I am no neuro scientist, but the way I understand is that each neuron make dozens of synapses per second, which means that the electric configuration of the brain changes 12-24-36 times per second?

Thats 100 billion brain cells having 12 configurations each that's 438.49 bits of entropy / second.

I'd say that would be physicall impossible to decypther in 1 second, since a 256 bit entropy is already quantum computer proof.

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So if you brain configuration changes that rapidly, then it's impossible to use it as a login, because it's changing.

If there are parts of the brain that don't change, then that is already public information (like brain size, etc...)

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Now if they were to implant a chip in our brains, that would be a whole another story....