We aspired to create a service that is similarly secure to a file upload service with SSL, but more usable at the same time by not tying the user to a partcular data exchange mechanism (you can sent the sharelock URL via e-mail, Tweet it, or publish in a New York Times).

Having said that, exposure of the user of sharelock.io can be argued to be lower than in case of a service which durably stores user's data. While sharelock.io keeps the cryptographic credentials, it does not durably store users' secrets or ciphertext.