Scientists at the University of Utah found that they could hide QR codes by using small rods printed with varying mixtures of silver and carbon and in varying lengths and orientations.
(From Daniel Akst/The Wall Street Journal)

Invisible ink has long been a staple of love, war and intrigue. In a new twist on the ancient concept of hidden writing, scientists have come up with a way to encode messages in innocuous-looking markings that can only be decoded with a specific type of electromagnetic radiation. And they managed it with cheap, off-the-shelf products available on the internet.

Researchers at the University of Utah used a standard inkjet printer costing less than $60 and equipped it with specialty ink cartridges, also readily available, containing either carbon or silver. In printing, the inkjet device mixed the two as required by instructions from a computer, just as it would mix different colors in normal printing. The resulting output enabled the scientists to exploit the differing electrical properties of the two materials; the more silver, the higher the conductivity.

Using this technique, the scientists printed sets of small rods—all looking the same to the naked eye—roughly a millimeter each in length, covering a square about 5 inches on each side. A collection of these rods, while visible, would seem to have no particular significance. But these markings contain an invisible image—one made visible by passing a device called a terahertz scanner over them. The scanner bombards the markings with harmless electromagnetic waves known as terahertz radiation and then displays the results on a screen.

Commercial printing technology can already embed invisible authentication codes into product packaging, but the new research relies on affordable everyday equipment—and adds an extra security wrinkle. To see the carbon-silver images created by the Utah scientists, you have to employ terahertz radiation with two precise characteristics: a particular frequency (wave size) and a particular polarization (orientation). Ajay Nahata, one of the scientists who wrote about the research in a new paper, says an expert could deduce these characteristics from the visible markings: “If you could see the rods, you could measure them and determine the frequency and polarization to use.”

But the system works even if the markings are covered with a layer of paint or ink, rendering them truly invisible. In that case, their very existence would be undetectable—and it would be impossible for an unauthorized person to deduce the necessary radiation characteristics, and the corresponding scanner settings, that would let someone read the hidden image. Knowing the settings thus becomes akin to knowing the combination of a lock.

There are about 200 possible frequency settings (some tweaking could produce more), and the existence of the rods can be hidden, but Dr. Nahata says that he doesn’t see his team’s creation as a form of encryption. The research is more in the venerable realm of steganography, which deals with hidden messages, than cryptography, he says.

The system, while simple, can carry hidden messages that are relatively complex. The scientists found that they could hide QR codes, for example, in both shades of gray or in 64 colors, all by using rods printed with varying mixtures of silver and carbon and in varying lengths and orientations. The scientists even embedded two QR codes in a single array of markings, with each code requiring different terahertz settings to become visible. Dr. Nahata says that the scientists hope their findings might have application in authenticating documents or products.

“Multi-level multi-color images in terahertz metasurfaces,” Ashish Chanana, Andrew Paulsen, Sivaraman Guruswamy and Ajay Nahata, Optica (Dec. 8)

Click here to read the whole article at The Wall Street Journal.