diff --git a/book/source/17-resources.md b/book/source/17-resources.md
index d831d92..ce828cf 100644
--- a/book/source/17-resources.md
+++ b/book/source/17-resources.md
@@ -1,8 +1,8 @@
 # External resources
 
-[RFC4880](https://www.rfc-editor.org/rfc/rfc4880.html) OpenPGP Message Format [2007]
+[OpenPGP version 6 draft specification](https://datatracker.ietf.org/doc/draft-ietf-openpgp-crypto-refresh/) draft-ietf-openpgp-crypto-refresh [2023]
 
-[An Advanced Introduction to GnuPG](https://gnupg.org/ftp/people/neal/an-advanced-introduction-to-gnupg/openpgp/openpgp.pdf) [Neal Walfield, 2017]
+[RFC4880](https://www.rfc-editor.org/rfc/rfc4880.html) OpenPGP Message Format [2007], specifies OpenPGP version 4
 
 
 ## Web of Trust
@@ -12,3 +12,7 @@
 
 [OpenPGP Web of Trust](https://sequoia-pgp.gitlab.io/sequoia-wot/) [Neal Walfield, 2022];
 implementation: <https://gitlab.com/sequoia-pgp/sequoia-wot/>.
+
+## GnuPG
+
+[An Advanced Introduction to GnuPG](https://gnupg.org/ftp/people/neal/an-advanced-introduction-to-gnupg/openpgp/openpgp.pdf) [Neal Walfield, 2017]