[xtacy]

This thread has a lot of discussion related to "convergent encryption."

http://news.ycombinator.com/item?id=2570538

EDIT: http://news.ycombinator.com/item?id=2461713 as well

EDIT2: Actually, there's more to this problem than just convergent encryption. If the storage provider knows which encrypted blobs belong to you, it can encrypt _some_ file and still figure out which users have copies of it. So, the storage provider, which stores a collection of encrypted blobs, should not know the blob -> list(users) association. I don't know if Bitcasa addresses this part.