Guess what? Dropbox is capable of telling whether you're sharing copyrighted files. In fact, it doesn't even have to look at what you're sharing to know.

A tweet this weekend from Darrell Whitelaw spoke of a DMCA takedown in his personal folders on Dropbox, which is a result of software that the cloud service has been using for the last two years.

Dropbox uses "file hashing against a blacklist" to block pre-selected files from being shared person to person over its cloud service. This avoids Dropbox from getting in trouble with the feds, as well as help them avoid anti-infringement policies.

The hashing uses a simple algorithm that maps data of arbitrary length to data of a fixed length to produce a unique identifier for every file you upload. It is unique to each and every file.

When DMCA complaints are sent to Dropbox from record labels or content producers, the files they relate are hashed as well. If you uploaded the exact same file that Dropbox received, it will match the hash to one on its list and stop you from sharing it.
"There have been some questions around how we handle copyright notices. We sometimes receive DMCA notices to remove links on copyright grounds. When we receive these, we process them according to the law and disable the identified link. We have an automated system that then prevents other users from sharing the identical material using another Dropbox link. This is done by comparing file hashes. We don't look at the files in your private folders and are committed to keeping your stuff safe."
Don't get mad if you're using the platform to share these sorts of things. [Engadget]