diff --git a/book/source/03-cryptography.md b/book/source/03-cryptography.md
index 29a47a4..5a2c02f 100644
--- a/book/source/03-cryptography.md
+++ b/book/source/03-cryptography.md
@@ -43,3 +43,6 @@ https://en.wikipedia.org/wiki/Symmetric-key_algorithm
 Symmetric encryption is a core concept in OpenPGP. It usually comes up involving the term "session key."
 
 "Session keys" in OpenPGP are symmetric cryptographic keys.
+
+## Cryptographic signatures
+