In my mind there is no question that they'll be doing de-duplication here. You don't need to rely on a header key, you just take something like a sha1 hash and

In my mind there is no question that they'll be doing de-duplication here. You don't need to rely on a header key, you just take something like a sha1 hash and the size. Services like dropbox do this - even making it so you don't have to upload non-unique files. I have always assumed that all of the non-encrypted remote backup solutions that live on s3 or elsewhere do this too. It's just too easy for a massive storage space savings to not do it.