Username & Password Algorithm

well i'm currentlly developing a messenger program like yahoo(not as goog ym).... anyway... if i use microsoft asp.net service to check login details from client system application.... and want to intrigrate with wwf.... then those users who already registered to wwf they can also login through there that account in messenger. anyway how to check that there username and password is valid through wwf database... as password and username are encrypted..... so, i think this password maybe check password through same algorithm...


anyway, any suggesion?