Although users of Dropbox and other cloud-based file storage and sharing systems have become accustomed to treating their files on these services as private, this is not actually the case. Darrell Whitelaw recently tried to share copyrighted material via Dropbox, and received a message that he could not share the content due to DMCA regulations. He tweeted his frustration, which received almost 4,000 retweets, and caused outrage throughout the Twittersphere.

Although users were angry that Dropbox was looking through their personal files, this is not true. According to TechCrunch, “The system is neither new, nor sketchy. It allows Dropbox to block pre-selected files from being shared from  person-to-person (thus keeping Dropbox from getting raided by the Feds), without their anti-infringement system having any idea what most of your [the user’s] files actually are.”

Dropbox is only able to use the anti-infringement system on files that are being shared, not files that are just sitting in a user’s cloud storage. Dropbox also does not delete a copyrighted file, but simply prevents a user from sharing it.

Dropbox’s automated system functions by comparing file hashes. Dropbox has also been using file hashing algorithms to avoid duplicating identical files among various users’ accounts. For example, if a user tries to upload a 20GB movie file to Dropbox, the website will give the user access to the movie in another users’ account, rather than having multiples of the same file stored on Dropbox’s cloud storage. This saves storage space for both the user and Dropbox.

Whitelaw did not expect his tweet to gain such traction, notes Ars Technica. “This isn’t a Dropbox problem,” he said. “They’re just following the laws laid out for them. Was just surprised to see it.”