4 ms·
So this may be visible on the spectrogram, right?
by megous 6y ago
So this may be visible on the spectrogram, right?
- cozzyd 6y agoMaybe, maybe not. You can probably make statistical changes to the lowest bits in certain samples or something that would be quite difficult to detect.
- megous 6y agoSuch small changes in sample value would probably get lost quite easily after being encoded with some codec. And I don't see them streaming un-encoded audio. Right?
- cozzyd 6y agoYeah that's probably true but you can do something similar with the encoded audio (change the bits of the quantized frequency domain representation in whatever codec they use). But that would get lost on reencoding in a different format, probably.
- roamingryan 6y agoNot necessarily. Watermarking an audio stream like this wouldn't require that high of a bit rate. It could easily be hidden "under" the content using coding techniques like direct-sequence spread spectrum. A real world example is GPS, which uses a spreading code to provide about 30 dB of gain. GPS signals aren't directly observable relative to the noise floor in many receivers. It's not until after the signal is "de-spread" that it becomes observable in a spectrogram. This process requires prior knowledge of the signal structure. In short, if you don't need to send data at high rate there are many ways to hide your signal.
- duckfang 6y agoWe can thank Michael Ossmann for his DC25 talk about detecting and pulling DSSS signals out of noise. He also posts proof of concept code to do just that, including also detecting the chip sequences and chiprate. https://hackaday.com/2017/07/29/michael-ossmann-pulls-dsss-out-of-nowhere/ https://hackaday.com/2017/07/29/michael-ossmann-pulls-dsss-o... We're also dealing with only 48KHz, which is 2-3 magnitudes less than what SDR people are accustomed to dealing with. And we can also make our own detection systems by having a 3 person "meeting" with fake audio, with this fingerprinting on, and then comparing the 3 recordings.
- a1369209993 6y ago> Michael Ossmann for his DC25 talk Link to the video? Searches aren't coming up with anything useful.
- zo1 6y agoStruggled to find it via google. Eventually Bing Video search worked... https://www.youtube.com/watch?v=z9RkPt6uVQ4 https://www.youtube.com/watch?v=z9RkPt6uVQ4 Note, it's not from the official Def Con channel, so some fuckery is afoot.
- zo1 6y agoMinor correction: The talk was made at "RECON" instead of DefCon.
- a1369209993 6y agoWell, that explains why I couldn't find it. Thanks for tracking it down.
- megous 6y agoYeah, GPS analogy seems interesting. Would defense against this then be sharing just a shortest possible compromising clip, to hamper de-spreading?
- eru 6y agoIn general, keeping the amount of data low makes the life of the steganographer and finger printer harder. Almost independent of technique used.
- anon946 6y agoMore here: https://ieeexplore.ieee.org/abstract/document/1188746 https://ieeexplore.ieee.org/abstract/document/1188746
- a-dub 6y agosee this one too: https://www.sciencedirect.com/science/article/abs/pii/S0165168498000140 https://www.sciencedirect.com/science/article/abs/pii/S01651...