Power/Performance Bits: Feb. 27

Encryption chip; simple TENG; thermoelectrics.

Encryption chip
A team at MIT developed a new chip to lower the power consumption of public-key cryptography for IoT devices. Software execution of encryption protocols require more energy and memory space than embedded IoT sensors can typically spare, given the need to maximize battery life.

The new chip is hardwired to perform public-key encryption and consumes only 1/400 as much power as software execution of the same protocols would. It also uses about 1/10 as much memory and executes 500 times faster, according to the team.

The chip uses a technique called elliptic-curve encryption. While previous chips have been built to handle specific elliptic curves or families of curves, the new one can handle any elliptic curve.

“Cryptographers are coming up with curves with different properties, and they use different primes,” says Utsav Banerjee, an MIT graduate student in electrical engineering and computer science. “There is a lot of debate regarding which curve is secure and which curve to use, and there are multiple governments with different standards coming up that talk about different curves. With this chip, we can support all of them, and hopefully, when new curves come along in the future, we can support them as well.”

Elliptic-curve cryptography relies on modular arithmetic. The values of the numbers that figure into the computation are assigned a limit. If the result of some calculation exceeds that limit, it’s divided by the limit, and only the remainder is preserved. The secrecy of the limit helps ensure cryptographic security.

Modular multiplication earned a special-purpose circuit in the chip, which can handle 256-bit numbers. Eliminating the extra circuitry for integrating smaller computations helped reduce the chip’s energy consumption and increase its speed.

Another key operation in elliptic-curve cryptography is inversion. Inversion is the calculation of a number that, when multiplied by a given number, will yield a modular product of 1. In previous chips, inversions were performed by the same circuits that did the modular multiplications, saving chip space. The researchers instead equipped their chip with a special-purpose inverter circuit, which increased the chip’s surface area by 10%, but cut the power consumption in half.

The datagram transport layer security protocol, which governs the elliptic-curve computations and the formatting, transmission, and handling of the encrypted data, is also hardwired in the chip to reduce the amount of memory required.

The chip also features a general-purpose processor that can be used in conjunction with the dedicated circuitry to execute other elliptic-curve-based security protocols. But it can be powered down when not in use, so it doesn’t compromise the chip’s energy efficiency.

Simple TENG
Researchers at the Chinese Academy of Science and the University at Buffalo propose a new construction for a triboelectric nanogenerator they say is cost effective and easy to manufacture.

The device consists of two thin layers of gold, with polydimethylsiloxane (also called PDMS, a silicon-based polymer used in contact lenses, Silly Putty and other products) sandwiched in between.

One layer of gold is stretched, causing it to crumple upon release and create what looks like a miniature mountain range. When that force is reapplied, for example from a finger bending, the motion leads to friction between the gold layers and PDMS.

A prototype of the triboelectric nanogenerator. (Source: Nano Energy)

“This causes electrons to flow back and forth between the gold layers. The more friction, the greater the amount of power is produced,” said Yun Xu, a professor at the Institute of Semiconductors at CAS.

The team created a small tab of the material, 1.5cm long by 1cm wide. It delivered a maximum voltage of 124 volts, a maximum current of 10 microamps and a maximum power density of 0.22 millwatts per square centimeter. While it’s not enough to quickly charge a smartphone, it lit 48 red LED lights simultaneously.

The team is working on improving the performance of the TENG and plans to use larger pieces of gold, which when stretched and folded together are expected to deliver more electricity. They are also working on developing a portable battery to store energy produced by the tab.

Heat into energy
Researchers at the University of Tsukuba and the National Institute of Technology, Gunma College developed a new kind of thermoelectric system that can harness small energy differences at low temperatures.

Thermoelectric devices can change heat energy into electricity, and vice versa. But to capture energy from heat efficiently, these devices typically need to work at high temperatures with a large temperature difference.

“Thermoelectric batteries, like ours, have been proposed before but those have been based on liquid-based cells, which are impractical for real-world applications. We created a thin-film device that operates on the same principle but with two types of solid redox materials that produce a change in the potential difference in the cell over a heating and cooling cycle,” said Takayuki Shibata of Gunma.

In the device, which utilizes two different analogs of the Prussian blue pigment, changing the temperature alters the ability of different layers in the device to hold onto electrons. If one layer has a greater affinity for electrons that another, this creates a potential difference. The flow of electrons from one layer to the other can then be harnessed to do work as the cell is discharged, in the same way that a normal battery works.

The researchers tested their devices for harvesting waste heat energy near room temperature. Their device produced an electrical energy of 2.3 meV per heat cycle between around 25 and 50 degrees Celsius. This result reflected an efficiency of around 1.0%, although the theoretical maximum for this device should be around 8.7%.

“We have shown that solid-state thermoelectric batteries are viable and our film deposition method could be extended to large areas,” said Yutaka Moritomo, a professor at the University of Tsukuba. “This technology offers realistic prospects for large-scale heat energy recovery, which could be help a range of industries become more efficient.”

The team plans to work on improving the thermoelectric battery by optimizing the anode and cathode materials.