Six Lines

Dropbox Encryption

Posted by Aaron Massey on 13 Apr 2011.

Yesterday, Christopher Soghoian, a security and privacy research with a growing reputation for finding serious practical threats in major online services, announced another such threat with the encryption scheme Dropbox uses for all of their user’s files:

Dropbox, the popular cloud based backup service deduplicates the files that its users have stored online. This means that if two different users store the same file in their respective accounts, Dropbox will only actually store a single copy of the file on its servers.

The service tells users that it “uses the same secure methods as banks and the military to send and store your data” and that “[a]ll files stored on Dropbox servers are encrypted (AES-256) and are inaccessible without your account password.” However, the company does in fact have access to the unencrypted data (if it didn’t, it wouldn’t be able to detect duplicate data across different accounts).

Read the whole post. It’s excellently written, and it explains the problems in detail. If you’re curious, here’s a section of Dropbox’s recently updated privacy policy:

Compliance with Laws and Law Enforcement Requests; Protection of Dropbox’s Rights.

We may disclose to parties outside Dropbox files stored in your Dropbox and information about you that we collect when we have a good faith belief that disclosure is reasonably necessary to (a) comply with a law, regulation or compulsory legal request; (b) protect the safety of any person from death or serious bodily injury; (c) prevent fraud or abuse of Dropbox or its users; or (d) to protect Dropbox’s property rights. If we provide your Dropbox files to a law enforcement agency as set forth above, we will remove Dropbox’s encryption from the files before providing them to law enforcement. However, Dropbox will not be able to decrypt any files that you encrypted prior to storing them on Dropbox.

In short, U.S. law enforcement can read your personal files stored on Dropbox without a warrant.

In some ways, this ‘new’ threat isn’t really all that new. Jim Harper identified the basic privacy problems with Dropbox way back in December 2009:

I homed right in on their “Policies” page, looking for assurance that they would protect the legal rights of users to control information placed in the care of their service. There’s precious little to be found.

There’s no promise that they would limit information they share with authorities to what is required by valid legal process. There’s no promise that they would notify users of a warrant or subpoena. They do reserve the right to monitor access and use of their site “to comply with applicable law or the order or requirement of a court, administrative agency or other governmental body.”

Is there protection in the fact that files are stored encrypted on their service? The site—though not the terms of service—says “All files stored on Dropbox servers are encrypted (AES-256) and are inaccessible without your account password.” Not if Dropbox is willing to monitor the use of the site on behalf of law enforcement. They can simply gather your password and hand it over.

Ideally, Dropbox would build this into their service. If users had the option of locally encrypting files, either individually or in a particular folder, prior to uploading them to Dropbox and without allowing Dropbox access to their encryption keys, this would be a much less pressing concern. This could even be a feature only available to paying customers. Certainly, the inability to deduplicate data would be an added expense for Dropbox.

However, as it stands, encryption must be done manually to ensure your data is protected. If you use Dropbox for anything remotely serious, such as business-critical documents, diaries, sensitive contact information, or calendars, then you should independently encrypt those files prior to uploading them to Dropbox. Some applications are designed to store data directly to Dropbox folders. These applications could actually encrypt that data and store the encryption keys locally. I strongly recommend that developers of such applications consider protecting their users by building in strong encryption.