Risks of Credit Cards
Apparently credit card numbers are linked to the expiration date MM/YY so that you can run an algorithm with just these two pieces of information offline and tell if the CC is valid or not.
Although a convenient feature, this design can be....i mean actually is being exploited.
Apparently, some Japanese hackers found out how to derive the full credit card number from just having the last four digits of the CC# and the expiration date.
Now... I wonder where they could the last four digits and exp. date from??
Oh yea! My monthly credit card statement has that information along with my address!!
So yes, apparently, bad guys are actively stealing people's monthly bills, calculating the full CC#, and using that for fraud.
By the way, most postal boxes in Japan are pretty easy to steal from and even if they have locks (most of the fancier apartments do), you can still stick your fingers in and take out all of the mail... so yea, the lock is more for show than anything practical.