In application/libraries/REST_Controller.php find the following function:

protected function _check_login($username = '', $password = NULL)</pre>
 {
 if (empty($username))
 {
 return FALSE;
 }

 $auth_source = strtolower($this->config->item('auth_source'));

if ($auth_source == 'ldap')
 {
 log_message('debug', 'performing LDAP authentication for $username');
 return $this->_perform_ldap_auth($username, $password);
 }

$valid_logins = & $this->config->item('rest_valid_logins');

if ( ! array_key_exists($username, $valid_logins))
 {
 return FALSE;
 }

// If actually NULL (not empty string) then do not check it
 if ($password !== NULL AND $valid_logins[$username] != $password)
 {
 return FALSE;
 }
 return TRUE;
 }

And replace it with the following:

protected function _check_login($username = '', $password = NULL)</pre>
{
 if (empty($username))
 {
 return FALSE;
 }
 return $this->tank_auth->login($username, $password, FALSE, FALSE, TRUE);
 }

If you are not auto loading the tank auth library, be sure to do that in your constructor as well.