Why are ATM PINs only 4 digits?
Most of the time we never really think about technology we’ve been using for a long time. The BBC article The man who invented the cash machine, takes a step back to look at the first ATMs.
This is a classic, about the first ATM “cards”:
Plastic cards had not been invented, so Mr Shepherd-Barron’s machine used cheques that were impregnated with carbon 14, a mildly radioactive substance.
The machine detected it, then matched the cheque against a Pin number.
But why only four digits?
One by-product of inventing the first cash machine was the concept of the Pin number.
Mr Shepherd-Barron came up with the idea when he realised that he could remember his six-figure army number. But he decided to check that with his wife, Caroline.
“Over the kitchen table, she said she could only remember four figures, so because of her, four figures became the world standard,” he laughs.
Mind you, some banks use 6 digits .. but as mentioned in the comments on Schneiers post (link below) maybe other banks don’t know how to use the extra digits?
So .. how many digits can people remember? I guess with four digits .. there only a 3 in 10,000 chance of guessing it before the card gets swallowed by the ATM .. that’s not too bad. So could most people memorise a 5 or 6 digit PIN to make it more secure?
( Via Why an ATM PIN Has Four Digits – Shneier )