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How long is a password string?

2011-10-18 by grahamlee. 1 comments

Password policy questions are a perennial fixture of IT Security stack exchange, and take many forms.

Take, for example, the recent XKCD comic on the subject:

Shortly after it was posted, user Billy ONeal asked directly whether the logic was sound: Is a short complex password or a long dictionary passphrase better? You will find answers that support either conclusion, as well as answers that put the trade-offs involved into context.

Of course, part of knowing how easy a password is to crack is knowing how a password cracker works. Are there state of the art techniques or theory specifically for attacking pass phrases? Are there lists of the most common words or ngrams used in passwords and pass phrases?

So we’ve got no consistent idea about what constitutes a “good” password, although we can probably guess at what weak passwords will fall first. But then who is responsible for deciding a user’s password strength? Pretend for a moment that we agree what a strong password is. Should we stop users from choosing “weak” passwords, or is it their own fault for not understanding information theory and the entropy content of their chosen string of characters?

The definitive answer is “it depends”. It depends on how valuable the assets being protected by password access are. It depends on whether the value is appreciated by you as service provider/systems administrator, or by the user as customer/asset owner (or by a combination of those roles, or by someone else). It depends on how much inconvenience you’re willing to put your users to, and on how much inconvenience your users are willing to accept.

So, you’ve decided that you do want to enforce a policy. How does that work? Some websites enforce a maximum password length, is that a good idea?. Should passwords be truly random?

To summarise the discussion this far: there are different ideas of what makes a good password, of who is responsible for deciding whether a password is good or bad, and of how to enforce good passwords once you do decide you want to. But all of this discussion has already put the cart before the horse. Are your passwords going to be brute-force, or will the attacker use a key logger? Maybe they’ll attack the password where it’s stored?.

Indeed, while we’re asking the big questions, why passwords at all? Might biometric identification be better? Why not just forget all of this complication over strong passwords and start taking fingerprints instead? Some reasons include the possibility of false positives from the biometrics system (who hasn’t tried holding up a photograph to a facial recognition system?), and the icky disgustingness of some other attacks.

Suffice it to say that the problem of password security is a complex one, but one that the denizens of enjoy tackling.