diff --git a/PyPDF2/pdf.py b/PyPDF2/pdf.py
index c5cae9e673..2974fef39c 100644
--- a/PyPDF2/pdf.py
+++ b/PyPDF2/pdf.py
@@ -420,7 +420,7 @@ def encrypt(self, user_pwd, owner_pwd = None, use_128bit = True):
             rev = 2
             keylen = int(40 / 8)
         # permit everything:
-        P = -1
+        P = -3904
         O = ByteStringObject(_alg33(owner_pwd, user_pwd, rev, keylen))
         ID_1 = ByteStringObject(md5(b_(repr(time.time()))).digest())
         ID_2 = ByteStringObject(md5(b_(repr(random.random()))).digest())