Question

Why do banks use 4 pin passcodes with ATM cards? I am wondering since I am thinking about pincode security.

Answer 1

They’re memorable:

The concept of a PIN originates with
the inventor of the ATM, John
Shepherd-Barron. One day in 1967,
while thinking about more efficient
ways banks could disburse cash to
their customers, it occurred to him
that the vending machine model was a
proven fit. For authentication
Shepherd-Barron at first envisioned a
six-digit numeric code, given what he
could reliably remember. His wife
however preferred four digits, which
became the most commonly used length.

http://en.wikipedia.org/wiki/Personal_identification_number

From the article, assuming:

• 4 Digits
• No information known about PIN
• No information known about PIN generation method

There is apparently a 0.06% probability of guessing the correct PIN before the card is blocked. However, I’m not sure this takes into account that PINs can typically not start with zeroes, and even then the figure seems higher than I calculate (0.0003% if PINs starting with zeroes are allowed)