In the early 1960s, David Kahn completed the now infamous and in-the-day controversial book The Code Breakers. I say controversial because of the reports that federal agencies were concerned about some of the historical content that was to be included in the book. This is a reference that should be on every serious code breaking researcher’s bookshelf, as the material and fundamental methods are still quite relevant today. In other words, don’t let the publication date concern you.

“Covered-Writing” Then
Here is one story that relates directly to steganography. Many believe the story led to the genesis of the word steganography, which comes from the Greek roots:

This gives us today’s translation: “covered-writing.” David Kahn tells the story of Demaratus, the son of Ariston who at the time was exiled in Persia. While he was there, he learned of the imminent attack on Greece by the Persians. Demaratus decided he must provide warning to Sparta of the plans, but doing so would have certainly put his life at risk, or worse. As the story goes, Demaratus turned to the writing instrument of the day, a simple folding wax tablet as shown in Figure 1.

Demaratus scraped off the wax and then engraved a message warning Sparta in the wood. He then covered the engraved tablet once again with wax concealing the hidden message, creating the physical form of “Covered-Writing.” The tablet easily passed by the sentries without notice and finally reached Cleomenes, who discovered the hidden message and warned Sparta of the Persian’s plans.

“Covered-Writing” Now
Taking a look at one of the popular writing instruments of today, (the iPad) the question is: can we duplicate Demaratus’ covert operation? I have chosen the iPad application InvisLetter as the closest match to the Demaratus wax tablet of earlier times. InvisiLetter is offered by Hideaki Tamori at the iTunes store.

InvisiLetter Application Steps
Step 1: Download and select the application.

Step 2: Select “Embedding Secret Image” (top panel) or “Extracting Secret Image” (bottom panel).

Step 3: Select the image in which you wish to store the covered writing. You can either take a photo with the camera or select an image from the photo album.

Step 4: You can see that I chose a photo of the Charles Bridge leading into Prague.

Step 5: At this point, as Demaratus might have done, write the secret message directly on top of the image with your stylus of choice. InvisiLetter then conceals the writing in the image.

Step 6: Finally, send the covert message via e-mail to anyone you wish. They need to have the same application if you wish for them to recover the hidden message. Or you could just send them a photo with a harmless secret message inside and see if anyone notices. If you decide to do this, please make sure the message is harmless, like “Hi” or “How are you?”

You might be thinking that our modern sentries, (application firewalls, e-mail gateways, content filters, and data leak prevention systems) would certainly stop such a simple exfiltration method ... Or have thousands of years passed and the same method used by Demaratus around 400 B.C. and described by David Kahn in the 1960s, work perfectly today? I think you will find that the sentries today are just as blind as those guarding the castles during the Persian war era.

(Click Image For A Larger Version)

(Click Image For A Larger Version)

(Click Image For A Larger Version)

Taking a Closer Look
Let’s take a closer look at the two images before and after we apply covered writing today. As many steganography applications on the iPad do, InvisiLetter utilizes .png files to carry out cover. Many of these apps perform hiding activities that provide few visual clues when examining the before and after image. InvisiLetter is different, as you can see in Figure 3, the app has changed the size of the photo slightly and the whole image seems slightly out of focus.

This is especially odd, since we did not hide very much information within the image. The AFTER image simply looks like a poorly taken, out-of-focus photograph. By examining other notational forms of the image before and after the hidden text is inserted, especially rendering saturation for both the BEFORE and AFTER images (Figure 4) you can see the lack of detail or sharpness that exists in the AFTER image.

At first you could jump to the conclusion that this is a very poor stego app. After taking a closer look however, we discovered that the app actually attempts to store the complete overwritten image (not just the handwriting) inside the BEFORE image. This causes the increase in size, used colors, and visual distortion. However, the result still works and may actually draw less attention, and be more difficult to detect as it would simply be set aside as a poorly taken photograph.

Finally, we exchanged a photograph containing the harmless hidden handwriting with over a dozen organizations of varying sizes and sophistication. Of course, none of them stopped or blocked the message or content … at least not yet.

In summary, the underlying technology has changed since 400 B.C., but covered writing is alive and well. We have just moved from wax to glass tablets. Unfortunately, it seems that the modern sentries are as overwhelmed, and possibly as oblivious, as they were then.

You can download the harmless AFTER.png image here: 

Keep discovering the hidden!

References

1. Kahn David, “The Code Breakers, “The comprehensive History of Secret Communication from Ancient Times to the Internet”, SCRIBNER 1967, 1966 ISBN 0- 684-83130-9
2. Kahn David, “The Code Breakers, “The comprehensive History of Secret Communication from Ancient Times to the Internet”, Page 81-82 SCRIBNER 1967, 1966 ISBN 0-684-83130-9

Chet Hosmer has been researching and developing technology and training surrounding data hiding, steganography, and watermarking for over a decade. He has made numerous appearances to discuss the threat steganography poses including National Public Radio's Kojo Nnamdi show, ABC's Primetime Thursday, NHK Japan, Cyber Crime TechTV, and ABC News Australia. He has also been a frequent contributor to technical and news stories relating to steganography and has been interviewed and quoted by IEEE, The New York Times, The Washington Post, Government Computer News, Salon.com, and Wired Magazine. Chet also delivers keynote and plenary talks on various cyber security related topics around the world every year.