Jack Reacher's Rules

Get into their minds, think like them.

Try birthdays, wedding anniversaries, house numbers; these are the passwords most people use.

Most people can’t remember all their passwords. If you look, you can find where they’ve written them down.

Try two-digit prime numbers, or the number whose square root is the sum of its digits.

Watch the position of their fingers when keying in a code so you can copy it.

The perfect PIN:

“I’d probably write out my birthday, month, day, year, and find the nearest prime number. Actually that would be a problem, because there would be two equally close, one exactly seven less and one exactly seven more. So I guess I’d use the square root instead, rounded to three decimal places. Ignore the decimal point; that would give me six numbers, all different.”