<P> Some systems set up default PINs and most allow the customer to set up a PIN or to change the default one, and on some a change of PIN on first access is mandatory . Customers are usually advised not to set up a PIN based on their or their spouse's birthdays, on driver license numbers, consecutive or repetitive numbers, or some other schemes . Some financial institutions do not give out or permit PINs where all digits are identical (such as 1111, 2222, ...), consecutive (1234, 2345, ...), numbers that start with one or more zeroes, or the last four digits of the cardholder's social security number or birth date . </P> <P> Many PIN verification systems allow three attempts, thereby giving a card thief a putative 0.03% probability of guessing the correct PIN before the card is blocked . This holds only if all PINs are equally likely and the attacker has no further information available, which has not been the case with some of the many PIN generation and verification algorithms that financial institutions and ATM manufacturers have used in the past . </P> <P> Research has been done on commonly used PINs . The result is that without forethought, a sizable portion of users may find their PIN vulnerable . "Armed with only four possibilities, hackers can crack 20% of all PINs . Allow them no more than fifteen numbers, and they can tap the accounts of more than a quarter of card - holders ." </P> <P> Breakable PINs can worsen with length, to wit: </P>

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