diff --git a/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 9019e83..7f38054 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -809,6 +809,34 @@
 either could not be updated or the authentication image descriptor indicates
 that it is not allowed to be updated.
 
+Function: plat_convert_pk()
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : void *, unsigned int, void **, unsigned int *
+    Return   : int
+
+This function is optional when Trusted Board Boot is enabled, and only
+used if the platform saves a hash of the ROTPK.
+First argument is the Distinguished Encoding Rules (DER) ROTPK.
+Second argument is its size.
+Third argument is used to return a pointer to a buffer, which hash should
+be the one saved in OTP.
+Fourth argument is a pointer to return its size.
+
+Most platforms save the hash of the ROTPK, but some may save slightly different
+information - e.g the hash of the ROTPK plus some related information.
+Defining this function allows to transform the ROTPK used to verify
+the signature to the buffer (a platform specific public key) which
+hash is saved in OTP.
+
+The default implementation copies the input key and length to the output without
+modification.
+
+The function returns 0 on success. Any other value means the expected
+public key buffer cannot be extracted.
+
 Dynamic Root of Trust for Measurement support (in BL31)
 -------------------------------------------------------