diff --git a/constraints.txt b/constraints.txt
new file mode 100644
index 0000000..ba7eb68
--- /dev/null
+++ b/constraints.txt
@@ -0,0 +1,3 @@
+# cryptography 1.4 requires OpenSSL > 0.9.8 which is not available on Debian
+# 6/"Squeeze".
+cryptography<1.4