Changing way UL NAS count is used to generate the K_enb.

lib/include/srslte/interfaces/ue_interfaces.h

@@ -138,7 +138,7 @@ public:
   virtual void      paging(LIBLTE_RRC_S_TMSI_STRUCT *ue_identiy) = 0;
   virtual bool      is_attached() = 0;
   virtual void      write_pdu(uint32_t lcid, srslte::byte_buffer_t *pdu) = 0;
-  virtual uint32_t  get_ul_count() = 0;
+  virtual uint32_t  get_k_enb_count() = 0;
   virtual bool      get_k_asme(uint8_t *k_asme_, uint32_t n) = 0;
   virtual uint32_t  get_ipv4_addr() = 0;
   virtual bool      get_ipv6_addr(uint8_t *ipv6_addr) = 0;

srsue/hdr/upper/nas.h

@@ -85,7 +85,8 @@ public:
   void paging(LIBLTE_RRC_S_TMSI_STRUCT *ue_identiy);
   void set_barring(barring_t barring);
   void write_pdu(uint32_t lcid, byte_buffer_t *pdu);
-  uint32_t get_ul_count();
+  void set_k_enb_count();
+  uint32_t get_k_enb_count();
   bool is_attached();
   bool get_k_asme(uint8_t *k_asme_, uint32_t n);
   uint32_t get_ipv4_addr();
@@ -125,6 +126,7 @@ private:
     uint8_t  k_asme[32];
     uint32_t tx_count;
     uint32_t rx_count;
+    uint32_t k_enb_count;
     srslte::CIPHERING_ALGORITHM_ID_ENUM  cipher_algo;
     srslte::INTEGRITY_ALGORITHM_ID_ENUM  integ_algo;
     LIBLTE_MME_EPS_MOBILE_ID_GUTI_STRUCT guti;

srsue/src/upper/nas.cc

@@ -388,14 +388,17 @@ void nas::write_pdu(uint32_t lcid, byte_buffer_t *pdu) {
   }
 }
 
-uint32_t nas::get_ul_count() {
+void nas::set_k_enb_count() {
-  // UL count for RRC key derivation depends on ESM information transfer procedure
+  // UL count for RRC key derivation depends on UL count of the Authentication Request or Service Request.
-  if (cfg.apn.empty()) {
+  // This function should be called after sending these messages, for later derivation of the keys.
-    // No ESM info transfer has been sent
+  ctxt.k_enb_count = ctxt.tx_count;
-    return ctxt.tx_count - 1;
+  return;
-  } else {
-    return ctxt.tx_count - 2;
 }
+
+uint32_t nas::get_k_enb_count() {
+  // UL count for RRC key derivation depends on UL count of the Authentication Request or Service Request.
+  // On the special case of Service Request without authentication, the UL count for the SR must be used.
+  return ctxt.k_enb_count;
 }
 
 bool nas::get_k_asme(uint8_t *k_asme_, uint32_t n) {
@@ -886,6 +889,7 @@ void nas::parse_authentication_request(uint32_t lcid, byte_buffer_t *pdu, const
     nas_log->info("Network authentication successful\n");
     send_authentication_response(res, res_len, sec_hdr_type);
     nas_log->info_hex(ctxt.k_asme, 32, "Generated k_asme:\n");
+    set_k_enb_count();
     auth_request = true;
   } else if (auth_result == AUTH_SYNCH_FAILURE) {
     nas_log->error("Network authentication synchronization failure.\n");
@@ -1214,6 +1218,7 @@ void nas::gen_service_request(byte_buffer_t *msg) {
   }
 
   ctxt.tx_count++;
+  set_k_enb_count();
 }
 
 void nas::gen_pdn_connectivity_request(LIBLTE_BYTE_MSG_STRUCT *msg) {

srsue/src/upper/rrc.cc

@@ -2075,8 +2075,8 @@ void rrc::parse_dl_dcch(uint32_t lcid, byte_buffer_t *pdu) {
       uint8_t k_asme[32];
       nas->get_k_asme(k_asme, 32);
       rrc_log->debug_hex(k_asme, 32, " UE K_asme");
-      rrc_log->debug("Generating K_enb. UL NAS COUNT %d\n", nas->get_ul_count());
+      rrc_log->debug("Generating K_enb. UL NAS COUNT %d\n", nas->get_k_enb_count());
-      usim->generate_as_keys(k_asme, nas->get_ul_count(), k_rrc_enc, k_rrc_int, k_up_enc, k_up_int, cipher_algo, integ_algo);
+      usim->generate_as_keys(k_asme, nas->get_k_enb_count(), k_rrc_enc, k_rrc_int, k_up_enc, k_up_int, cipher_algo, integ_algo);
       rrc_log->debug_hex(k_rrc_enc, 32, "RRC encryption key - k_rrc_enc");
       rrc_log->debug_hex(k_rrc_int, 32, "RRC integrity key  - k_rrc_int");
       rrc_log->debug_hex(k_up_enc, 32,  "UP encryption key  - k_up_enc");