EAP-AKA: Add Session-Id derivation during fast-reauth

The Session-Id derivation for EAP-AKA in RFC 5247 only explained how the
Session-Id is derived for regular authentication. Jouni reported it as
an errata with text explaining how to derive it during fast
reauthentication.

This patch now exports the Session-Id for EAP-AKA during fast
reauthentication based on this Session-Id = 0x17 || NONCE_S || MAC
construction.

Also documented by Alan Dekok in draft-dekok-emu-eap-session-id.

Signed-off-by: Mohit Sethi <mohit.sethi@aalto.fi>

src/eap_peer/eap_aka.c

@@ -31,6 +31,7 @@ struct eap_aka_data {
 	u8 emsk[EAP_EMSK_LEN];
 	u8 rand[EAP_AKA_RAND_LEN], autn[EAP_AKA_AUTN_LEN];
 	u8 auts[EAP_AKA_AUTS_LEN];
+	u8 reauth_mac[EAP_SIM_MAC_LEN];
 
 	int num_id_req, num_notification;
 	u8 *pseudonym;
@@ -1226,6 +1227,14 @@ static struct wpabuf * eap_aka_process_reauthentication(
 					    EAP_AKA_UNABLE_TO_PROCESS_PACKET);
 	}
 
+	/* At this stage the received MAC has been verified. Use this MAC for
+	 * reauth Session-Id calculation if all other checks pass.
+	 * The peer does not use the local MAC but the received MAC in deriving
+	 * Session-Id. */
+	os_memcpy(data->reauth_mac, attr->mac, EAP_SIM_MAC_LEN);
+	wpa_hexdump(MSG_DEBUG, "EAP-AKA: Server MAC",
+		    data->reauth_mac, EAP_SIM_MAC_LEN);
+
 	if (attr->encr_data == NULL || attr->iv == NULL) {
 		wpa_printf(MSG_WARNING, "EAP-AKA: Reauthentication "
 			   "message did not include encrypted data");
@@ -1497,14 +1506,24 @@ static u8 * eap_aka_get_session_id(struct eap_sm *sm, void *priv, size_t *len)
 	if (data->state != SUCCESS)
 		return NULL;
 
+	if (!data->reauth)
 		*len = 1 + EAP_AKA_RAND_LEN + EAP_AKA_AUTN_LEN;
+	else
+		*len = 1 + EAP_SIM_NONCE_S_LEN + EAP_SIM_MAC_LEN;
 	id = os_malloc(*len);
 	if (id == NULL)
 		return NULL;
 
 	id[0] = data->eap_method;
+	if (!data->reauth) {
 		os_memcpy(id + 1, data->rand, EAP_AKA_RAND_LEN);
-	os_memcpy(id + 1 + EAP_AKA_RAND_LEN, data->autn, EAP_AKA_AUTN_LEN);
+		os_memcpy(id + 1 + EAP_AKA_RAND_LEN, data->autn,
+			  EAP_AKA_AUTN_LEN);
+	} else {
+		os_memcpy(id + 1, data->nonce_s, EAP_SIM_NONCE_S_LEN);
+		os_memcpy(id + 1 + EAP_SIM_NONCE_S_LEN, data->reauth_mac,
+			  EAP_SIM_MAC_LEN);
+	}
 	wpa_hexdump(MSG_DEBUG, "EAP-AKA: Derived Session-Id", id, *len);
 
 	return id;

src/eap_server/eap_server_aka.c

@@ -30,6 +30,7 @@ struct eap_aka_data {
 	u8 ck[EAP_AKA_CK_LEN];
 	u8 ik[EAP_AKA_IK_LEN];
 	u8 res[EAP_AKA_RES_MAX_LEN];
+	u8 reauth_mac[EAP_SIM_MAC_LEN];
 	size_t res_len;
 	enum {
 		IDENTITY, CHALLENGE, REAUTH, NOTIFICATION, SUCCESS, FAILURE
@@ -542,6 +543,7 @@ static struct wpabuf * eap_aka_build_reauth(struct eap_sm *sm,
 					    struct eap_aka_data *data, u8 id)
 {
 	struct eap_sim_msg *msg;
+	struct wpabuf *buf;
 
 	wpa_printf(MSG_DEBUG, "EAP-AKA: Generating Re-authentication");
 
@@ -581,7 +583,16 @@ static struct wpabuf * eap_aka_build_reauth(struct eap_sm *sm,
 
 	wpa_printf(MSG_DEBUG, "   AT_MAC");
 	eap_sim_msg_add_mac(msg, EAP_SIM_AT_MAC);
-	return eap_sim_msg_finish(msg, data->eap_method, data->k_aut, NULL, 0);
+	buf = eap_sim_msg_finish(msg, data->eap_method, data->k_aut, NULL, 0);
+
+	/* Remember this MAC before sending it to the peer. This MAC is used for
+	 * Session-Id calculation after receiving response from the peer and
+	 * after all other checks pass. */
+	os_memcpy(data->reauth_mac,
+		  wpabuf_head(buf) + wpabuf_len(buf) - EAP_SIM_MAC_LEN,
+		  EAP_SIM_MAC_LEN);
+
+	return buf;
 }
 
 
@@ -1304,14 +1315,24 @@ static u8 * eap_aka_get_session_id(struct eap_sm *sm, void *priv, size_t *len)
 	if (data->state != SUCCESS)
 		return NULL;
 
+	if (!data->reauth)
 		*len = 1 + EAP_AKA_RAND_LEN + EAP_AKA_AUTN_LEN;
+	else
+		*len = 1 + EAP_SIM_NONCE_S_LEN + EAP_SIM_MAC_LEN;
 	id = os_malloc(*len);
 	if (id == NULL)
 		return NULL;
 
 	id[0] = data->eap_method;
+	if (!data->reauth) {
 		os_memcpy(id + 1, data->rand, EAP_AKA_RAND_LEN);
-	os_memcpy(id + 1 + EAP_AKA_RAND_LEN, data->autn, EAP_AKA_AUTN_LEN);
+		os_memcpy(id + 1 + EAP_AKA_RAND_LEN, data->autn,
+			  EAP_AKA_AUTN_LEN);
+	} else {
+		os_memcpy(id + 1, data->nonce_s, EAP_SIM_NONCE_S_LEN);
+		os_memcpy(id + 1 + EAP_SIM_NONCE_S_LEN, data->reauth_mac,
+			  EAP_SIM_MAC_LEN);
+	}
 	wpa_hexdump(MSG_DEBUG, "EAP-AKA: Derived Session-Id", id, *len);
 
 	return id;