Cryptographic authentication could be coming to wireless chargers

Researchers from the Massachusetts Institute of Technology’s Microsystems Technology Labs have built a chip that blocks attempts to wireless charging unless the charger first provides cryptographic authentication.

The MIT researchers say their work is in response to a counterfeit goods problem that now plagues consumer electronics. The researchers presented the new chip at the recent International Solid-State Circuits Conference. They also claim the same technology also solves a problem that can arise when two or more devices share the same wireless charger: Two devices sharing a single charger will charge at much different rates if they are different distances from the charger’s electrical coil. The closest might charge fully while the other might remain virtually uncharged. In this case, the researchers’ new chip can slow the power transfer to the device nearer the charging coil, ensuring more equitable charge rates.

Anantha Chandrakasan, the Vannevar Bush Professor of Electrical Engineering and Computer Science, is the senior author on the conference paper. The first author is Nachiket Desai, an MIT graduate student in electrical engineering and computer science (EECS) when the work was done. They’re joined by Chiraag Juvekar, also an EECS graduate student at MIT, and Shubham Chandak, a graduate student in electrical engineering at Stanford University.

In a wireless charging system, both the charger and the target device contain metal coils and use magnetic induction to pass energy. The device’s coil must be tuned to the transmission frequency in order to receive power. The MIT researchers’ chief innovation is a more compact and efficient circuit for tuning the frequency of the receiving coil. A standard tuning circuit connects the coil to a series of capacitors for storing charge. Between each pair of capacitors is a switch; switching the capacitors on and off changes the receiver frequency.

“Those switches have severe requirements,” Juvekar says. “They either have to block a large voltage when they’re off, or they have to carry a large current when they’re on, or in some cases both. If a switch must block a big voltage, then it’s hard to put that on the chip. So it has to be a discrete component on the PCB, outside the chip. Or if it’s on the chip, it requires a specialized [manufacturing] process that might be expensive.”

Instead of a single coil attached to a bank of capacitors, the MIT researchers’ design uses a pair of coils attached to one capacitor each — no switches required. “The fact that those switches aren’t there anymore is a big advantage,” Juvekar says.

In the researchers’ chip, one of the coils — the main coil — is much larger than the other — the auxiliary coil. The main coil carries the chief responsibility for charging a device’s battery. A current flowing through the auxiliary coil produces a magnetic field that changes the tuning frequency of the main coil.

In the circuit connected to the auxiliary coil, the resistance can be continuously varied. When the resistance is low, the auxiliary coil produces a strong magnetic field, which changes the main coil’s tuning frequency so drastically that charging is impossible.

When the resistance in the auxiliary coil circuit is higher, the magnetic field is weaker, and the detuning is less drastic. There will still be some power transfer, but the charge rate is lower. That permits other, more distant devices to harvest more of the power transmitted by the charger coil.

The chip uses an authentication technique called elliptic curve cryptography, which is a public-key cryptographic technique. Using publicly available information, the chip can generate — and verify the response to — a question that only a charger with valid private information can answer. The chip doesn’t need to store a secret key of its own.

Elliptic curve cryptography is a well-established technique. But Chandrakasan’s group has developed a battery of methods for reducing chips’ power consumption, and the researchers found a way to simplify the encryption circuit so it takes up less space on the chip and consumes less power.