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Can MEMS be a sensor and a cryptographic device at the same time?

MEMS developers invest enormous effort as they struggle to eliminate process variations which result in imperfect devices, manufacturing yield loss and lower profit margins.Yet it’s ironic that the very flaws that plague MEMS sensors are in fact features for another application – cryptography.

MEMS gyroscopes, it turns out, are useful as a Physical Unclonable Function (PUF), a device used to create a cryptographic key. When interrogated, a PUF produces a unique response (‘physical function’) that cannot be predicted by inspecting the physical device, nor can it be duplicated (‘unclonable’). Simply put, a PUF is a tamper proof, copy proof, black box that generates a unique code.

MEMS devices with electrostatic comb fingers (see image) – such as accelerometers and gyroscopes – have large surface areas, which make them vulnerable to silicon etch process variation.

Subtle variations in the etch process make each gyro uniquely imperfect in a random, unpredictable way. And even if one could thoroughly measure and inspect it, a gyro could not be perfectly duplicated because silicon etch processes cannot be controlled that finely. All of which are excellent characteristics for a PUF.

A research group at Bosch recently demonstrated how to use a commercially available gyro as a PUF to generate a 128bit cryptographic key. Further optimisation of a MEMS PUF design could produce a longer, and therefore more secure, key.

Should you think this cryptography application for a gyro is merely an academic curiosity, think again: All data generating devices in the IoT will need encryption – even seemingly innocent wearables – to protect them from hacking. Only recently, researchers determined that sensor data intercepted from smart watches and fitness trackers can reveal an ATM PIN.

A dual-purpose MEMS device which not only offers sensing functionality, but which can also produce its own encryption key is a very exciting emerging technology with huge potential in future IoT markets.

Author profile:
Alissa Fitzgerald is founder of MEMS engineering consultancy AM Fitzgerald and Associates

Alissa Fitzgerald

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