decrypt password from aspnet_membership table

hi,
how can decrypt password like this:
 var check = from t1 in db.aspnet_Memberships                     join t2 in db.aspnet_Users                     on t1.UserId equals t2.UserId                     where  t2.UserName=="test"                     select new { t1, t2 }; Membership.GetUser("test").GetPassword(); I need to get password from membership.

Thanks