Steganography, Electronic Disappearing Ink
Article by Rick Leinecker, May 1, 2006
When I was a kid I loved writing messages with disappearing ink. It was a cool way to write to someone without the fear of having my secret read by anyone else. All I had to do was let the recipient know there was a secret message and how to make it visible. Then only the intended recipient could read the message.
One of my favorite movies is National Treasure. The characters discover that there's an invisible map on the back of the Declaration of Independence that leads to a huge treasure. Imagine if a famous historical document has something in disappearing ink and you discover it after hundreds of years. That's an exciting thought!
Today there is a new version of hidden ink known as Steganography. This technique embeds messages in electronic files and communications. Just as happened in the National Treasure movie where a historical document had a hidden map for years that nobody saw, publicly accessible files such as images and sounds can easily have hidden messages. You may have seen an image on the Internet that had a top secret code intended for a spy. And nobody would even suspect that there was something hidden in a publicly available image. It's kind of like Edgar Allan Poe's The Purloined Letter story - if you leave it in plain sight, they're more likely to miss it.
Just think about the implications. A web site with thousands of images can contain one single image with an embedded message. Any time that the webmaster wants to communicate with his associates, he simply embeds a message in a predetermined image. Then the associates use their decoding software to retrieve the message. You could view the image with the embedded message a thousand times and never know there was a hidden message. Just think how hard it is to find and decode such images, especially if law enforcement doesn't even know that they exist.
Message can be hidden in just about any kind of electronic file or communication. I've been talking about images, but messages can be hidden in sound files, document files, spreadsheet files, and anything that anyone can imagine. Messages can be hidden in instant message communications, emails, and even phone conversations.
Organizations such as the NSA are getting a handle on the problem. They are developing techniques for identifying files that contain hidden messages, and techniques for extracting these messages. Neither of these, however, is a trivial task. And as fast as they develop these defensive strategies, the opponents develop additional methods of hiding messages.
There's a great web site where you can create your own images that contain hidden messages. You can use the web site to create images for your web site, or images to email to friends. Intended recipients can then use the web site to extract the messages. You can also password protect the hidden messages. To try it out, go to SteganographyGuru.com. You will find instructions there on how to use the web site's Steganography process.
You might be wondering how images are embedded within electronic files. While there's not a single, easy answer; I'll describe a simple system for embedding messages within image files.
Let's start with an image. Each dot of the image can be slightly degraded without the human eye noticing. The slight degradation is done by removing some of the information that describes a dot to the computer's video system. Each degraded dot makes room for a small amount of information that can replace what was originally part of the dot description - all without the human eye being able to notice. With enough of these dots, a message can be stored.
As you might have guessed, the amount of information that can be stored in an image is far less than the size of the image itself. For instance, the SteganographyGuru.com web site uses a methodology that requires seven dots of an image for each letter of a message. If your message has 20 characters, then you need at least 140 dots to hide it.
Those are the basics of Steganography.