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How many possibilities for a numerical PIN with exactly 9 digits? How about a PIN with no more than 9 digits?

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    $\begingroup$ This is a basic mathematic question. It might have been better suited on math.SE. $\endgroup$ – A. Hersean Jul 26 '19 at 14:56
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    $\begingroup$ I'm voting to close this question as off-topic because it's a basic math question. $\endgroup$ – AleksanderRas Jul 27 '19 at 15:10
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For a PIN of length 9, you have exactly 10^9 = 1000000000 combinations (which can be brute-forced by a computer in less than one second). That's 29.9 bits of entropy.

For a PIN of at most 9 digits, you have exactly 10^9 + 10^8 + ... + 10^1 = 1111111110 combinations, which is not that greater than the previous answer. That's 30.0 bits of entropy.

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