[lotsoflumens]

Since backscatter systems depend on on an RF source, I wonder if work has been done on encrypting the source signal (pseudorandom bit pattern for example), then modulating the random stream at the device and decoding the backscattered signal at the receiver - essentially a streaming XOR cypher.

[junkcollector]

In this system the RF source you are sending is effectively the IF component for the sensor's Tx chain. Because it is so high power and such a clean pilot tone, the sensor can eliminate it's IF generating oscillator and output amplifier by just mixing the data onto the receive pilot tone and retransmitting it. This is how the sensor is made so cheap and low power. If the pilot tone was cyphered, it wouldn't mean the data was. That is, you could recover the unencrypted data if you recorded a copy of the pilot by mixing. There is also the issue that, because the pilot tone provides the power to the sensor's carrier, any spreading of it's spectrum, which is what random xor encoding does, makes it harder to recover and use as the IF component for the sensor's radio chain. The last issue here is that the CSS standard does not allow this sort of non-linear modulation.

All that said, it's not a fundamentally bad idea. Pseudo-random xor encoding of signals is a technique used all the time to increase orthoganality between signals for a variety of encoding schemes.

[lotsoflumens]

>>> That is, you could recover the unencrypted data if you recorded a copy of the pilot by mixing.

Yes, thanks for the response!