Max password length... WTF?!?


I wonder why setting a (little) maximum password length.

Passwords (or their grown-up cousins passphrases) should be strong, right?

Up to a few years ago, this was basically translated into recommending a minimum width (usually around 8 characters) and forcing users to use uppercase, lowercase and special characters.

In 2017, NIST updated their guidelines based on a few more years of experience, with NIST Special Publication 800-63. Among other suggestions, it’s pretty clear that length matters (e.g. Appendix A in Digital Identity Guidelines - Authentication and Lifecycle Management):

Password length has been found to be a primary factor in characterizing password strength [Strength] [Composition]. Passwords that are too short yield to brute force attacks as well as to dictionary attacks using words and commonly chosen passwords.

xkcd gets the gist of the updated view about complexity in 936:

xkcd on password strength

I still see places that put a low limit to password length and I wonder… WHAT THE FUTZ?!? (By low I mean around 16 characters. 16 charactersxkcd 936’s password would be out of luck.)

Appendix A has something to say to this regard:

Users should be encouraged to make their passwords as lengthy as they want, within reason. Since the size of a hashed password is independent of its length, there is no reason not to permit the use of lengthy passwords (or pass phrases) if the user wishes. Extremely long passwords (perhaps megabytes in length) could conceivably require excessive processing time to hash, so it is reasonable to have some limit.

So… please everybody let’s not limit users’ creativeness in choosing strong passphrases that will be easy for them to remember!

I hope no more rants in the few days ahead!

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