Power/Performance Bits: June 1

Stronger PUFs
Researchers from Ohio State University and Potomac Research propose a new version of physical unclonable functions, or PUFs, that could be used to create secure ID cards, to track goods in supply chains, and as part of authentication applications.

“There’s a wealth of information in even the smallest differences found on computers chips that we can exploit to create PUFs,” said Noeloikeau Charlot, a doctoral student in physics at Ohio State.

In PUFs, the slight manufacturing variations are used to create unique sequences, or secrets. However, they can be compromised when there aren’t enough of them. “If you have a PUF where this number is 1,000 or 10,000 or even a million, a hacker with the right technology and enough time can learn all the secrets on the chip,” said Daniel Gauthier, professor of physics at Ohio State. “We believe we have found a way to produce an uncountably large number of secrets to use that will make it next to impossible for hackers to figure them out, even if they had direct access to the computer chip.”

The key in improving PUFs is chaos. The researchers created a complex network in their PUFs using a web of randomly interconnected logic gates. “We are using the gates in a non-standard way that creates unreliable behavior. But that’s what we want. We are exploiting that unreliable behavior to create a type of deterministic chaos,” Gauthier said.

Chaos amplifies the manufacturing variation of a chip, Charlot explained. “Chaos really expands the number of secrets that are available on a chip. This will likely confuse any attempts at predicting the secrets.”

However, limiting the chaos is also important, Gauthier added. “We want the process to run long enough to create patterns that are too complex for hackers to attack and guess. But the pattern must be reproducible so we can use it for authentication tasks.”

The researchers calculated that their PUF could create 10⁷⁷ secrets. Gauthier said if an attacker could guess one secret every microsecond, it would take the hacker longer than the life of the universe, about 20 billion years, to guess every secret available in that microchip.

As part of the study, the researchers attacked their PUF to see if it could be successfully hacked. They attempted machine learning attacks, including deep learning-based methods and model-based attacks, all of which failed. They are now offering their data to other research groups to see if they can find a way to hack it.

The researchers have applied for a patent for their PUF device and founded a startup, Verilock, with a goal of bringing a product to market within a year.

High mobility p-type oxide
Researchers from ARC Center of Excellence in Future Low-Energy Electronics Technologies (FLEET), RMIT University, Australian National University, University of New South Wales, University of Melbourne, and Deakin University proposed a new oxide-based semiconductor material, 2D beta-tellurite, that shows promise as a high mobility p-type semiconductor and potential for transparent electronics.

“This new, high-mobility p-type oxide fills a crucial gap in the materials spectrum to enable fast, transparent circuits,” said Dr Torben Daeneke, a FLEET associate investigator.

In 2018, a computational study pointed to beta-tellurite (β-TeO2) as an attractive p-type oxide candidate. Tellurium can behave as both a metal and a non-metal, providing its oxide with useful properties. “This prediction encouraged our group at RMIT University to explore its properties and applications,” said Daeneke.

The team isolated the material using a synthesis technique relying on liquid metal chemistry. “A molten mixture of tellurium (Te) and selenium (Se) is prepared and allowed to roll over a surface,” said Patjaree Aukarasereenont, a FLEET PhD student at RMIT. “Thanks to the oxygen in ambient air, the molten droplet naturally forms a thin surface oxide layer of beta-tellurite. As the liquid droplet is rolled over the surface, this oxide layer sticks to it, depositing atomically thin oxide sheets in its way. The process is similar to drawing: you use a glass rod as a pen and the liquid metal is your ink.”

Selenium is added to create an alloy with a lower melting point.

“The ultrathin sheets we obtained are just 1.5 nanometers thick – corresponding to only few atoms. The material was highly transparent across the visible spectrum, having a bandgap of 3.7 eV which means that they are essentially invisible to the human eye,” said Dr Ali Zavabeti of the University of Melbourne. “Having a fast, transparent p-type semiconductor at our disposal has the potential to revolutionize transparent electronics, while also enabling better displays and improved energy-efficient devices.”

“These devices showed characteristic p-type switching as well as a high hole mobility (roughly 140 cm2V-1s-1), showing that beta-tellurite is ten to one hundred times faster than existing p-type oxide semiconductors. The excellent on/off ratio (over 106) also attests the material is suitable for power efficient, fast devices,” added Aukarasereenont.

The team plans to continue work with the material, including integration into consumer electronics.