diff --git a/docs/design/trusted-board-boot.rst b/docs/design/trusted-board-boot.rst
index 46177d7..fed202a 100644
--- a/docs/design/trusted-board-boot.rst
+++ b/docs/design/trusted-board-boot.rst
@@ -216,10 +216,11 @@
 
 The ``cert_create`` tool is built and runs on the host machine as part of the
 TF-A build process when ``GENERATE_COT=1``. It takes the boot loader images
-and keys as inputs (keys must be in PEM format) and generates the
-certificates (in DER format) required to establish the CoT. New keys can be
-generated by the tool in case they are not provided. The certificates are then
-passed as inputs to the ``fiptool`` utility for creating the FIP.
+and keys as inputs and generates the certificates (in DER format) required to
+establish the CoT. The input keys must either be a file in PEM format or a
+PKCS11 URI in case a HSM is used. New keys can be generated by the tool in
+case they are not provided. The certificates are then passed as inputs to
+the ``fiptool`` utility for creating the FIP.
 
 The certificates are also stored individually in the output build directory.