
“Fingerprinting” chips to fight counterfeiting - ohjeez
http://newsoffice.mit.edu/2015/fingerprinting-chips-fight-counterfeiting-0501
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TheLoneWolfling
Something to note: the whole "96 of 128 bits" thing _drastically_ reduces
entropy. By my count it's only got ~27.2 (!) bits of entropy, as in, if you
have _one_ number in the database it's take ~2^26.2 tries on average to get a
valid one.

There are n choose k bitstrings with exactly k bits set to the correct value.
Hence, there are sum(n=96 to 128 of 128 choose n) length-128 bitstrings with
at least 96 bits set to the correct value. There are 2^128 length-128
bitstrings. Hence, your chance of randomly generating a bitstring with at
least 96 bits set correctly from a length-128 bitstring is sum(n=96 to 128 of
128 choose n) / 2^(128), which works out to ~1 / 2^(27.2).

If I did the math correctly, that is.

