diff --git a/lib/trait/Login.class.php b/lib/trait/Login.class.php
index d03b94a..545a83f 100644
--- a/lib/trait/Login.class.php
+++ b/lib/trait/Login.class.php
@@ -175,8 +175,7 @@ private function getDerviedk ($password) {
 		// calculate derivedk
 		if (!function_exists('hash_pbkdf2')) {
 			$pbkdf2 = new CryptLib\Key\Derivation\PBKDF\PBKDF2(array('hash' => 'sha1'));
-			$derivedk = bin2hex($pbkdf2->derive(hash('sha256', $password), substr($this->challenge, 0, 16), 1000, 32));
-			$derivedk = substr($derivedk, 0, 32);
+			$derivedk = bin2hex($pbkdf2->derive(hash('sha256', $password), substr($this->challenge, 0, 16), 1000, 16));
 		}
 		else {
 			$derivedk = hash_pbkdf2('sha1', hash('sha256', $password), substr($this->challenge, 0, 16), 1000, 32);