cosmetic prep: publish vlr_use_umts_aka() decision

During Set Ciphering Mode on GERAN, it is required to know whether UMTS AKA is
used to decide which Kc to pick. Change static function is_umts_auth() into
public vlr_use_umts_aka(), so future patches can re-use it.

Prepares: If04e405426c55a81341747a9b450a69188525d5c
Change-Id: I85d784c62ecbabdb6186a3dae4dcd554e7921041

include/osmocom/msc/vlr.h

@@ -435,5 +435,7 @@ int vlr_set_ciph_mode(struct vlr_instance *vlr,
 		      enum vlr_ciph ciph_mode,
 		      bool retrieve_imeisv);
 
+bool vlr_use_umts_aka(struct osmo_auth_vector *vec, bool is_r99);
+
 void log_set_filter_vlr_subscr(struct log_target *target,
 			       struct vlr_subscr *vlr_subscr);

src/libvlr/vlr.c

@@ -1122,6 +1122,23 @@ int vlr_set_ciph_mode(struct vlr_instance *vlr,
 	}
 }
 
+/* Decide whether UMTS AKA should be used.
+ * UTRAN networks are by definition R99 capable, and the auth vector is required to contain UMTS AKA
+ * tokens. This is expected to be verified by the caller. On GERAN, UMTS AKA must be used iff MS and
+ * GERAN are R99 capable and UMTS AKA tokens are available.
+ * \param[in] vec  Auth tokens (received from the HLR).
+ * \param[in] is_r99  True when BTS and GERAN are R99 capable.
+ * \returns true to use UMTS AKA, false to use pre-R99 GSM AKA.
+ */
+bool vlr_use_umts_aka(struct osmo_auth_vector *vec, bool is_r99)
+{
+	if (!is_r99)
+		return false;
+	if (!(vec->auth_types & OSMO_AUTH_TYPE_UMTS))
+		return false;
+	return true;
+}
+
 void log_set_filter_vlr_subscr(struct log_target *target,
 			       struct vlr_subscr *vlr_subscr)
 {

src/libvlr/vlr_auth_fsm.c

@@ -211,16 +211,6 @@ static void auth_fsm_onenter_failed(struct osmo_fsm_inst *fi, uint32_t prev_stat
 		vlr_subscr_tx_auth_fail_rep(vsub);
 }
 
-static bool is_umts_auth(struct auth_fsm_priv *afp,
-			 uint32_t auth_types)
-{
-	if (!afp->is_r99)
-		return false;
-	if (!(auth_types & OSMO_AUTH_TYPE_UMTS))
-		return false;
-	return true;
-}
-
 /* Terminate the Auth FSM Instance and notify parent */
 static void auth_fsm_term(struct osmo_fsm_inst *fi, enum vlr_auth_fsm_result res)
 {
@@ -268,7 +258,7 @@ static int _vlr_subscr_authenticate(struct osmo_fsm_inst *fi)
 	afp->auth_requested = true;
 	vsub->last_tuple = at;
 	vsub->vlr->ops.tx_auth_req(vsub->msc_conn_ref, at,
-				   is_umts_auth(afp, at->vec.auth_types));
+				   vlr_use_umts_aka(&at->vec, afp->is_r99));
 	return 0;
 }