通过POST使用SecurityServiceProvider进行用户身份验证

What I'm looking to do is

Authenticate users on bar.com and
Post their credentials to foo.com/login and re-authenticate them without needing to log in again.

Currently, to GET secure pages on foo.com I'm using form-based access via the SecurityServiceProvider and a db-backed UserProvider to authenticate. Works great: any attempt to load a secured route is intercepted by the firewall and then redirected after successful authentication.

What I can't figure out is how to pass the POST variables (username and password) on to the provider instance and forward the user to the supplied route.

Stub POST route:

$app->post('/login', function(Request $req) use ($app) {
    $route    = $req->request->filter('route');
    $username = $req->get('username');
    $password = $req->get('password');

    /* magic happens...? */ 
});

Here is an example of using the user provider to load a user check the password matches then setting the token in the security service. So if you put this code into a route you can get access to the Request for your username and password.

$userProvider = $app['security.user_provider.default'];

$user = null;
try {
   $user = $userProvider->loadUserByUsername($username);
} catch (UsernameNotFoundException $e)
{
   ;
}
$encoder = $app['security.encoder_factory']->getEncoder($user);

// compute the encoded password
$encodedPassword = $encoder->encodePassword($password, $user->getSalt());

// compare passwords
if ($user->password == $encodedPassword)
{
   // set security token into security
   $token = new UsernamePasswordToken($user, $password, 'yourProviderKeyHere', array('ROLE_USER'));
   $app['security']->setToken($token);

   // redirect or give response here
} else {
   // error feedback
}