[HN Gopher] Audio Masking
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Audio Masking
Author : goles
Score : 93 points
Date : 2024-09-25 14:27 UTC (8 hours ago)
(HTM) web link (www.cryptomuseum.com)
(TXT) w3m dump (www.cryptomuseum.com)
| amelius wrote:
| In practice most audio channels are low-pass filtered and
| bandwidth limited, so I'm guessing that these modulation
| techniques are not going to work. Also, we have digital methods
| now.
| KK7NIL wrote:
| These are techniques for modulating audio onto a radio signal,
| I think the article didn't make that very clear.
| PaulHoule wrote:
| I'll point out that a common method of detecting bugs at the time
| was to set up a radio receiver with a speaker and then sweep the
| frequency, if you managed to hit you would get a feedback sound
| between the speaker and bug. These oddball modulation schemes
| would prevent that from working.
|
| I like it a lot that many of techniques have a hybrid
| analog/digital structure that would involve sample-and-hold,
| sweeps and comparators like the Triple Pulse scheme.
|
| Today I can't believe you wouldn't use some digital solution but
| at that point in time you'd be lucky to be able to use a small
| IC.
| bgnn wrote:
| Audio ADCs are incredibly small and the digital functionality
| needed isn't much for this. One can fit everything in a sub mm2
| IC I think. Minus the antenna.
| fortran77 wrote:
| Today with spread spectrum, it's probably much easier to hide a
| covert radio signal.
| dbcurtis wrote:
| Yes and no. SS still has an energy signature, which you can
| recognize if you go looking for SS. And the transmitting
| antenna can be RDF'ed.
| r2_pilot wrote:
| Plus if the electronics aren't shielded you can use a non-
| linear junction detector even if it's turned off.
| knodi123 wrote:
| Ah, unless your spy is clever....
|
| > As a countermeasure against an NLJD, professional covert
| listening devices (bugs) of the Central Intelligence Agency
| were equipped from 1968 onwards with a so-called isolator.
| An isolator is a 3-port circulator of which the return port
| is terminated with a resistor. Any energy injected into the
| bug by an NLJD will be absorbed by the resistor, resulting
| in no (or very little) reflected energy. An example of such
| a bug is the CIA's SRT-107.
|
| (or my favorite:)
|
| > A means to hinder isolating a non linear junction is to
| add inexpensive diodes to places where the NLJD is expected
| to sweep. This masks the true listening device against a
| field of false alerts when the many diodes are detected.
| Such a technique was used in the 1980s construction of the
| U.S. embassy in Moscow. Thousands of diodes were mixed by
| the Soviets into the building's structural concrete, making
| detection and removal of the true listening devices by its
| American occupants nearly impossible.
| Jugurtha wrote:
| Are there any bugs that masquerade as normal devices such as
| phones in a time-frequency sense, such that they blend in the
| environment as phones. Polymorphic bugs? Bugs that change
| their signature.
|
| One more question: are there any bugs that shut down if there
| is no chatter in the spectrum... Say, if it's a noisy
| environment (frequency wise) with many phones and devices,
| the bug blends in and transmits. If it gets quiet, such as
| when phones are being turned off or distant, then there's
| something fishy and the bug suspends its operation?
| knodi123 wrote:
| > are there any bugs
|
| They aren't usually off-the-shelf. They're custom, and can
| be as smart as the builder wants to make them. (working
| within constraints like size and power supply)
| motohagiography wrote:
| it would be interesting to see what the waterfall charts of these
| looked like, and I can't tell if there is enough info in the
| article to produce a gnuradio flowgraph for any of them. it could
| be a fun retro spy tech project.
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