diff --git a/common/utils/nr/nr_common.c b/common/utils/nr/nr_common.c
index b370b8da66..a3a8520c86 100644
--- a/common/utils/nr/nr_common.c
+++ b/common/utils/nr/nr_common.c
@@ -1463,6 +1463,18 @@ uint64_t nr_construct_5g_s_tmsi(uint16_t amf_set_id, uint8_t amf_pointer, uint32
   return ((uint64_t)amf_set_id << 38) | ((uint64_t)amf_pointer << 32) | m_tmsi;
 }
 
+/** @brief Extract 5G-S-TMSI-Part1 from full 5G-S-TMSI
+ * @param fiveg_s_tmsi Full 5G-S-TMSI (48 bits)
+ * @return 5G-S-TMSI-Part1 (rightmost 39 bits)
+ * @note 5G-S-TMSI-Part1 is the rightmost 39 bits of the full 5G-S-TMSI:
+ *       - Bits 32-37: AMF Pointer (6 bits)
+ *       - Bits 0-31:  5G-TMSI (32 bits)
+ * @ref 3GPP TS 23.003 */
+uint64_t nr_extract_5g_s_tmsi_part1(const uint64_t fiveg_s_tmsi)
+{
+  return fiveg_s_tmsi & ((1ULL << 39) - 1);
+}
+
 /** @brief Construct 5G-S-TMSI-Part1 from 5G-S-TMSI components
  * @param amf_set_id AMF Set ID (10 bits)
  * @param amf_pointer AMF Pointer (6 bits)
@@ -1476,5 +1488,48 @@ uint64_t nr_construct_5g_s_tmsi_part1(uint16_t amf_set_id, uint8_t amf_pointer,
 {
   // Construct full 5G-S-TMSI and extract Part1: rightmost 39 bits
   uint64_t full_s_tmsi = nr_construct_5g_s_tmsi(amf_set_id, amf_pointer, m_tmsi);
-  return full_s_tmsi & ((1ULL << 39) - 1);
+  return nr_extract_5g_s_tmsi_part1(full_s_tmsi);
+}
+
+/** @brief Extract 5G-S-TMSI-Part2 from full 5G-S-TMSI
+ * @param fiveg_s_tmsi Full 5G-S-TMSI (48 bits)
+ * @return 5G-S-TMSI-Part2 (leftmost 9 bits)
+ * @note 5G-S-TMSI-Part2 is the leftmost 9 bits of the full 5G-S-TMSI:
+ *       - Bits 39-47: AMF Set ID (9 bits)
+ * @ref 3GPP TS 23.003 */
+uint16_t nr_extract_5g_s_tmsi_part2(const uint64_t fiveg_s_tmsi)
+{
+  return (fiveg_s_tmsi >> 39) & ((1ULL << 9) - 1);
+}
+
+/** @brief Build full 5G-S-TMSI from Part1 and Part2
+ * @param part1 5G-S-TMSI-Part1 (rightmost 39 bits)
+ * @param part2 5G-S-TMSI-Part2 (leftmost 9 bits)
+ * @return Full 5G-S-TMSI (48 bits)
+ * @note Combines Part2 (leftmost 9 bits) and Part1 (rightmost 39 bits)
+ * @ref 3GPP TS 23.003 */
+uint64_t nr_build_full_5g_s_tmsi(const uint64_t part1, const uint16_t part2)
+{
+  return ((uint64_t)part2 << 39) | part1;
+}
+
+/** @brief Deconstruct full 5G-S-TMSI into its components
+ * @param fiveg_s_tmsi Full 5G-S-TMSI (48 bits)
+ * @param[out] amf_set_id Pointer to store AMF Set ID (10 bits)
+ * @param[out] amf_pointer Pointer to store AMF Pointer (6 bits)
+ * @param[out] m_tmsi Pointer to store 5G-TMSI (32 bits)
+ * @note The 5G-S-TMSI is deconstructed from a 48-bit value:
+ *       - Bits 38-47: AMF Set ID (10 bits)
+ *       - Bits 32-37: AMF Pointer (6 bits)
+ *       - Bits 0-31:  5G-TMSI (32 bits)
+ * @ref 3GPP TS 23.003 */
+void nr_deconstruct_5g_s_tmsi(const uint64_t fiveg_s_tmsi, uint16_t *amf_set_id, uint8_t *amf_pointer, uint32_t *m_tmsi)
+{
+  DevAssert(amf_set_id != NULL);
+  DevAssert(amf_pointer != NULL);
+  DevAssert(m_tmsi != NULL);
+
+  *amf_set_id = fiveg_s_tmsi >> 38;
+  *amf_pointer = (fiveg_s_tmsi >> 32) & 0x3F;
+  *m_tmsi = fiveg_s_tmsi;
 }
diff --git a/common/utils/nr/nr_common.h b/common/utils/nr/nr_common.h
index ada9813373..ca867fa694 100644
--- a/common/utils/nr/nr_common.h
+++ b/common/utils/nr/nr_common.h
@@ -360,6 +360,18 @@ uint64_t nr_construct_5g_s_tmsi(uint16_t amf_set_id, uint8_t amf_pointer, uint32
 /** @brief Construct 5G-S-TMSI-Part1 from 5G-S-TMSI components */
 uint64_t nr_construct_5g_s_tmsi_part1(uint16_t amf_set_id, uint8_t amf_pointer, uint32_t m_tmsi);
 
+/** @brief Extract 5G-S-TMSI-Part1 from full 5G-S-TMSI */
+uint64_t nr_extract_5g_s_tmsi_part1(const uint64_t fiveg_s_tmsi);
+
+/** @brief Extract 5G-S-TMSI-Part2 from full 5G-S-TMSI */
+uint16_t nr_extract_5g_s_tmsi_part2(const uint64_t fiveg_s_tmsi);
+
+/** @brief Build full 5G-S-TMSI from Part1 and Part2 */
+uint64_t nr_build_full_5g_s_tmsi(const uint64_t part1, const uint16_t part2);
+
+/** @brief Deconstruct full 5G-S-TMSI into its components */
+void nr_deconstruct_5g_s_tmsi(const uint64_t fiveg_s_tmsi, uint16_t *amf_set_id, uint8_t *amf_pointer, uint32_t *m_tmsi);
+
 #define CEILIDIV(a,b) ((a+b-1)/b)
 #define ROUNDIDIV(a,b) (((a<<1)+b)/(b<<1))
 #define BOUNDED_EVAL(a, b, c) (min(c, max(a, b)))
diff --git a/openair2/RRC/NR/MESSAGES/asn1_msg.c b/openair2/RRC/NR/MESSAGES/asn1_msg.c
index 57fb3daf12..a377236485 100644
--- a/openair2/RRC/NR/MESSAGES/asn1_msg.c
+++ b/openair2/RRC/NR/MESSAGES/asn1_msg.c
@@ -716,7 +716,7 @@ int do_RRCSetupRequest(uint8_t *buffer, size_t buffer_size, uint8_t *rv, uint64_
     str->buf[3] = rv[3];
     str->buf[4] = rv[4] & 0xfe;
   } else {
-    uint64_t fiveG_S_TMSI_part1 = fiveG_S_TMSI & ((1ULL << 39) - 1);
+    uint64_t fiveG_S_TMSI_part1 = nr_extract_5g_s_tmsi_part1(fiveG_S_TMSI);
     /** set the ue-Identity to ng-5G-S-TMSI-Part1
      * ng-5G-S-TMSI-Part1: the rightmost 39 bits of 5G-S-TMSI
      * BIT STRING (SIZE (39)) - 3GPP TS 38.331 */
@@ -840,7 +840,7 @@ int do_RRCSetupComplete(uint8_t *buffer,
       str->size = 2;
       str->bits_unused = 7;
       str->buf = calloc_or_fail(str->size, sizeof(str->buf[0]));
-      uint16_t fiveG_s_tmsi_part2 = (fiveG_s_tmsi >> 39) & ((1ULL << 9) - 1);
+      uint16_t fiveG_s_tmsi_part2 = nr_extract_5g_s_tmsi_part2(fiveG_s_tmsi);
       str->buf[0] = (fiveG_s_tmsi_part2 >> (8 - str->bits_unused)) & 0xFF;
       str->buf[1] = (fiveG_s_tmsi_part2 << str->bits_unused) & 0xFF;
       LOG_D(NR_RRC, "5G-S-TMSI part 2 %d in RRCSetupComplete (5G-S-TMSI %ld)\n", fiveG_s_tmsi_part2, fiveG_s_tmsi);
diff --git a/openair2/RRC/NR/MESSAGES/tests/CMakeLists.txt b/openair2/RRC/NR/MESSAGES/tests/CMakeLists.txt
index 980d6160e8..ee462ba7a6 100644
--- a/openair2/RRC/NR/MESSAGES/tests/CMakeLists.txt
+++ b/openair2/RRC/NR/MESSAGES/tests/CMakeLists.txt
@@ -2,7 +2,7 @@ add_executable(test_asn1_msg
   test_asn1_msg.cpp
   ${OPENAIR2_DIR}/LAYER2/nr_pdcp/asn1_utils.c
   ${OPENAIR2_DIR}/LAYER2/nr_rlc/asn1_utils.c)
-target_link_libraries(test_asn1_msg PRIVATE nr_rrc SECURITY minimal_lib GTest::gtest UTIL)
+target_link_libraries(test_asn1_msg PRIVATE nr_rrc SECURITY minimal_lib GTest::gtest UTIL nr_common)
 add_dependencies(tests test_asn1_msg)
 add_test(NAME test_asn1_msg
     COMMAND ./test_asn1_msg)
diff --git a/openair2/RRC/NR/rrc_gNB.c b/openair2/RRC/NR/rrc_gNB.c
index a68ac6691e..4b0acd44d4 100644
--- a/openair2/RRC/NR/rrc_gNB.c
+++ b/openair2/RRC/NR/rrc_gNB.c
@@ -60,6 +60,7 @@
 #include "common/openairinterface5g_limits.h"
 #include "common/platform_constants.h"
 #include "common/ran_context.h"
+#include "common/utils/nr/nr_common.h"
 #include "common_lib.h"
 #include "constr_SEQUENCE.h"
 #include "constr_TYPE.h"
@@ -2073,7 +2074,7 @@ static void handle_rrcSetupComplete(gNB_RRC_INST *rrc, gNB_RRC_UE_t *UE, const N
         uint16_t stmsi_part2 = BIT_STRING_to_uint16(part2);
         LOG_I(RRC, "s_tmsi part2 %d (%02x %02x)\n", stmsi_part2, part2->buf[0], part2->buf[1]);
         // Part2 is leftmost 9, Part1 is rightmost 39 bits of 5G-S-TMSI
-        fiveg_s_TMSI = ((uint64_t) stmsi_part2) << 39 | UE->ng_5G_S_TMSI_Part1;
+        fiveg_s_TMSI = nr_build_full_5g_s_tmsi(UE->ng_5G_S_TMSI_Part1, stmsi_part2);
       } else {
         LOG_W(RRC, "UE %d received 5G-S-TMSI-Part2, but no 5G-S-TMSI-Part1 present, won't send 5G-S-TMSI to core\n", UE->rrc_ue_id);
         UE->Initialue_identity_5g_s_TMSI.presence = false;
@@ -2090,9 +2091,10 @@ static void handle_rrcSetupComplete(gNB_RRC_INST *rrc, gNB_RRC_UE_t *UE, const N
     }
 
     if (UE->Initialue_identity_5g_s_TMSI.presence) {
-      uint16_t amf_set_id = fiveg_s_TMSI >> 38;
-      uint8_t amf_pointer = (fiveg_s_TMSI >> 32) & 0x3F;
-      uint32_t fiveg_tmsi = (uint32_t) fiveg_s_TMSI;
+      uint16_t amf_set_id;
+      uint8_t amf_pointer;
+      uint32_t fiveg_tmsi;
+      nr_deconstruct_5g_s_tmsi(fiveg_s_TMSI, &amf_set_id, &amf_pointer, &fiveg_tmsi);
       LOG_I(NR_RRC,
             "5g_s_TMSI: 0x%lX, amf_set_id: 0x%X (%d), amf_pointer: 0x%X (%d), 5g TMSI: 0x%X \n",
             fiveg_s_TMSI,