More details on storing 'kdcchallenge' at the KDC

Author: marco-tiloca-sics

draft-ietf-ace-key-groupcomm.md

@@ -420,7 +420,7 @@ If the access token contains a role that requires the Client to send its own aut
 
 Later, when joining the group (see {{ssec-key-distribution-exchange}}), the Client uses the 'kdcchallenge' value and additional information to build a proof-of-possession (PoP) input. This is in turn used to compute a PoP evidence, which the Client also provides to the KDC in order to prove possession of its own private key (see the 'client_cred_verify' parameter in {{gid-post}}).
 
-The KDC MUST store the 'kdcchallenge' value associated with the Client at least until it receives a Join Request from it (see {{ssec-key-distribution-exchange}}), to be able to verify the PoP evidence provided during the join process, and thus that the Client possesses its own private key.
+While storing the access token, the KDC MUST store the 'kdcchallenge' value associated with the Client at least until it receives a Join Request from it (see {{ssec-key-distribution-exchange}}), to be able to verify the PoP evidence provided during the join process, and thus that the Client possesses its own private key. The KDC deletes the 'kdcchallenge' value associated with the Client upon deleting the access token (e.g., upon its expiration, see {{Section 5.10.3 of RFC9200}}).
 
 The same 'kdcchallenge' value MAY be reused several times by the Client, to generate a new PoP evidence, e.g., in case the Client provides the KDC with a new authentication credential while being a group member (see {{update-pub-key}}), or joins a different group where it intends to use a different authentication credential. Therefore, it is RECOMMENDED that the KDC keeps storing the 'kdcchallenge' value after the first join is processed as well. If the KDC has already discarded the 'kdcchallenge' value, that will trigger an error response with a newly generated 'kdcchallenge' value that the Client can use to restart the join process, as specified in {{ssec-key-distribution-exchange}}.