How do I measure the strength of a password?

Question

I was looking for an effective algorithm that can give me an accurate idea of how strong a password is.

I found that several different websites use several different algorithms as I get different password strength ratings on different websites.

Answer 1

This has grown to my general brain dump of best practices for working with passwords in PHP/MySQL. The ideas presented here are generally not my own, but the best of what I've found to date.

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Ensure you are using SSL for all operations involving user information. All pages that involve these forms should check they are being called via HTTPS, and refuse to work otherwise.

You can eliminate most attacks by simply limiting the number of failed logins allowed.

Allow for relatively weak passwords, but store the number of failed logins per user and require a captcha or password verification by email if you exceed it. I set my max failures to 5.

Presenting login failures to the user needs to be carefully thought out as to not provide information to attackers. A failed login due to a non existent user should return the same message as a failed login due to a bad password. Providing a different message will allow attackers to determine valid user logins.

Also make sure you return exactly the same message in the event of a failure for too many logins with a valid password, and a failure with too many logins and a bad password. Providing a different message will allow attackers to determine valid user passwords. A fair number of users when forced to reset their password will simply put it back to what it was.

Unfortunately limiting the number of logins allowed per IP address is not practical. Several providers such as AOL and most companies proxy their web requests. Imposing this limit will effectively eliminate these users.

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I've found checking for dictionary words before submit to be inefficient as either you have to send a dictionary to the client in javascript, or send an ajax request per field change. I did this for a while and it worked ok, but didn't like the traffic it generated.

Checking for inherently weak passwords minus dictionary words IS practical client side with some simple javascript.

After submit, I check for dictionary words, and username containing password and vice versa server side. Very good dictionaries are readily downloadable and the testing against them is simple. One gotcha here is that to test for a dictionary word, you need to send a query against the database, which again contains the password. The way I got around this was to encrypt my dictionary before hand with a simple encryption and end positioned SALT and then test for the encrypted password. Not ideal, but better than plain text and only on the wire for people on your physical machines and subnet.

Once you are happy with the password they have picked encrypt it with PHP first, then store. The following password encryption function is not my idea either, but solves a number of problems. Encrypting within PHP prevents people on a shared server from intercepting your unencrypted passwords. Adding something per user that won't change (I use email as this is the username for my sites) and add a hash (SALT is a short constant string I change per site) increases resistance to attacks. Because the SALT is located within the password, and the password can be any length, it becomes almost impossible to attack this with a rainbow table. Alternately it also means that people can't change their email and you can't change the SALT without invalidating everyone's password though.

EDIT: I would now recommend using PhPass instead of my roll your own function here, or just forget user logins altogether and use OpenID instead.

function password_crypt($email,$toHash) {
  $password = str_split($toHash,(strlen($toHash)/2)+1);
  return hash('sha256', $email.$password[0].SALT.$password[1]); 
}

My Jqueryish client side password meter. Target should be a div. It's width will change between 0 and 100 and background color will change based on the classes denoted in the script. Again mostly stolen from other things I've found:

$.updatePasswordMeter = function(password,username,target) {
$.updatePasswordMeter._checkRepetition = function(pLen,str) {
res = ""
for ( i=0; i<str.length ; i++ ) {
    repeated=true;
    for (j=0;j < pLen && (j+i+pLen) < str.length;j++)
        repeated=repeated && (str.charAt(j+i)==str.charAt(j+i+pLen));
    if (j<pLen) repeated=false;
    if (repeated) {
        i+=pLen-1;
        repeated=false;
    }
    else {
        res+=str.charAt(i);
    };
};
return res;
};
var score = 0;
var r_class = 'weak-password';
//password < 4
if (password.length < 4 || password.toLowerCase()==username.toLowerCase()) { 
target.width(score + '%').removeClass("weak-password okay-password good-password strong-password"
).addClass(r_class);
  return true;
} 
//password length
score += password.length * 4;
score += ( $.updatePasswordMeter._checkRepetition(1,password).length - password.length ) * 1;
score += ( $.updatePasswordMeter._checkRepetition(2,password).length - password.length ) * 1;
score += ( $.updatePasswordMeter._checkRepetition(3,password).length - password.length ) * 1;
score += ( $.updatePasswordMeter._checkRepetition(4,password).length - password.length ) * 1;
//password has 3 numbers
if (password.match(/(.*[0-9].*[0-9].*[0-9])/))  score += 5; 

//password has 2 symbols
if (password.match(/(.*[!,@,#,$,%,^,&,*,?,_,~].*[!,@,#,$,%,^,&,*,?,_,~])/)) score += 5; 

//password has Upper and Lower chars
if (password.match(/([a-z].*[A-Z])|([A-Z].*[a-z])/))  score += 10; 

//password has number and chars
if (password.match(/([a-zA-Z])/) && password.match(/([0-9])/))  score += 15; 
//
//password has number and symbol
if (password.match(/([!,@,#,$,%,^,&,*,?,_,~])/) && password.match(/([0-9])/))  score += 15; 

//password has char and symbol
if (password.match(/([!,@,#,$,%,^,&,*,?,_,~])/) && password.match(/([a-zA-Z])/))  score += 15; 

//password is just a nubers or chars
if (password.match(/^\w+$/) || password.match(/^\d+$/) )  score -= 10; 

//verifing 0 < score < 100
score = score * 2;
if ( score < 0 )  score = 0;
if ( score > 100 )  score = 100;
if (score > 25 ) r_class = 'okay-password';
if (score > 50  ) r_class = 'good-password';
if (score > 75 ) r_class = 'strong-password';
target.width(score + '%').removeClass("weak-password okay-password good-password strong-password"
).addClass(r_class);
return true;
};

Answer 2

Fundamentally you want to prevent to major types of attacks

• Dictionary attacks
• Brute force attacks

To prevent the first, you want to consider passwords containing common words weak. To prevent the second, you want to encourage passwords of reasonable length (8+ characters is common) and with a reasonably large character set (include letters, numbers, and special characters). If you consider lower case and upper case letters to be different, that increases the character set substantially. However, this creates a usability issue for some user communities so you need to balance that consideration.

A quick google search turned up solutions that account for brute force attacks (complex password) but not for dictionary attacks. PHP Password Strength Meter from this list of strength checkers runs the check server-side, so it could be extended to check a dictionary.

EDIT:

By the way... you should also limit the number of login attempts per user. This will make both types of attacks less likely. Effective but not-user-friendly is to lock an account after X bad attempts and require a password reset. More user friendly but more effort is to throttle time between login attempts. You can also require CAPTCHA after the first few login attempts (which is something that Stack Overflow requires after too many edits, or for very new users).