diff --git a/srsepc/hdr/mme/s1ap_nas_transport.h b/srsepc/hdr/mme/s1ap_nas_transport.h
index 03444cee7..f4e207ce6 100644
--- a/srsepc/hdr/mme/s1ap_nas_transport.h
+++ b/srsepc/hdr/mme/s1ap_nas_transport.h
@@ -41,6 +41,8 @@ public:
   void set_log(srslte::log *s1ap_logger);
   bool unpack_initial_ue_message(LIBLTE_S1AP_MESSAGE_INITIALUEMESSAGE_STRUCT *init_ue, LIBLTE_MME_ATTACH_REQUEST_MSG_STRUCT *attach_req, LIBLTE_MME_PDN_CONNECTIVITY_REQUEST_MSG_STRUCT *pdn_con_req);
   bool pack_authentication_request(srslte::byte_buffer_t *reply_msg, uint32_t enb_ue_s1ap_id, uint32_t next_mme_ue_s1ap_id, uint8_t *autn,uint8_t *rand);
+  bool pack_authentication_reject(srslte::byte_buffer_t *reply_msg, uint32_t enb_ue_s1ap_id, uint32_t mme_ue_s1ap_id);
+
   bool unpack_authentication_response(LIBLTE_S1AP_MESSAGE_UPLINKNASTRANSPORT_STRUCT *ul_xport, LIBLTE_MME_AUTHENTICATION_RESPONSE_MSG_STRUCT *auth_resp);
   bool pack_security_mode_command(srslte::byte_buffer_t *reply_msg, ue_ctx_t *ue_ctx);
 
diff --git a/srsepc/src/mme/s1ap.cc b/srsepc/src/mme/s1ap.cc
index 0b5312db7..5c9ab9093 100644
--- a/srsepc/src/mme/s1ap.cc
+++ b/srsepc/src/mme/s1ap.cc
@@ -320,7 +320,8 @@ s1ap::handle_initial_ue_message(LIBLTE_S1AP_MESSAGE_INITIALUEMESSAGE_STRUCT *ini
   /*Get NAS Attach Request and PDN connectivity request messages*/
   if(!m_s1ap_nas_transport.unpack_initial_ue_message(init_ue, &attach_req,&pdn_con_req))
   {
-    //TODO set up error reply
+    //Could not decode the attach request and the PDN connectivity request.
+    m_s1ap_log->error("Could not unpack NAS Attach Request and PDN connectivity request.\n");
     return false;
   }
 
@@ -472,6 +473,7 @@ s1ap::handle_nas_authentication_response(srslte::byte_buffer_t *nas_msg, srslte:
     m_s1ap_log->console("UE Authentication Rejected. IMSI: %lu\n", ue_ctx->imsi);
     m_s1ap_log->warning("UE Authentication Rejected. IMSI: %lu\n", ue_ctx->imsi);
     //TODO send back error reply
+    m_s1ap_nas_transport.pack_authentication_reject(reply_msg, ue_ctx->enb_ue_s1ap_id, ue_ctx->mme_ue_s1ap_id);
     return false;
   }
   m_s1ap_log->console("UE Authentication Accepted. IMSI: %lu\n", ue_ctx->imsi);
diff --git a/srsepc/src/mme/s1ap_nas_transport.cc b/srsepc/src/mme/s1ap_nas_transport.cc
index f3b20f022..2400edc17 100644
--- a/srsepc/src/mme/s1ap_nas_transport.cc
+++ b/srsepc/src/mme/s1ap_nas_transport.cc
@@ -118,7 +118,7 @@ s1ap_nas_transport::pack_authentication_request(srslte::byte_buffer_t *reply_msg
   //Setup Dw NAS structure
   LIBLTE_S1AP_MESSAGE_DOWNLINKNASTRANSPORT_STRUCT *dw_nas = &init->choice.DownlinkNASTransport;
   dw_nas->ext=false;
-  dw_nas->MME_UE_S1AP_ID.MME_UE_S1AP_ID = next_mme_ue_s1ap_id;
+  dw_nas->MME_UE_S1AP_ID.MME_UE_S1AP_ID = next_mme_ue_s1ap_id;//FIXME Change name
   dw_nas->eNB_UE_S1AP_ID.ENB_UE_S1AP_ID = enb_ue_s1ap_id;
   dw_nas->HandoverRestrictionList_present=false;
   dw_nas->SubscriberProfileIDforRFP_present=false;
@@ -156,6 +156,56 @@ s1ap_nas_transport::pack_authentication_request(srslte::byte_buffer_t *reply_msg
   return true;
 }
 
+bool
+s1ap_nas_transport::pack_authentication_reject(srslte::byte_buffer_t *reply_msg, uint32_t enb_ue_s1ap_id, uint32_t mme_ue_s1ap_id)
+{
+  srslte::byte_buffer_t *nas_buffer = m_pool->allocate();
+
+  //Setup initiating message
+  LIBLTE_S1AP_S1AP_PDU_STRUCT tx_pdu;
+  bzero(&tx_pdu, sizeof(LIBLTE_S1AP_S1AP_PDU_STRUCT));
+
+  tx_pdu.ext          = false;
+  tx_pdu.choice_type  = LIBLTE_S1AP_S1AP_PDU_CHOICE_INITIATINGMESSAGE;
+
+  LIBLTE_S1AP_INITIATINGMESSAGE_STRUCT *init = &tx_pdu.choice.initiatingMessage;
+  init->procedureCode = LIBLTE_S1AP_PROC_ID_DOWNLINKNASTRANSPORT;
+  init->choice_type   = LIBLTE_S1AP_INITIATINGMESSAGE_CHOICE_DOWNLINKNASTRANSPORT;
+
+  //Setup Dw NAS structure
+  LIBLTE_S1AP_MESSAGE_DOWNLINKNASTRANSPORT_STRUCT *dw_nas = &init->choice.DownlinkNASTransport;
+  dw_nas->ext=false;
+  dw_nas->MME_UE_S1AP_ID.MME_UE_S1AP_ID = mme_ue_s1ap_id;//FIXME Change name
+  dw_nas->eNB_UE_S1AP_ID.ENB_UE_S1AP_ID = enb_ue_s1ap_id;
+  dw_nas->HandoverRestrictionList_present=false;
+  dw_nas->SubscriberProfileIDforRFP_present=false;
+
+  LIBLTE_MME_AUTHENTICATION_REQUEST_MSG_STRUCT auth_rej;
+  LIBLTE_ERROR_ENUM err = liblte_mme_pack_authentication_request_msg(&auth_rej, (LIBLTE_BYTE_MSG_STRUCT *) nas_buffer);
+  if(err != LIBLTE_SUCCESS)
+  {
+    m_s1ap_log->error("Error packing Athentication Request\n");
+    m_s1ap_log->console("Error packing Athentication Request\n");
+    return false;
+  }
+
+  //Copy NAS PDU to Downlink NAS Trasport message buffer
+  memcpy(dw_nas->NAS_PDU.buffer, nas_buffer->msg, nas_buffer->N_bytes);
+  dw_nas->NAS_PDU.n_octets = nas_buffer->N_bytes;
+
+  //Pack Downlink NAS Transport Message
+  err = liblte_s1ap_pack_s1ap_pdu(&tx_pdu, (LIBLTE_BYTE_MSG_STRUCT *) reply_msg);
+  if(err != LIBLTE_SUCCESS)
+  {
+    m_s1ap_log->error("Error packing Athentication Reject\n");
+    m_s1ap_log->console("Error packing Athentication Reject\n");
+    return false;
+  } 
+
+  m_pool->deallocate(nas_buffer);
+  return true;
+}
+
 bool
 s1ap_nas_transport::unpack_authentication_response(LIBLTE_S1AP_MESSAGE_UPLINKNASTRANSPORT_STRUCT *ul_xport,
                                               LIBLTE_MME_AUTHENTICATION_RESPONSE_MSG_STRUCT *auth_resp )