I would expect that the PIN digits have a quite good random distribution. Although impractical at the ATM point a smartened brute force attack could effectively work provided you have some machinery at home.Assuming that the distribution of the dirt per finger (or the distribution of cleaning factor per finger) is flat then the ATM pad at the gasoline station where I usually refuel my car provides some odd evidences.
The numeric pad has some keys clearly cleaner than others – ‘7’ and the square ‘5’, ‘6’, ‘2’ and ‘3’ are much more clean than the rest.
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