If you've got a website aimed at people that are not neccessarily technically proficient, letting new users choose their own password isn't very secure because of the simple fact that a lot of people don't choose secure passwords. Some methods of dealing with this problem are:
- Most common: have a 'password strenght checker' or demand that the password contains a certain number and different kinds of characters. These often aren't very good and result in passwords that are not actually very secure and/or difficult to remember (of course this doesn't mean you can make a very good password checker, although that is pretty difficult).
- Generate a random sequence of characters: very impractical and insecure because people tend to write these down because they can't remember them.
- Two phase authentication through SMS or e-mail: not very convenient for the user.
I've thought of a different approach that is inspired by this XKCD comic. It works like this:
When a new user registers, they are presented with 20 randomly picked dictionary words. They are asked to make a secret sentence (let's call it a passphrase) that contains at least five of these words. They are encouraged to create a nonsensical sentence that is hard to guess but easy for them to remember. They have an option to refresh the 20 random words multiple times if they can't think of anything.
This passphrase will simply function as their passwords. You only have to check they use at least five of the provided words and take the usual security measures you would for regular passwords.
I personally think this is a pretty user-friendly way of letting people choose strong passwords; and the problems I mentioned for the other methods appear to be avoided. Of course I could be completely wrong and might be overseeing something important. So I'd really like to hear what the smart people here think about this system.