I have a confession to make.  I have been carrying the PIN number to my credit cards in my wallet for the last five years!

In my wallet I have a slip of paper right next each of my cards that looks like this…

A B C D E F G H I
6 9 2 1 6 2 4 0 1
J K L M N O P Q R
8 7 9 1 7 2 3 2 4
S T U V W X Y Z
7 7 2 8 1 7 2 0

… I can remember a four letter word easier than four random numbers and have challenged many people to guess my pin number from it.  e.g. if the four letter word was MOVE then the PIN would be 1286 and if the four letter word was CHIP then the PIN would be 2013.

I cryptographic terminology this is classed as a one way hash, a terrible idea for encrypting data on the internet but for data as small as 4 numbers it works quite well.  4 numbers only gives (10^4 =) 10000 combinations at the best of times although there are things that can be done to try and break it.

If we take a standard dictionary file (/usr/dict/words) there are 1778 four letter words that could be used. Based on the example matrix above that equates to 907 different PIN numbers. While this is still too many to guess at random we are down a long way from the 10000 original possibilities. I’ll let you decide if that is an acceptable risk.

It’s worth noting that while most people would probably use a four letter dictionary word there’s nothing from stopping you using things like “A DOG” or “I RUN” or even a pass phase “Am I Nicely Secure?” = AINS.

What are your views? Foolish or clever? Are there any other wallet tricks that people know?