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

Answer 1

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).