A few years ago, MIT researchers invented a cryptographic identification tag that is several times smaller and significantly cheaper than traditional radio frequency (RFID) tags that are often placed on products to verify their authenticity.
This small tag, which offers greater security than RFID, uses terahertz waves, which are smaller and travel much faster than radio waves. But this terahertz tag shared a major security vulnerability with traditional RFIDs: a counterfeiter could remove the tag from a genuine item and reattach it to a fake one, and the authentication system wouldn't notice.
Researchers have now overcome this security vulnerability by harnessing terahertz waves to develop an anti-tamper identification tag that still offers the benefits of being small, cheap and secure.
They mix microscopic metal particles with the glue that sticks the label to an object and then use terahertz waves to detect the unique pattern those particles form on the surface of the item. Similar to a fingerprint, this random glue pattern is used to authenticate the item, explains Eunseok Lee, a graduate student in electrical engineering and computer science (EECS) and lead author of a paper on the anti-tamper label.
“These metal particles are essentially like mirrors of terahertz waves. If I spread a bunch of mirror pieces on a surface and then light them, depending on the orientation, size and location of those mirrors, I would get a different reflected pattern. But if you remove the chip and put it back in, you destroy that pattern,” adds Ruonan Han, associate professor at EECS, who heads the Terahertz Integrated Electronics Group at the Electronics Research Laboratory.
The researchers produced a light-powered anti-tamper tag that is approximately 4 square millimeters in size. They also demonstrated a machine learning model that helps detect tampering by identifying fingerprints of similar glue patterns with more than 99 percent accuracy.
Because the terahertz tag is so cheap to produce, it could be deployed across a massive supply chain. And its small size allows the tag to attach to items too small for traditional RFIDs, such as certain medical devices.
The paper, which will be presented at the IEEE Solid State Circuits Conference, is a collaboration between Han's group and the Energy-Efficient Circuits and Systems Group of Anantha P. Chandrakasan, MIT's director of strategy and innovation, dean of the MIT School of Engineering. and Professor Vannevar Bush of EECS. Co-authors include EECS graduate students Xibi Chen, Maitryi Ashok, and Jaeyeon Won.
Prevent tampering
This research project was inspired in part by Han's favorite car wash. The company placed an RFID tag on his windshield to authenticate his car wash membership. For added security, the sticker was made of fragile paper so that it could be destroyed if a less than honest customer tried to remove it and stick it on a different windshield.
But that's not a very reliable way to avoid tampering. For example, someone could use a solution to dissolve the glue and safely remove the fragile label.
Instead of authenticating the tag, a better security solution is to authenticate the item itself, says Han. To accomplish this, the researchers directed the glue to the interface between the tag and the surface of the item.
Its tamper-resistant label contains a series of tiny slots that allow terahertz waves to pass through the label and hit microscopic metal particles that have mixed with the glue.
Terahertz waves are small enough to detect particles, while larger radio waves would not have enough sensitivity to see them. Additionally, using terahertz waves with a wavelength of 1 millimeter allowed the researchers to make a chip that does not need a larger antenna off the chip.
After passing through the tag and hitting the surface of the object, the terahertz waves are reflected or backscattered toward a receiver for authentication. The way these waves are backscattered depends on the distribution of the metal particles that reflect them.
The researchers placed multiple slots in the chip so that the waves can hit different points on the object's surface, capturing more information about the random distribution of the particles.
“These responses are impossible to duplicate, as long as a forger destroys the glue interface,” Han says.
A supplier would perform an initial reading of the anti-tamper label once attached to an item and then store that data in the cloud, using it later for verification.
ai for authentication
But when it came time to test the tamper-proof label, Lee ran into a problem: It was very difficult and time-consuming to take measurements accurate enough to determine whether two glue patterns matched.
He contacted a friend at MIT's Computer Science and artificial intelligence Laboratory (CSAIL) and together they tackled the problem using ai. They trained a machine learning model that could compare glue patterns and calculate their similarity with more than 99 percent accuracy.
“One drawback is that we had a limited data sample for this demonstration, but we could improve the neural network in the future if a large number of these tags were deployed in a supply chain, which would give us many more data samples,” Lee says. .
The authentication system is also limited by the fact that terahertz waves suffer high levels of loss during transmission, so the sensor can only be about 4 centimeters from the tag to get an accurate reading. This distance wouldn't be a problem for an application like barcode scanning, but it would be too short for some potential uses, such as in an automated toll booth on a highway. Additionally, the angle between the sensor and the tag must be less than 10 degrees or the terahertz signal will degrade too much.
They plan to address these limitations in future work and hope to inspire other researchers to be more optimistic about what can be achieved with terahertz waves, despite the many technical challenges, Han says.
“Something we really want to show here is that the application of terahertz spectrum can go far beyond wireless broadband. In this case, you can use terahertz for identification, security and authentication. There are many possibilities,” he adds.
This work is supported, in part, by the US National Science Foundation and the Korean Foundation for Advanced Studies.
