Clarify when key derivation entry points are mandatory/permitted
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/docs/proposed/psa-driver-interface.md b/docs/proposed/psa-driver-interface.md
index bbd1f58..b6a6508 100644
--- a/docs/proposed/psa-driver-interface.md
+++ b/docs/proposed/psa-driver-interface.md
@@ -322,10 +322,10 @@
A key derivation driver has the following entry points:
* `"key_derivation_setup"` (mandatory): always the first entry point to be called. This entry point provides the [initial inputs](#key-derivation-driver-initial-inputs). See [“Key derivation driver setup”](#key-derivation-driver-setup).
-* `"key_derivation_input_step"` (optional): provide an extra input for the key derivation. This entry point is only mandatory in drivers that support algorithms that have extra inputs. See [“Key derivation driver long inputs”](#key-derivation-driver-long-inputs).
+* `"key_derivation_input_step"` (mandatory if the driver supports a key derivation algorithm with long inputs, otherwise ignored): provide an extra input for the key derivation. This entry point is only mandatory in drivers that support algorithms that have extra inputs. See [“Key derivation driver long inputs”](#key-derivation-driver-long-inputs).
* `"key_derivation_output_bytes"` (mandatory): derive cryptographic material and output it. See [“Key derivation driver outputs”](#key-derivation-driver-outputs).
* `"key_derivation_output_key"`, `"key_derivation_verify_bytes"`, `"key_derivation_verify_key"` (optional, opaque drivers only): derive key material which remains inside the same secure element. See [“Key derivation driver outputs”](#key-derivation-driver-outputs).
-* `"key_derivation_set_capacity"` (mandatory for opaque drivers that implement `"key_derivation_output_key"` for “cooked”, i.e. non-raw-data key types): update the capacity policy on the operation. See [“Key derivation driver operation capacity”](#key-derivation-driver-operation-capacity).
+* `"key_derivation_set_capacity"` (mandatory for opaque drivers that implement `"key_derivation_output_key"` for “cooked”, i.e. non-raw-data key types; ignored for other opaque drivers; not permitted for transparent drivers): update the capacity policy on the operation. See [“Key derivation driver operation capacity”](#key-derivation-driver-operation-capacity).
* `"key_derivation_abort"` (mandatory): always the last entry point to be called.
For naming purposes, here and in the following subsection, this specification takes the example of a driver with the prefix `"acme"` that implements the `"key_derivation"` entry point family with a capability that does not use the `"names"` property to declare different type and entry point names. Such a driver must implement the following type and functions, as well as the entry points listed above and described in the following subsections: