.