Snowden: Dropbox is an NSA surveillance target, use Spideroak instead

.... or simply don't dump sensitive files onto cloud storage. Personally, I love Dropbox. It's ideal for sharing the details of my kitchen extension with my builders. It is not suitable for sharing documents with my patent attorney.

Seems to be the relevant bit: https://spideroak.com/zero-knowledge/

Doesn't Dropbox have a master key? How else could they do deduplication?