protected function _digest($request) {
		$realm = $this->_config['realm'];
		$data = array(
			'username' => null, 'nonce' => null, 'nc' => null,
			'cnonce' => null, 'qop' => null, 'uri' => null,
			'response' => null
		);
		$result = array_map(function ($string) use (&$data) {
			$parts = explode('=', trim($string), 2) + array('', '');
			$data[$parts[0]] = trim($parts[1], '"');
		}, explode(',', $request->env('PHP_AUTH_DIGEST')));

		$users = $this->_config['users'];
		$password = !empty($users[$data['username']]) ? $users[$data['username']] : null;
		$user = md5("{$data['username']}:{$realm}:{$password}");
		$nonce = "{$data['nonce']}:{$data['nc']}:{$data['cnonce']}:{$data['qop']}";
		$req = md5($request->env('REQUEST_METHOD') . ':' . $data['uri']);
		$hash = md5("{$user}:{$nonce}:{$req}");

		if (!$data['username'] || $hash !== $data['response']) {
			$nonce = uniqid();
			$opaque = md5($realm);

			$message = "WWW-Authenticate: Digest realm=\"{$realm}\",qop=\"auth\",";
			$message .= "nonce=\"{$nonce}\",opaque=\"{$opaque}\"";
			$this->_writeHeader($message);
			return false;
		}
		return array('username' => $data['username'], 'password' => $password);
	}