diff --git a/lib/openpgp.php b/lib/openpgp.php
index b3a2fd6..4a417e8 100644
--- a/lib/openpgp.php
+++ b/lib/openpgp.php
@@ -476,7 +476,7 @@ class OpenPGP_SignaturePacket extends OpenPGP_Packet {
     $body .= pack('n', strlen($unhashed_subpackets)).$unhashed_subpackets;
 
     $body .= pack('n', $this->hash_head);
-    $body .= pack('n', strlen($this->data)*8).$this->data;
+    $body .= pack('n', OpenPGP::bitlength($this->data)).$this->data;
 
     return $body;
   }