Additional measure at that point: the server could challenge the client to provide the values of bytes at a couple of arbitrarily chosen byte offsets of the original file. (Could precompute that, provided the queries don't repeat often).
Client A wants the file that Client B has so when Dropbox asks Client A for some random offset, Client A asks Client B in the background and relays the result to Dropbox.
It really depends on how far pirates would be willing to go.
For some reason, this inspired me to write a blog post: http://a3nm.net/blog/deduplication_attacks.html and http://news.ycombinator.com/item?id=2489594
For example if some web site charges per download of a file, but still has the hash posted publicly, you can try to "steal" it from someone who has it stored privately in Dropbox?
IOW, the file hash is equivalent to your account login/password combo [restricted to any given file]?
If you have a sub-4MB sensitive file, and you publish its SHA256, and the Dropbox protocol applies the hash function in the same way as file hashing tools (e.g. doesn't include a tag meaning "this hash is computed particularly for Dropbox deduplication" into the SHA computation), yes, then apparently people can download your file.
However, I rarely see SHA256 checksums along with download links; more SHA1 and MD5.
Another related attack could be to start with a known file (say, your employment contract), swap out the name with a colleague and generate a bunch of files with different salary amounts, essentially bruteforcing sha256 sums. If dropbox suddenly coughs up a file, you've revealed his salary!
Dropbox effectively acts as an "existence oracle". You can't ask it to cough up a file you don't have, but you can ask it if a given file exists anywhere in the system.
This would be an effective way for law enforcement or copyright civil enforcement to check for content that is clearly illegal or a certainly copyright violation to possess. They would need to query for a set of hashes of the given illegal content. If any matches returned positive data, they would be able to issue a subpoena for all users who stored the given content in their dropbox folder and pursue them further.
How can something be "clearly" a violation? If I have an album, but copy someone else's rip instead of making my own - is that "clearly" a violation? Alternatively if I used the same application, I'd probably obtain the exact same file - is that clearly a violation too?
(grooveshark kind of operates on the assumption that it's ok)
The following dropship file was assembled using only shasum, ls and vi:
{"blocks": ["f3f754a5dcd93f271ad013a5ee84f495a36da84f152e0a1fec4646345b0c10d6"], "name": "ostseestrand.jpg", "size": 514779}