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Comment by akoboldfrying | Hacker News Reader

Really interesting concept! I would not have thought of EMR variation as a usable side channel for this.

IIUC, the EMR increases following scene changes (like a light being turned on or off) are a result of the fact that video codecs try hard to compress similar images. So, I speculate that an "extremely bad codec" that simply dumps raw pixels to storage would not produce such variation, and therefore remain undetectable?

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Potentially, although raw pixel data presents its own challenges - one 1080p60 camera will fill a terabyte of storage in less than an hour.

A better mitigation might be to take a leaf from cryptography and develop a constant-time codec.

Like all I frames in H264 ?

Non-branching memory accesses?

Makes sense. It might be possible to develop a codec that, in addition to being constant-time, has the property that every prefix of the bits encoding a unit (e.g., a frame or block within a frame) encodes an approximate version of that unit, where the longer the prefix the more faithful the approximation is. Then each unit could be output in full to storage, and the "output file pointer" could then be wound back to a point dependent on how many bits are actually required to represent it accurately -- so keeping the full amount for a full scene change, or just a few bits for the "same as last time" common case.

Oh, that is fascinating - I think a scheme like that could very well give you constant-space as well as constant-time, while calculating and preserving the desired compression information for use in transmission (which itself could separately be made constant for detectable channels).

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