If you have multiple bank cards and have difficulty remembering all your PINs, especially as Hong Kong banks use the more secure 6-digit PIN, I've come up with a simple cypher that may help.
1. Determine the month and year of a date important to you. Do not use your birth date; choose a more secret date. For example, if your grandmother was born in June 1955, the date code would be 061955.
This becomes your Master PIN; the only one to memorise.
2. If you have six cards, order them from 1 to 6, based on the most often used card, followed by the next most often used card, and so on.
Take your most often used card, say, your personal bank account card, which usually gets a lot of use for ATM withdrawals and purchases: that would be Card #1.
3. Take your Master PIN and subtract 1 from each digit. If the result is a negative number, simply ignore the minus sign.
Based on grandmother's birth date code, the result would be 150844. This would the new PIN for your main card, which you should be able to change at the ATM or in the branch. If you forget the PIN next time you use the machine, simply remember grandma's birth date and subtract 1.
For your second-busiest card, subtract 2 from each digit, as above. The new PIN would therefore be 241733. Card #3 would be 332622, Card #4 would be 423511, Card #5 would be 514400, Card #6 would be 605311.
This system is simple because it requires only that you remember one PIN, and your card order. If you're absent-minded you could write a number on each card (i.e. 1, 2, 3, etc.), or dots or lines; even if a thief or pickpocket somehow got your cards, there is no way he would know your Master PIN or how to use the digit, dot(s) or line(s) on each card to figure out the card's PIN.
As long as you don't tell anyone your secret code, there is little chance anyone could crack your cypher. Again, don't use your own birth date, as it's on your Hong Kong ID card and probably your driver's license.
And as most bank ATMs are programmed to eat a card after three failed passcode attempts, the chance of thief guessing your PIN is pretty much nil.
But don't blame me if you forget grandma's birthday.