"Secretbook" allows people to encode hidden messages in their Facebook pictures.

Now there’s a browser extension that lets you encode those images with secret, hard-to-detect messages. Secretbook,is a browser extension released by a 21-year-old Oxford University computer science student and former Google intern Owen-Campbell Moore. With the extension, anyone could share messages hidden in JPEG images uploaded to Facebook without the prying eyes of the company, the government or anyone else noticing or figuring out what the messages say. The only way to unlock them is through a password you create.

“The goal of this research was to demonstrate that JPEG steganography can be performed on social media where it has previously been impossible,” Campbell-Moore tells Danger Room. He says he spent about two months spread out over the last year working on the extension as a research project for the university.

The extension is only available for the Google Chrome browser — Campbell-Moore cites its developer tools and popularity — and the messages are restricted to 140 characters. Less certain is what Facebook thinks; a spokesman declined to comment. But it’s still the first time anyone’s managed to figure out how to automate digital steganography — the practice of concealing messages inside computer files — through Facebook, the world’s biggest social media platform. Unlike cryptography, which uses ciphertext to encrypt messages, steganographic messages are simply hidden where no one would think to look.

Secretbook has to be subtle. It uses Google Chrome’s web extension platform, since Facebook’s in-house apps publicly list their users — which would defeat the purpose of a secrecy tool. Since the extension runs through a web browser without a server connection, the users can’t be detected by network analysis. It’s also hard for Facebook to block or remove permissions, as the extension doesn’t rely on a Facebook API key.

Since the images contain a large number of changes, someone looking for them could conceivably write an algorithm that tracks down manipulated images. That could limit the extension to “hobbyists and researchers,” he says, rather than militants, or maybe not.

“A researcher could certainly build a simple system for detecting which images have secret messages hidden in them although they would first require access to all 300+ million photos being uploaded to Facebook every day,” Campbell-Moore says. “Which I suspect even the NSA doesn’t currently have, and performing detection on that scale would be very difficult.”
http://www.wired.com/dangerroom/2013/04/secretbook/