Schneier on Security

Clive Robinson • November 20, 2016 2:51 PM

@ Matija,

In most cases, it is very easy to check if anything what should not be inside is actually there.

Only if you know what should and should not be there, and you can see all of it. Further that what should be there, has not been configured in a way that causes information leakage, that you are not aware of.

For instance some transformer cores are “gapped” for sound electrical reasons[1], however you can arange for the core to change the reluctance thus effect the value of the inductance. That is you could make it behave like a microphone or other transducer. Thus that ferrite core you think is there for EMC reasons may not be. Likewise what looks like an earth shield, could be a slot radiator. Using just such techniques you can build the modern day equivalent of the “great seal bug” etc.

Further how do you know that the plastic casing of an electricity plug does not contain an embbeded micro chip or similar. You can design microwave fets to be incredibly small, and have a chip smaller than a grain of sand all it needs to do is bridge a narrow slot in the earth shield and have it’s gate driven by a signal that changes it’s impedence. Illuminating the slot at it’s natural frequency will cause the slot to re-radiate the signal phase and amplitude modulated by the gate signal…

[1] http://info.ee.surrey.ac.uk/Workshop/advice/coils/gap/