[hobby] [notes] [sdr] [spam] was Re: SDR projects have been developing

Karl gmkarl at gmail.com
Wed May 5 09:27:41 PDT 2021


I thought I already was posting against my will, but now it's like another
bubble of struggle inside that.

I'm not a real special agent or anything, obviously.  It was just a weird
thing to say some time ago.  I ran into "classified" technology many years
ago and have been losing my mind severely, obviously.  I'm aware of
hundreds of other people in that situation.

A cheap imu has too much drift to be a good source of truth on its own.  So
you need like a gymbal or the phone camera.  There are free
visual-inertial-odometry software systems that can get highly accurate
location data from a camera and an imu.  They are used for drones and 3d

I'm not a radio engineer, but if I wanted to send an undetectable radio
signal, I would measure noise already being scattered by my equipment, and
emit rare signals that appeared to be that noise without amplifying it.
The other party would know the timing and incoming direction of my signals,
and integrate them to amplify my signal over the real noise. You could also
split your weak noise into different parts to be reconstructed together
from how they interfered at the receiver.

Usually when I think of anonymous radio I just think of putting a
transmitter somewhere distant from its owner.  Seems simplest.

To start a covert communication project, I would try to just demodulate any
old signal you send to yourself.  Make sure you understand working with the
raw i/q data so you can do tricks later to hide it.  I'm guessing it might
work fine if you think of EM as being complex-valued sinusoids that fill
space and get summed by antennas, but don't mistake those numbers for being
the electric and magnetic field components, they're just the electric
voltages at your antenna sampled in such a way that they can be frequency
shifted with an fft.  Then set up a shared key, synchronise your
transmitter and receiver clocks, and send secret messages by tuning at
cryptographically determined times. Then make your data signal a little too
weak to be received, but send redundant data, and recover the data at the
receiver by summing the redundant timepoints together so that the signals
add and the noise cancels.  Then work on making your signal be
indistinguishable from existing signals unless you are the intended
recipient, even if an adversary knows exactly what you are doing.

I'm not a cryptographer, but it seems if there were some cryptographic
function that worked like xor or a stream cipher, but sustained its
properties under different linear transformations of each datapoint, such
that data could be recovered after decryption of data transformed into
noise if the linear transformations were then inverted, it would be
incredibly helpful.  Then you could decrypt redundant noise and combine it
to amplify a signal.  Decrypting the signal after amplification leaves a
path for others to identify the redundancy in the air.

add to and organise gnuradio

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