Is the password weak under dictionary attack

Question

Thanks for looking. All sincerely helpful answers are voted up.

I use a password strength meter to let the user know how strong the password they've chosen is. But this password checker obviously doesn't cover how weak under a dictionary attack the password is. How can I check for that, and is it worth it?

Also my regular password checker runs initially with javascript in the browser (no transmission required). If I want to check for dictionary attack weakness, I'd have to transmit it to a script. My understanding is that I shouldn't transmit it in the clear.

Can someone help me sort this out. How do I check the password isn't weak under a dictionary attack and how do I encrypt it before transmitting to my script?

Extra info:

Why do I think I need the dictionary attack check in addition to the regular password meter? As some of you have pointed out, users can choose passwords like P@ssword or Yellow12. But most password strength checkers I've come across will treat this as a good password. At least I'm using Yet Another Password Meter and it does (and I actually think it's one of the better password checkers.) If anyone knows of a stronger password checker, please mention it, but only if you know for sure based on experience that it's stronger ;)

But my question really is: how do I conduct a dictionary attack check on the password? I read somewhere that it's done against the hash, but where do I do the search? Once I find out how to do it, I will then decide whether it's worth it or not.

thanks to everyone who's helped out so far :)

Answer 1

Opinions are going to vary and some people will say that checking for dictionary words is important. I disagree and instead favor requiring different cases of letters, numbers and special characters like !@#$%^&()-=+. Obviously passwords should be case sensitive.

Dictionary attacks are much less likely to succeed with the presence of numbers and special characters. Lets say that there are 1000 common passwords. Now with the addition of a required upper case letter and special character lets assume the user is "lazy" and they choose to make the first letter capital and add a special character to the end. That 1000 sized dictionary is now over 30,000.

Additionally there should be account lockouts in place to avoid dictionary attacks. And possibly a throttle on how often an IP address can attempt to login depending on your application.

There may still be a case to avoid some very common passwords while running your script. I would for example not allow the word password p@ssword or any variation of password.

Edit: A captcha, while hated by most (including me) may be appropriate as well after a few failed logins to avoid brute force login attempts.

Answer 2

If you are using proper complexity requirements (length, mix of casing, numbers, symbols, and perhaps forbid repeat a char consecutively) then I'd say it's not really worth it. If you're in a situation where that would be required then probably password authentication would not be good enough for your situation anyway.

Answer 3

SSL

If your website in any way or on any page requests sensitive personal information, including passwords, then you should enable and enforce SSL across the entire site. This will ensure that all passwords are transmitted in encrypted form from the browser to the server, and that nobody can sniff the passwords off the network or modify the pages in transit (and alter the form postback url's).

Password Meter

You should run your password meter entirely in the browser. You should accept any and all passwords (with a min length of, for example, 6 characters) that the user enters, but feel free to hint to the user, from within the browser, whether they have entered a weak or strong password.

Answer 4

I'm coming to this question later than the others, and I'm surprised that no-one has pointed out that a dictionary check might not be exhaustive. At least no-one has said it in so many words.

I think you need a large dictionary, where each entry is hashed and compared to the hashed password. This will allow you to say the user's chosen password is not in your dictionary, but how will you be sure it's complete?

Obviously, you can't be sure. Do you include foreign words? Technical words?

Do password crackers have access to better dictionaries?

I think all you can do is advise users how to create a good password — show them a few examples — but let it be their choice.

And do the SSL thing.

Answer 5

One additional point - if you control the site, you can stop dictionary attacks by limiting the number of times a user can try a user/pass.

It is great you want your users to have better passwords and you should continue in that direction but a better solution for the dictionary/brute force attack would be an exponential backoff solution to failed login attempts. No real user will try and login 1000 times in 10 seconds with all different passwords.