diff --git a/openair3/NAS/NR_UE/5GS/5GMM/MSG/CMakeLists.txt b/openair3/NAS/NR_UE/5GS/5GMM/MSG/CMakeLists.txt
index c1a28a18f3..3b51ec748c 100644
--- a/openair3/NAS/NR_UE/5GS/5GMM/MSG/CMakeLists.txt
+++ b/openair3/NAS/NR_UE/5GS/5GMM/MSG/CMakeLists.txt
@@ -11,6 +11,7 @@ add_library(fgs_5gmm_lib OBJECT
     RegistrationComplete.c
     FGSUplinkNasTransport.c
     FGSDeregistrationRequestUEOriginating.c
+    fgmm_authentication_failure.c
 )
 
 target_link_libraries(fgs_5gmm_lib PUBLIC fgs_5gmm_ies_lib)
diff --git a/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_authentication_failure.c b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_authentication_failure.c
new file mode 100644
index 0000000000..2dc865ab4c
--- /dev/null
+++ b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_authentication_failure.c
@@ -0,0 +1,101 @@
+/*
+ * Licensed to the OpenAirInterface (OAI) Software Alliance under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The OpenAirInterface Software Alliance licenses this file to You under
+ * the OAI Public License, Version 1.1  (the "License"); you may not use this file
+ * except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.openairinterface.org/?page_id=698
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *-------------------------------------------------------------------------------
+ * For more information about the OpenAirInterface (OAI) Software Alliance:
+ *      contact@openairinterface.org
+ */
+
+#include "fgmm_authentication_failure.h"
+#include <string.h>
+#include <arpa/inet.h> // For htons and ntohs
+#include <stdlib.h> // For malloc and free
+#include "fgmm_lib.h"
+#include "common/utils/ds/byte_array.h"
+#include "common/utils/utils.h"
+
+#define MIN_AUTH_FAILURE_LEN 1
+#define AUTH_FAILURE_PARAM_LEN 16
+#define AUTH_FAIL_PARAM_IEI 0x30
+
+/** @brief Encode Authentication Failure (8.2.4 of 3GPP TS 24.501) */
+int encode_fgmm_auth_failure(byte_array_t *buffer, const fgmm_auth_failure_t *msg)
+{
+  if (buffer->len < MIN_AUTH_FAILURE_LEN) {
+    PRINT_NAS_ERROR("Missing mandatory IE in Authentication Failure");
+    return -1; // missing mandatory IE
+  }
+  // 5GMM cause (Mandatory)
+  int encoded = encode_fgs_nas_cause(buffer, &msg->cause);
+  // Authentication failure parameter (see 8.2.4.2 3GPP TS 24.501)
+  if (msg->cause == Synch_failure) {
+    // Authentication failure parameter shall be present with cause Synch Failure
+    if (msg->authentication_failure_param.len == 0) {
+      PRINT_NAS_ERROR("Authentication Failure Cause is synch failure but Authentication Failure Parameter is not available");
+      return -1;
+    }
+    if (buffer->len - encoded < msg->authentication_failure_param.len + 2) {
+      PRINT_NAS_ERROR("Invalid buffer for encoding of Authentication Failure Parameter");
+      return -1;
+    }
+    // Encode the IEI
+    buffer->buf[encoded++] = AUTH_FAIL_PARAM_IEI;
+    buffer->buf[encoded++] = msg->authentication_failure_param.len;
+    for (int i = 0; i < msg->authentication_failure_param.len; i++) {
+      buffer->buf[encoded++] = msg->authentication_failure_param.buf[i];
+    }
+  }
+  return encoded;
+}
+
+/** @brief Decode Authentication Failure (8.2.4 of 3GPP TS 24.501) */
+int decode_fgmm_auth_failure(fgmm_auth_failure_t *msg, const byte_array_t *buffer)
+{
+  if (buffer->len < MIN_AUTH_FAILURE_LEN) {
+    return -1; // missing mandatory IE
+  }
+  byte_array_t ba = *buffer; // Local copy of buffer
+  // 5GMM Cause (Mandatory)
+  uint32_t decoded = decode_fgs_nas_cause(&msg->cause, buffer);
+  // Authentication Failure Parameters (Optional)
+  if (msg->cause == Synch_failure) {
+    uint8_t iei = buffer->buf[decoded++];
+    if (iei == AUTH_FAIL_PARAM_IEI) {
+      msg->authentication_failure_param.len = ba.buf[decoded++];
+      if (msg->authentication_failure_param.len > AUTH_FAILURE_PARAM_LEN - 2) { // maximum length of contents is 14 octets
+        PRINT_NAS_ERROR("Length of Authentication Failure Parameter contents is too large");
+        return -1;
+      }
+      msg->authentication_failure_param.buf = malloc_or_fail(msg->authentication_failure_param.len);
+      for (int i = 0; i < msg->authentication_failure_param.len; i++) {
+        msg->authentication_failure_param.buf[i] = buffer->buf[decoded++];
+      }
+    }
+  }
+  return decoded;
+}
+
+bool eq_fgmm_auth_failure(const fgmm_auth_failure_t *a, const fgmm_auth_failure_t *b)
+{
+  _NAS_EQ_CHECK_INT(a->cause, b->cause);
+  eq_byte_array(&a->authentication_failure_param, &b->authentication_failure_param);
+  return true;
+}
+
+void free_fgmm_auth_failure(const fgmm_auth_failure_t *msg)
+{
+  free(msg->authentication_failure_param.buf);
+}
\ No newline at end of file
diff --git a/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_authentication_failure.h b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_authentication_failure.h
new file mode 100644
index 0000000000..71d047b656
--- /dev/null
+++ b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_authentication_failure.h
@@ -0,0 +1,41 @@
+/*
+ * Licensed to the OpenAirInterface (OAI) Software Alliance under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The OpenAirInterface Software Alliance licenses this file to You under
+ * the OAI Public License, Version 1.1 (the "License"); you may not use this file
+ * except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.openairinterface.org/?page_id=698
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *-------------------------------------------------------------------------------
+ * For more information about the OpenAirInterface (OAI) Software Alliance:
+ *      contact@openairinterface.org
+ */
+
+#ifndef FGMM_AUTH_FAILURE_H
+#define FGMM_AUTH_FAILURE_H
+
+#include <stdint.h>
+#include "fgmm_lib.h"
+#include "common/utils/ds/byte_array.h"
+
+typedef struct {
+  // 5GMM cause (Mandatory)
+  cause_id_t cause;
+  // Authentication failure parameter (Optional)
+  byte_array_t authentication_failure_param;
+} fgmm_auth_failure_t;
+
+int decode_fgmm_auth_failure(fgmm_auth_failure_t *msg, const byte_array_t *buffer);
+int encode_fgmm_auth_failure(byte_array_t *buffer, const fgmm_auth_failure_t *msg);
+bool eq_fgmm_auth_failure(const fgmm_auth_failure_t *a, const fgmm_auth_failure_t *b);
+void free_fgmm_auth_failure(const fgmm_auth_failure_t *msg);
+
+#endif /* FGMM_AUTH_FAILURE_H */
diff --git a/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.c b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.c
index 323cffe7e7..1b607589d9 100644
--- a/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.c
+++ b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.c
@@ -26,6 +26,7 @@
 #include "common/utils/ds/byte_array.h"
 
 #define GPRS_TIMER_LENGTH 3 // octets
+#define FGS_NAS_CAUSE_LENGTH 1
 
 /**
  * Encode PDU Session Status and PDU session reactivation result (optional IEs)
@@ -158,3 +159,27 @@ int decode_eap_msg_ie(const byte_array_t *buffer)
   PRINT_NAS_ERROR("EAP message IE in NAS Service Reject is not handled\n");
   return eap_msg_len + 2; // content length + 2 octets for the length IE
 }
+
+int encode_fgs_nas_cause(byte_array_t *buffer, const cause_id_t *cause)
+{
+  uint32_t encoded = 0;
+
+  if (buffer->len < FGS_NAS_CAUSE_LENGTH)
+    return -1;
+
+  buffer->buf[encoded++] = *cause;
+
+  return encoded;
+}
+
+int decode_fgs_nas_cause(cause_id_t *cause, const byte_array_t *buffer)
+{
+  int decoded = 0;
+
+  if (buffer->len < FGS_NAS_CAUSE_LENGTH)
+    return -1; // nothing to decode
+
+  *cause = buffer->buf[decoded++];
+
+  return decoded;
+}
diff --git a/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.h b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.h
index b757620506..25118f2742 100644
--- a/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.h
+++ b/openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.h
@@ -165,5 +165,7 @@ int encode_gprs_timer_ie(byte_array_t *buffer, nas_service_IEI_t iei, const gprs
 int decode_gprs_timer_ie(gprs_timer_t *timer, const byte_array_t *buffer);
 bool eq_gprs_timer(const gprs_timer_t *a, const gprs_timer_t *b);
 int decode_eap_msg_ie(const byte_array_t *buffer);
+int encode_fgs_nas_cause(byte_array_t *buffer, const cause_id_t *cause);
+int decode_fgs_nas_cause(cause_id_t *cause, const byte_array_t *buffer);
 
 #endif /* FGS_LIB_H */
\ No newline at end of file
diff --git a/openair3/NAS/NR_UE/5GS/CMakeLists.txt b/openair3/NAS/NR_UE/5GS/CMakeLists.txt
index e04abb02db..9a7feabb72 100644
--- a/openair3/NAS/NR_UE/5GS/CMakeLists.txt
+++ b/openair3/NAS/NR_UE/5GS/CMakeLists.txt
@@ -12,6 +12,7 @@ target_link_libraries(fgs_lib PUBLIC
     fgs_5gsm_lib
     fgs_5gmm_ies_lib
     nas_common_ies_lib
+    ds
 )
 
 target_include_directories(fgs_lib PUBLIC . 5GMM/MSG 5GMM/IES)
diff --git a/openair3/NAS/NR_UE/5GS/tests/nas_lib_test.c b/openair3/NAS/NR_UE/5GS/tests/nas_lib_test.c
index a73ee9b2be..faea1c8df0 100644
--- a/openair3/NAS/NR_UE/5GS/tests/nas_lib_test.c
+++ b/openair3/NAS/NR_UE/5GS/tests/nas_lib_test.c
@@ -6,6 +6,7 @@
 #include "fgs_service_request.h"
 #include "fgmm_service_accept.h"
 #include "fgmm_service_reject.h"
+#include "fgmm_authentication_failure.h"
 #include "nr_nas_msg.h"
 
 void exit_function(const char *file, const char *function, const int line, const char *s, const int assert)
@@ -170,10 +171,46 @@ static void test_service_reject(void)
   free_fgs_service_reject(&orig);
 }
 
+/** @brief Test NAS Authentication Failure enc/dec */
+static void test_auth_failure(void)
+{
+  // Dummy NAS Authentication Failure message
+  uint8_t afp[] = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06};
+  const fgmm_auth_failure_t orig = {
+      .cause = Synch_failure,
+      .authentication_failure_param.len = 6,
+      .authentication_failure_param.buf = afp,
+  };
+
+  // Expected encoded data
+  uint8_t expected_enc[] = {0x15, 0x30, 0x06, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06};
+
+  // Buffer
+  uint8_t buf[16] = {0};
+  byte_array_t buffer = {.buf = buf, .len = sizeof(buf)};
+
+  // Encode
+  int encoded_length = encode_fgmm_auth_failure(&buffer, &orig);
+  AssertFatal(encoded_length == sizeofArray(expected_enc), "encode_fgmm_auth_failure() failed\n");
+
+  // Compare the raw encoded buffer with expected encoded data
+  AssertFatal(memcmp(buffer.buf, expected_enc, encoded_length) == 0, "Encoding mismatch!\n");
+
+  // Decode
+  fgmm_auth_failure_t dec = {0};
+  int decoded_length = decode_fgmm_auth_failure(&dec, &buffer);
+  AssertFatal(decoded_length >= 0, "decode_fgmm_auth_failure() failed\n");
+
+  // Compare original and decoded messages
+  AssertFatal(eq_fgmm_auth_failure(&orig, &dec), "test_auth_failure() failed: original and decoded messages do not match\n");
+  free_fgmm_auth_failure(&dec);
+}
+
 int main()
 {
   test_service_request();
   test_service_accept();
   test_service_reject();
+  test_auth_failure();
   return 0;
 }
diff --git a/openair3/NAS/NR_UE/CMakeLists.txt b/openair3/NAS/NR_UE/CMakeLists.txt
index 5e7d5da428..ec391092e5 100644
--- a/openair3/NAS/NR_UE/CMakeLists.txt
+++ b/openair3/NAS/NR_UE/CMakeLists.txt
@@ -10,6 +10,7 @@ target_link_libraries(nr_nas PUBLIC
     fgs_5gmm_ies_lib
     nas_common_ies_lib
     nas_utils
+    ds
 )
 
 target_include_directories(nr_nas PUBLIC . 5GS/)