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.
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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.