Hackers could be hampered by a crystal oscillator transmitter

29-08-2018 |   |  By Rob Coppinger

Hackers could be stopped intercepting transmitted data with a transmitter that changes its frequency with each individual 1 or 0 bit within a data packet.

Hackers can intercept data being transmitted and jam signals or corrupt the packets of data sent wirelessly from a device. A defence against this is to change the transmission frequency with every packet, but, according to researchers, hackers can locate a transmission channel in one microsecond and make a successful attack. By changing the frequency with every digital one and zero in the packet the hackers are not expected to be able to respond quickly enough. Critical devices, such as medical implants whose performance can be altered wirelessly, could therefore be secured.

A data packet contains the packet’s individual number and destination for routing purposes and reassembly, and up to 1,500 bytes of the data itself, for example the text of an email message. “By developing this protocol and radio frequency architecture together, we offer physical-layer security for connectivity of everything,” said Massachusetts Institute of Technology post-doctoral researcher, Rabia Tugce Yazicigil. “The transmitter could help secure medical devices, such as insulin pumps and pacemakers, that could be attacked if a hacker wants to harm someone.”

To create this ultrafast hopping for every digital one and zero in the packet, Yazicigil and her colleagues used a bulk acoustic wave (BAW) resonator-based oscillator instead of a typical transmitter’s crystal oscillator. A bulk acoustic wave resonator uses piezoelectric material instead of a crystal.


MIT researchers developed a transmitter that frequency hops data bits ultrafast to prevent signal jamming on wireless devices. The transmitter’s design (pictured) features bulk acoustic wave resonators (side boxes) that rapidly switch between radio frequency channels, sending data bits with each hop. A channel generator (top box) each microsecond selects the random channels to send bits. Two transmitters work in alternating paths (center boxes), so one receives channel selection, while the other sends data, to ensure ultrafast speeds. Credit: MIT 

The crystal oscillator vibrates to create an electrical signal at the frequency to be transmitted, but it is slow. It can send a packet of data, for example, on one channel among 80 possible channels on a one-megahertz frequency. Which channel it sends the packet on is based on a predetermined sequence shared previously with the receiver. A packet can take more than 600 microseconds to be sent on one of these channels, which is why it is too slow for hackers who can respond in a microsecond.

Another constraint with conventional transmitters is that one channel will always be 250 kilohertz apart from another. Trying to use this system with the frequency hopping ones and zeros approach would make it easier for the hackers to find the channels, as the one and the zero will only be 250 kilohertz apart. For the BAW resonator-based transmitter, to defeat the hackers, Yazicigil and her colleagues are also randomizing the possible channels for a one or a zero bit.

To achieve this randomness, Yazicigil and her colleagues have pairs of separate channels, for those ones and zeros, generated every microsecond and the receiver has already been sent a secret key so it to can calculate the randomised frequencies for the bits of data. A drawback with the use of BAW is that it only has a range of four to five megahertz, far less than crystal’s 80. The solution is to use what the researchers call a mixer component to divide those four to five megahertz frequencies into the 80 potential channels.


Read more: Can ARM-based Thin Clients Provide a Secure Alternative to Windows Desktops?

By Rob Coppinger

Rob Coppinger is a freelance science and engineering journalist. Originally a car industry production engineer, he jumped into journalism and has written about all sorts of technologies from fusion power to quantum computing and military drones. He lives in France.

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