[MME] Gn: Introduce initial support for 4G->2G cell reselection

In an Inter-RAT setup a UE could perform a RAU coming from a 4G network.
In that case the UE/MS is unknown to the SGSN and it should request the
SGSN context (MM, PDP) from the MME. This is done through the following
GTPv1C message exchange on the Gn interface of SGSN and MME:
SGSN -> MME: SGSN Context Request
SGSN <- MME: SGSN Context Response
SGSN -> MME: SGSN Context Acknowledge

This commit doesn't aim to be a complete implementation of the mentioned
procedure, since it's quite a complex one, with lots of fields and logic
required. This so far only implements in general the minimally
successful case by filling as much as possible the required set of
fields.
This will allow for a base onto which do incremental improvements and
fixes while testing against UEs and SGSNs (such as osmo-sgsn, which
doesn't yet support this procedure but will potentially earn it soon).

This commit doesn't implement the reverse direction, aka UE issuing cell
reselection 2G->4G. Initial support for this scenario will hopefully be
added soon as a follow-up patch, similar to this one.

Related: https://osmocom.org/issues/6294

lib/crypt/ogs-kdf.c

@@ -34,6 +34,9 @@
 #define FC_FOR_KENB_DERIVATION                  0x11
 #define FC_FOR_NH_ENB_DERIVATION                0x12
 #define FC_FOR_EPS_ALGORITHM_KEY_DERIVATION     0x15
+#define FC_FOR_CK_IK_DERIVATION_HANDOVER        0x16
+#define FC_FOR_NAS_TOKEN_DERIVATION             0x17
+#define FC_FOR_CK_IK_DERIVATION_IDLE_MOBILITY   0x1B
 
 typedef struct kdf_param_s {
     const uint8_t *buf;
@@ -379,6 +382,56 @@ void ogs_kdf_nas_eps(uint8_t algorithm_type_distinguishers,
     memcpy(knas, output+16, 16);
 }
 
+/* TS33.401 Annex A.8: KASME to CK', IK' derivation at handover */
+void ogs_kdf_ck_ik_handover(
+    uint32_t dl_count, const uint8_t *kasme, uint8_t *ck, uint8_t *ik)
+{
+    kdf_param_t param;
+    uint8_t output[OGS_SHA256_DIGEST_SIZE];
+
+    memset(param, 0, sizeof(param));
+    param[0].buf = (uint8_t *)&dl_count;
+    param[0].len = 4;
+
+    ogs_kdf_common(kasme, OGS_SHA256_DIGEST_SIZE,
+            FC_FOR_CK_IK_DERIVATION_HANDOVER, param, output);
+    memcpy(ck, output, 16);
+    memcpy(ik, output+16, 16);
+}
+
+/* TS33.401 Annex A.9: NAS token derivation for inter-RAT mobility */
+void ogs_kdf_nas_token(
+    uint32_t ul_count, const uint8_t *kasme, uint8_t *nas_token)
+{
+    kdf_param_t param;
+    uint8_t output[OGS_SHA256_DIGEST_SIZE];
+
+    memset(param, 0, sizeof(param));
+    param[0].buf = (uint8_t *)&ul_count;
+    param[0].len = 4;
+
+    ogs_kdf_common(kasme, OGS_SHA256_DIGEST_SIZE,
+            FC_FOR_NAS_TOKEN_DERIVATION, param, output);
+    memcpy(nas_token, output, 2);
+}
+
+/* TS33.401 Annex A.13: KASME to CK', IK' derivation at idle mobility */
+void ogs_kdf_ck_ik_idle_mobility(
+    uint32_t ul_count, const uint8_t *kasme, uint8_t *ck, uint8_t *ik)
+{
+    kdf_param_t param;
+    uint8_t output[OGS_SHA256_DIGEST_SIZE];
+
+    memset(param, 0, sizeof(param));
+    param[0].buf = (uint8_t *)&ul_count;
+    param[0].len = 4;
+
+    ogs_kdf_common(kasme, OGS_SHA256_DIGEST_SIZE,
+            FC_FOR_CK_IK_DERIVATION_IDLE_MOBILITY, param, output);
+    memcpy(ck, output, 16);
+    memcpy(ik, output+16, 16);
+}
+
 /*
  * TS33.401 Annex I Hash Functions
  * Use the KDF given in TS33.220

lib/crypt/ogs-kdf.h

@@ -101,6 +101,18 @@ void ogs_kdf_nh_enb(const uint8_t *kasme, const uint8_t *sync_input, uint8_t *ke
 void ogs_kdf_nas_eps(uint8_t algorithm_type_distinguishers,
     uint8_t algorithm_identity, const uint8_t *kasme, uint8_t *knas);
 
+/* TS33.401 Annex A.8: KASME to CK', IK' derivation at handover */
+void ogs_kdf_ck_ik_handover(
+    uint32_t dl_count, const uint8_t *kasme, uint8_t *ck, uint8_t *ik);
+
+/* TS33.401 Annex A.9: NAS token derivation for inter-RAT mobility */
+void ogs_kdf_nas_token(
+    uint32_t ul_count, const uint8_t *kasme, uint8_t *nas_token);
+
+/* TS33.401 Annex A.13: KASME to CK', IK' derivation at idle mobility */
+void ogs_kdf_ck_ik_idle_mobility(
+    uint32_t ul_count, const uint8_t *kasme, uint8_t *ck, uint8_t *ik);
+
 /*
  * TS33.401 Annex I Hash Functions
  * Use the KDF given in TS33.220

lib/gtp/v1/conv.c

@@ -104,6 +104,34 @@ int ogs_gtp1_gsn_addr_to_ip(const ogs_gtp1_gsn_addr_t *gsnaddr, uint16_t gsnaddr
     return OGS_OK;
 }
 
+int ogs_gtp1_pdu_session_type_to_eua_ietf_type(uint8_t session_type)
+{
+    switch (session_type) {
+    case OGS_PDU_SESSION_TYPE_IPV4:
+        return OGS_PDP_EUA_IETF_IPV4;
+    case OGS_PDU_SESSION_TYPE_IPV6:
+        return OGS_PDP_EUA_IETF_IPV6;
+    case OGS_PDU_SESSION_TYPE_IPV4V6:
+        return OGS_PDP_EUA_IETF_IPV4V6;
+    default:
+        return OGS_ERROR;
+    }
+}
+
+int ogs_gtp1_eua_ietf_type_to_pdu_session_type(uint8_t eua_ietf_type)
+{
+    switch (eua_ietf_type) {
+    case OGS_PDP_EUA_IETF_IPV4:
+        return OGS_PDU_SESSION_TYPE_IPV4;
+    case OGS_PDP_EUA_IETF_IPV6:
+        return OGS_PDU_SESSION_TYPE_IPV6;
+    case OGS_PDP_EUA_IETF_IPV4V6:
+        return OGS_PDU_SESSION_TYPE_IPV4V6;
+    default:
+        return OGS_ERROR;
+    }
+}
+
 int ogs_gtp1_eua_to_ip(const ogs_eua_t *eua, uint16_t eua_len, ogs_ip_t *ip,
         uint8_t *pdu_session_type)
 {

lib/gtp/v1/conv.h

@@ -35,6 +35,9 @@ int ogs_gtp1_sockaddr_to_gsn_addr(const ogs_sockaddr_t *addr,
 int ogs_gtp1_gsn_addr_to_ip(const ogs_gtp1_gsn_addr_t *gsnaddr, uint16_t gsnaddr_len,
         ogs_ip_t *ip);
 
+int ogs_gtp1_pdu_session_type_to_eua_ietf_type(uint8_t session_type);
+int ogs_gtp1_eua_ietf_type_to_pdu_session_type(uint8_t eua_ietf_type);
+
 int ogs_gtp1_eua_to_ip(const ogs_eua_t *eua, uint16_t eua_len, ogs_ip_t *ip,
         uint8_t *pdu_session_type);