diff --git a/tox.ini b/tox.ini
index 6987847f8..c7e1e43c9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -38,6 +38,7 @@ deps =
 # https://packages.debian.org/en/jessie/zope/
 basepython = python2.7
 deps =
+    cryptography==0.6.1
     pyOpenSSL==0.14
     lxml==3.4.0
     Twisted==14.0.2