diff --git a/requirements.txt b/requirements.txt
index 1f3bda1d7..192b6a0a8 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 Twisted==16.6.0
+cryptography==1.9
 appdirs==1.4.3
 argparse==1.2.1
 docopt==0.6.2