diff --git a/docs/glossary.rst b/docs/glossary.rst
index b80b4b2..e4fc828 100644
--- a/docs/glossary.rst
+++ b/docs/glossary.rst
@@ -18,7 +18,8 @@
 
     key
         Secret data is encoded with a function using this key. Sometimes
-        multiple keys are used.
+        multiple keys are used. These **must** be kept secret, if a key is
+        exposed to an attacker, any data encrypted with it will be exposed.
 
     symmetric cryptography
         Cryptographic operations where encryption and decryption use the same