SF Technotes

Decryption Using Only One Hand

By Michael Castelluccio
April 14, 2021

In the current catalog of biometric connections to our computers, we have digital assistants that know the individual sound of our voices, laptop cameras that recognize us and let us in, and fingerprint readers that do the same. There are iris scanners, hand geometry recognition, palm vein recognition, and even devices that can recall the uniquely irregular folds of cartilage of the ear to keep track of patients.


These are all identification devices, but what about something, equally biometric, that might actively do something as complicated as decrypt text or data and create passcodes? Something the size of your hand that requires no batteries or power connection and is completely mobile. Well, today there’s a team of engineers working on a most unusual version of this, and the basic device is complete and already rolled out. It’s your hand, and what it can do with a wave before their hardware and programming is decrypt messages and data and make very secure passkeys.




On Monday, April 13, 2021, a team of engineers in China published a research paper with the rather detailed title “Human hand as a powerless and multiplexed infrared light source for information decryption and complex signal generation.” It appeared in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).


In simple terms, the team has found ways to use the human hand as a remote control over encryption and decryption, and for generating complex signals. It does this not by holding another device, but by directing, at sensors, the infrared (IR) light that the human hand naturally generates. The “multiplexed” reference in the title refers to the team’s research into the use of all four digits and thumb as individual sources of that light, and the complex signal generation they’re designing for the digits is a little like sign language using specific gestures for numbers and instructions.


There are many advantages for using your hand to control these functions. They can be safely used to direct decryption and create passwords because they’re as unique as your fingerprints are. They’re always with you, and they don’t require any kind of extraneous power source. As an interface for integration with intelligent machine, they’re ideal. But what sort of light are you able to get from just your hands?




Click to enlarge.

(A) An IR image of a human hand. (B) Radiation spectra of a hand and the ambient temperature. (C) The simulated mapping of the irradiance of the hand in the plane perpendicular to the hand. (D) Illustration of using a hand as a single light source and a multiplexed light source. Images courtesy PNAS


The researchers explain, “The human hand is a natural IR emitter due to the thermal emission from the hand…. In this work, we demonstrate that the human hand is not just a natural and powerless IR light source, but also a multiplexed light source with each finger serving as an independent light source.”


IR light is invisible to us because it’s just outside the visible spectrum, but we do feel it as heat. The electromagnetic radiation of IR light has longer wavelengths just beyond the red edge of the spectrum visible to us. And it isn’t just our hands emitting IR light—our entire body is a source for this irradiance. In fact, anything above absolute zero emits IR radiation. And we do have devices, motion detectors, and night-vision equipment that can see in this band. The researchers discovered, “The IR radiation peak of the human hand is well within the atmospheric transmission window as well as the detection range of the IR detector (FLIR T620) used in this work.”




So, how are we able to see with our hands in this invisible world? And on what do we write the hidden messages or keys? Conventional encryption uses complex mathematics to encode text or data that can be sent to a privileged reader who has a key that can unlock it. Intruders along the transmission’s path don’t have the key and can’t open the message. Because the key is done with math, it theoretically can be guessed. But the random trials would take too long to crack (a good passkey might take thousands of years) that we shouldn’t have any problems until quantum computers arrive with their epic math skills.


The hand’s IR signature is unique for each of us, so we have a good key. Next, what do you write on? The researchers explain, “We deposited polydimethylsiloxane (PDMS), which has a low IR reflectivity, on the surface of aluminum (Al), which has a high IR reflectivity, to form the coding pattern. PDMS was diluted with hexane and sprayed through a stencil to generate the coding pattern…. The measured reflectance of the PDMS layer coated on the Al is lower than that of the Al, and such difference provides the possibility of pattern differentiation in the IR reflection-based decryption process.”



Click to enlarge.

(A) Schematic illustration of using the human hand as an IR light source for reflection-based decryption. (B) Mechanism of IR reflection-based decryption. Images courtesy PNAS


In the illustration, section A shows the aluminum reflecting nothing in the ambient light of the surroundings until the added IR radiation from the hand reveals the painted image. The relationship between ambient light and IR radiation from a hand is also shown in the first illustration sections B and C.


Further experiments found that the thickness of the PDMS pattern showed a profound impact on the image contrast. This led to trials involving layers of PDMS and the possibility of levels of encryption. “The pattern composed of PDMS with different thickness can thus be decrypted by the IR detector due to the difference in IR radiance, which can be shown as the color contrast in the IR image…. Such thickness-dependent contrast provides opportunity for multilevel coding as the change of relative IR radiance can be shown with color distribution under IR detection.” The use of more levels of coding equals greater levels of security.


And there was one other fascinating area of inquiry inspired by the American Sign Language. The development of a set of unique finger gestures could become an entire code of encryption keys. To give you an idea of some of the complications of hand signals in this format, here’s a proposed chart of 26 alphabet gestures and the 26 different IR diffraction gestures. You can’t just leave it at the symbolic gestures without accounting for the illumination levels.


IR alphabet based on hand-grating interactions (gratings are line patterns that show light diffraction). (A) Interaction of gratings and different hand gestures leads to different IR diffraction patterns. (B) The different combinations of three fingers, the corresponding IR patterns and alphabet letters. Images courtesy PNAS


How far this idea will travel on its way to general deployment is completely unknown today. But given its flexibility and the ubiquity of an ever-present key system, along with the possibility of strengthening encryption as quantum computing appears on the horizon, who knows? It wasn’t very long ago that if you wanted to find a fingerprint reader you had to visit a police station, and anyone who might have asked to photograph your retinas instead of just giving you a key to the door might have occasioned some serious suspicions.


Michael Castelluccio has been the technology editor for Strategic Finance for 26 years. His SF TechNotes blog is in its 23rd year. You can contact Mike at mcastelluccio@imanet.org.

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