diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index d7faa78..644d1a5 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -254,6 +254,14 @@ For v6, this type of approach is discouraged, but a replacement mechanism is sti
 Wiktor suggests to check: https://blogs.gentoo.org/mgorny/2018/08/13/openpgp-key-expiration-is-not-a-security-measure/ for important material
 ```
 
+### Metadata about the primary key: In Direct Key Signature vs. in Primary User ID, in v4 and v6
+
+```{admonition} TODO
+:class: warning
+
+write
+```
+
 ### Metadata leak of Social Graph
 
 (unbound_user_ids)=