From 727406cbacd23c97f489344e1f9a82c742f97ecc Mon Sep 17 00:00:00 2001
From: Kushal Kumaran <kushal.kumaran+github@gmail.com>
Date: Mon, 27 Dec 2010 16:03:35 +0530
Subject: [PATCH] Only compare first 16 bytes of U entry when decrypting
 revision 3 or greater.

---
 pyPdf/pdf.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/pyPdf/pdf.py b/pyPdf/pdf.py
index bf60d01..57e738b 100644
--- a/pyPdf/pdf.py
+++ b/pyPdf/pdf.py
@@ -936,15 +936,16 @@ def _authenticateUserPassword(self, password):
         p_entry = encrypt['/P'].getObject()
         id_entry = self.trailer['/ID'].getObject()
         id1_entry = id_entry[0].getObject()
+        real_U = encrypt['/U'].getObject().original_bytes
         if rev == 2:
             U, key = _alg34(password, owner_entry, p_entry, id1_entry)
+            return U == real_U, key
         elif rev >= 3:
             U, key = _alg35(password, rev,
                     encrypt["/Length"].getObject() / 8, owner_entry,
                     p_entry, id1_entry,
                     encrypt.get("/EncryptMetadata", BooleanObject(False)).getObject())
-        real_U = encrypt['/U'].getObject().original_bytes
-        return U == real_U, key
+            return U[:16] == real_U[:16], key
 
     def getIsEncrypted(self):
         return self.trailer.has_key("/Encrypt")