>>4 We already have tripcodes, this is how you'd 'create a password'. There's no need for anything else. Just try it out in test. Look at my name, that tripcode beside Inuhakka is made from a password, and only that password can make it.
I think what you want already exists on the site. There's a thread in the Test board under Random to help you out there. Note, if you want actual security from these, you'll have to use two starters (!! or ##) instead of one, since they are insecure without.