It's in Niels Ferguson's public comments to NIST regarding XTS-AES and storage that's not on physical hardware. tptacek explained it in his comments how this at

It's in Niels Ferguson's public comments to NIST regarding XTS-AES and storage that's not on physical hardware. tptacek explained it in his comments how this attack works if you search them https://news.ycombinator.com/item?id=7675698#up_7676864 https://news.ycombinator.com/item?id=7675698#up_7676864 This service if I remember encrypts files/container with user keys then they encrypt it again on their cloud backup with their key, so it's not an encrypted backup sitting on a dropbox server. Of course you have to trust their keys won't be stolen by somebody wanting at those XTS-AES encrypted backups.