nrUE NAS: accept some messages without integrity+ciphering

OAI core network is sending Deregistation Accept in clear, even
after security context activation.

The standards is not clear to me, so let's accept it.

To be removed if incorrect with respect to the standards.
(The core network will need to be fixed then.)

openair3/NAS/NR_UE/nr_nas_msg_sim.c

@@ -62,6 +62,7 @@ static nr_nas_msg_snssai_t nas_allowed_nssai[8];
 
 typedef enum {
   NAS_SECURITY_NO_SECURITY_CONTEXT,
+  NAS_SECURITY_UNPROTECTED,
   NAS_SECURITY_INTEGRITY_FAILED,
   NAS_SECURITY_INTEGRITY_PASSED,
   NAS_SECURITY_BAD_INPUT
@@ -71,6 +72,8 @@ security_state_t nas_security_rx_process(nr_ue_nas_t *nas, uint8_t *pdu_buffer,
 {
   if (nas->security_container == NULL)
     return NAS_SECURITY_NO_SECURITY_CONTEXT;
+  if (pdu_buffer[1] == 0)
+    return NAS_SECURITY_UNPROTECTED;
   /* header is 7 bytes, require at least one byte of payload */
   if (pdu_length < 8)
     return NAS_SECURITY_BAD_INPUT;
@@ -1371,6 +1374,14 @@ void *nas_nrue(void *args_p)
         int pdu_length = NAS_DOWNLINK_DATA_IND(msg_p).nasMsg.length;
 
         security_state_t security_state = nas_security_rx_process(nas, pdu_buffer, pdu_length);
+        /* special cases accepted without protection */
+        if (security_state == NAS_SECURITY_UNPROTECTED) {
+          int msg_type = get_msg_type(pdu_buffer, pdu_length);
+          /* for the moment, only FGS_DEREGISTRATION_ACCEPT is accepted */
+          if (msg_type == FGS_DEREGISTRATION_ACCEPT)
+            security_state = NAS_SECURITY_INTEGRITY_PASSED;
+        }
+
         if (security_state != NAS_SECURITY_INTEGRITY_PASSED
             && security_state != NAS_SECURITY_NO_SECURITY_CONTEXT) {
           LOG_E(NAS, "NAS integrity failed, discard incoming message\n");