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Comment by phendrenad2 | Hacker News Reader

Is compressed audio/video actually high-entropy (in the time domain) though?

Compressed anything is high entropy

Depends on the compression. Lossy compression discards a lot of noise vs the signal we care about and would in a sense reduce entropy.

Untrue. Many performance trade-offs have to be made and the entropy has to vary drastically with time. See for example B-Frames vs I-frames in compressed video. Couple that with the very low entropy video conference data and bam.

Two words: sensor noise.

Even uncompressed video will be hard to see that "penguin image effect" in, because the pixels that make up each block will be constantly changing in a random way, and unlike that synthetically generated image, it's highly unlikely for a block to be the exact same as any other one in any given frame.

You greatly overestimate both the image quality of crappy videoconferencing streamed video, the amount of pixel-wise sensor noise after noise reduction (pretty low actually), while underestimating the ingenuity of crypt-analysts and the power of having a lot of data. Like seriously, the only way the shitty 1mm or less sensors on webcams are able to deliver HD video is through an abject amount of noise reudction, sharpening and filtering. All of which greatly reduce entropy.

Hint: You don't need to know the plaintext exactly. you just need to be able to build a reasonably precise probability distribution.

I'm actually not sure - that's a good point. The whole point of compressing audio for video conferencing is to preserve human speech, so things that produce radically different waveforms but "sound the same" to us might show up as patterns. I guess it's better to avoid the question entirely and use an appropriate stream cipher!

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