One of my closely held beliefs is that expiring passwords reduce rather than increase security because the more often you have to change your passwords, the less likely you are to remember them. That is offset by the fact that people tend to use one password everywhere, so if you force people to change them, that pattern can be broken to some extent.
This week, Bruce Schneier has an essay on the subject. Here’s his bottom line, but read the whole thing:
So in general: you don’t need to regularly change the password to your computer or online financial accounts (including the accounts at retail sites); definitely not for low-security accounts. You should change your corporate login password occasionally, and you need to take a good hard look at your friends, relatives, and paparazzi before deciding how often to change your Facebook password. But if you break up with someone you’ve shared a computer with, change them all.