diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index a256b8e..48f943a 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -1,7 +1,7 @@
 (certificates_chapter)=
 # Certificates
 
-OpenPGP fundamentally hinges on the concept of "OpenPGP certificates," often referred to as "OpenPGP keys." These certificates are complex data structures essential for identity verification and data encryption. Understanding their structure and functionality is pivotal for effective application of the OpenPGP standard.
+OpenPGP fundamentally hinges on the concept of "OpenPGP certificates," often referred to as "OpenPGP keys." These certificates are complex data structures essential for identity verification, data encryption, and digital signatures. Understanding their structure and functionality is pivotal for effective application of the OpenPGP standard.
 
 ## Terminology: Understanding "keys"