Password Strength Estimation with zxcvbn

Following up my last post about Troy Hunt and breached passwords, I thought I’d look at something it mentioned – zxcvbn password strength estimation. Password strength estimators are, as a rule, crap. Usually it’s simply “is there a number, is there a capital”, etc., so passwords like “P@ssword1234” can come out as being ‘strong’, which is clearly wrong.

zxcvbn (awful name) attempts to make a smarter estimate of password strength. Basically, it looks at your password, and tries to estimate its strength based on appearances in dictionaries, structure, and entropy. It’ll then give an estimated “time to crack”, and tell you any matches that were achieved. For example (and neither of these are good passwords – score goes up to 4, and you want to get at least 3):


Yup, that’s pretty cool.

It’s available as a Nuget package (and is implemented in some form in most frameworks). The original zxcvbn was intended to run solely within the browser – but it still had a 0.5 Mb of dictionary files to download. That’s built into the assembly for the Nuget.

Testing a password – that’s simple:

var result = Zxcvbn.MatchPassword(“Squeamish Ossifrage”);

Neat. Worth considering if you’re implementing a password change or registration feature.

Bonus observation: http://gavinmiller.io/2016/a-tale-of-security-gone-wrong/

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Password Strength Estimation with zxcvbn