Aleation is meant to stay simple and is not designed to store passwords.
The user name of a player is only used to recognize his draws. Unless you PHP session is hi-jacked, nobody can steal your identity after you joined a game. On the contrary, someone will be able to start a game with your favorite user name or join another one if your user name is not already used.
If a player ever looses his session (browser crashes, power cut...) he won't be able to join the game again. Anyway, it's quiet easy to restart one.
Please note that the characters '/\?%*:|"<>«»'.&=' and 'space' will be replaced with '_'.
It's better to always use the same user name as in some 'templates', a list of previous games can be displayed. Then it's easier for someone to find your current game or for you to find old games you played.