fix #5111 AttributeError occured with >=pyOpenSSL-17.2.0 (#5112)

acme/acme/crypto_util.py

@@ -2,6 +2,7 @@
 import binascii
 import contextlib
 import logging
+import os
 import re
 import socket
 import sys
@@ -243,7 +244,7 @@ def gen_ss_cert(key, domains, not_before=None,
     """
     assert domains, "Must provide one or more hostnames for the cert."
     cert = OpenSSL.crypto.X509()
-    cert.set_serial_number(int(binascii.hexlify(OpenSSL.rand.bytes(16)), 16))
+    cert.set_serial_number(int(binascii.hexlify(os.urandom(16)), 16))
     cert.set_version(2)
 
     extensions = [