Hiding Information in Silver and Carbon Ink


"We used silver and carbon ink to print an image consisting of small rods that are about a millimeter long and a couple of hundred microns wide," said Ajay Nahata from the University of Utah, leader of the research team. "We found that changing the fraction of silver and carbon in each rod changes the conductivity in each rod just slightly, but visually, you can't see this modification. Passing terahertz radiation at the correct frequency and polarization through the array allows extraction of information encoded into the conductivity."

Research paper.

Posted on December 13, 2016 at 6:21 AM • 13 Comments


Nutty ProfessorDecember 13, 2016 9:33 AM

Maybe that explains the fancy copper-colored ink on the new US $100 bills... an efficient way to scan and track serial numbers?

{}December 13, 2016 11:05 AM

@Nutty Professor
"an efficient way to scan and track serial numbers?"
Magnetic ink has been in use for this purpose on personal cheques since the late 1950's.

The copper-coloured ink on $100 bills is an example of structural colour*. The ink contains chips of plastic that have alternating layers of differing refractive index. The spacing of the layers is in the same range as the the wavelength of light. The material reflects a distinctive pattern of wavelengths of light, making it easier to detect forgery.
Structural colour in banknotes came into widespread international use in the late 1980's and early 1990's, but the US lagged behind because of the bureaucratic and political inertia that is so typical of the US.
As a result, US dollars became – by a wide margin – the easiest currency in the world to forge. By the late 1990's, no one internationally would accept US $100 bills, because most of them were printed by North Korea.
An argument could be made that North Korea's mass production US $100 bills was for a good cause – the country had fallen on such hard times that the population was literally stunted by malnutrition. Even today North Korean adults tend to be significantly shorter than South Koreans.
Anyway, the irony is that the technology for these structural colour inks was developed in the US, and for several years a company in New York made the worldwide supply.


{}December 13, 2016 11:21 AM

@Nutty Professor
These days, most higher-denomination banknotes worldwide have moved beyond simple structural colour to contain holograms. The holographic features in post-2013 US $100 bills are subtle, but they are there.
Of course, you can also print holograms onto gold bars. Pure gold is quite soft. All you need to do is press regularly-spaced pits into the surface:

Clive RobinsonDecember 13, 2016 2:33 PM

@ {},

All you need to do is press regularly-spaced pits into the surface...

For some reason that reminds me of the idea of using quantum traps for making unforgeable bank notes thought up by Stephen Wiesner, an idea so ahead of it's time it was not published for a decade. When it was published it gave the idea for what we now call Quantum Key Distribution BB84 to Gilles Brassard and Charles Bennett.

TedDecember 13, 2016 2:43 PM

The Optica article says the special inks are readable under the terahertz wavelength.

From highest to lowest frequency: gamma > x-rays > ultraviolet > visible > infrared > terahertz > microwave > radio waves > television and fm radio > am radio

Terahertz radiation also has other uses, one of those being for security purposes such as screening. Just like visible light, terahertz radiation can penetrate certain mediums like fabrics and plastics and can be used to create unique images of many materials based on the wavelength each object emits. It can "see" objects on a body such as metals, liquids, gels, powders, ceramics, etc. even under many layers of clothing. A Phys.org article from several years ago says such a specialized camera can successfully distinguish between sugar and cocaine.

According to the Optica research summary, silver and carbon ink have different conductivity characteristics, although they appear similar to the naked eye. Since the inkjet printers in the study can print with 4 colors to a pixel, there are 64 conductivity variations possible. For those with the desire, you can make your own conductive ink and paint with supplies you or your kids may already have at home.

gustavoDecember 13, 2016 4:33 PM

I think the point of the article is using the technique to hide secret messages in plain sight.
The technique could be useful if you want to track something - such as to prove someone is in possession of a very specific copy of a document.
But for transmission of secret messages, it could be defeated by simply scanning the document and only allowing access to the scanned image.

WhiskersInMenloDecember 13, 2016 4:52 PM

The density of the data seems low. It does lend itself to stenography.
Readers seem bulky, printers not so much so one way messaging?

Since all currency has serial numbers all transactions
can be tracked and may already be tracked at the central
bank level. Optical character recognition is easy and could move this
to local banks, casinos, etc. with ease. International banks may
already do this to track piles of cash for TLAs. As meta-data goes
currency is an ideal object to track.

Counterfeiting depends a lot of features humans can detect.
Intaglio printing and paper quality was and is a first line of
defense for US currency and yes nation states and big crime
can counterfeit it.

Lazy agencies already track transactions by amount and
perhaps by individual. We worry about email and cell phones
but money never seems to get the press. Perhaps the lack
of press is a result of asset seizure and abuses.

"A bank must electronically file a Currency Transaction Report (CTR) for
each transaction in currency (deposit, withdrawal, exchange, or other
payment or transfer) of more than $10,000 by, through, or to the bank.
Certain types of currency transactions need not be reported, such as those
involving "exempt persons,"...

I wonder who is exempt?

JDecember 13, 2016 11:16 PM

There is only one way to prevent currency Counterfeiting: make the currency more expensive to produce than the currency itself is worth. If it takes $60 to produce a $50 note, then there is no point forging it.

albertDecember 14, 2016 11:56 AM


We used to have silver and gold coins. How about a $20 silver piece and a $1000 gold piece?

. .. . .. --- ....

chrisDecember 14, 2016 2:31 PM

@albert "How about a $20 silver piece and a $1000 gold piece?"

Interesting technology, requiring that they change mass based on daily (or shorter time period) prices of (silver, gold).

Money is an abstraction, precious metals are physical objects whose price varies over time. You can buy a "$20 gold piece", but it'll cost a lot more than $20.

Check the web for discussion of "gold standard" and consequences of doing what you suggest.

Trung DoanDecember 19, 2016 2:38 AM

Usually steganography uses digital bits, but these reseachers used physical bits - in this case metal rods. And each physical bit doesn't have to be binary but in principle can be analog. Steganography is always clever, and this invention is innovative!

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