Snowden: Dropbox is an NSA surveillance target, use Spideroak instead (boingboing.net)

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

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

.... 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.