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Comment by nathancahill | Hacker News Reader

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nathancahill·

Not feasible. Digital signatures are a one way operation, like hashes:

Let's say the audio signature of "Kill the president" is 9a0c0a6868a2a784059d9a634e4a58c0.

"kill the president" in a different tone would be entirely different: ed3e3cd6be6eeb2de9b480e2c9590cc9

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The server has to compare the signature of the sound recorded by your computer to the signature of the sound recorded by your phone. The sounds will not be identical, so the signature comparison has to be able to detect whether two sound recordings are sufficiently similar. It can't use, for example, SHA1 hashes for this purpose.

Couldn't they ignore noise below a certain threshold and create the signature just based off of the "peaks" and "troughs" so they could expect an exact match, and/or hash several samples from the recording in case some portions don't produce an exact match? Seems like there should be a way to this without revealing information about the recorded sound

If a slightly different sound resulted in an entirely different signature, the system would be useless for comparing recordings of ambient noise, which will never be identical. You've taken the analogy with hashing too far.

Eh, these are bound to be more like Shazam guess-that-tune signatures, with behavior wildly more loose than any cryptographic hash like an MD5.

Conside that the web browser's desktop/laptop microphone and the cellphone/telephone microphone will probably never produce identical audio samples, if not due to the actual transducers, then simply due to proximity to various ambient audio sources like the user's breathing versus fan motors attached to a laptop shell or desktop case.

So you're saying they'll have to have a database of these hashes to match against?

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