Wipe out Secure Element on region change

When the application changes region using AT+BAND=, restore all keys
stored in the Secure Element to factory defaults. This is necessary at
least for the session keys, since switching regions resets uplink and
downlink counters.

Strictly speaking, it is not necessary to wipe out the root keys on
region change, but we do that anyway to match the behavior of the
original Murata modem firmware.

Note that even though we wipe out the Secure Element, we preserve the
DevEUI value (which is stored in the Secure Element).

Closes #101

src/lrw.c

@@ -960,7 +960,6 @@ int lrw_set_region(unsigned int region)
     if (!RegionIsActive(region))
         return LORAMAC_STATUS_REGION_NOT_SUPPORTED;
 
-    // Store the new region id in the NVM state in group MacGroup2
     LoRaMacNvmData_t *state = lrw_get_state();
 
     // Region did not change, nothing to do
@@ -971,17 +970,25 @@ int lrw_set_region(unsigned int region)
     int rv = LoRaMacDeInitialization();
     if (rv != LORAMAC_STATUS_OK) return rv;
 
-    // The crypto group needs special handling to preserve the DevNonce value
-    // across the partial factory reset performed here.
+    // The crypto group needs special handling to preserve the DevNonce value.
     uint16_t nonce = state->Crypto.DevNonce;
     LoRaMacCryptoInit(&state->Crypto);
     state->Crypto.DevNonce = nonce;
     update_block_crc(&state->Crypto, sizeof(state->Crypto));
 
-    // Reset all other configuration parameters except the secure element. Note
-    // that we intentionally do not recompute the CRC32 checksums here (except
-    // for MacGroup2) since we don't want the state to be reloaded upon reboot.
-    // We want the LoRaMac to initialize itself from defaults.
+    // The secure element group also needs special handling to preserve DevEUI.
+    uint8_t dev_eui[SE_EUI_SIZE];
+    memcpy(dev_eui, SecureElementGetDevEui(), SE_EUI_SIZE);
+    if (SecureElementInit(&state->SecureElement) != SECURE_ELEMENT_SUCCESS)
+        log_error("Error while reinitializing secure element NVM partition");
+
+    memcpy(state->SecureElement.DevEui, dev_eui, SE_EUI_SIZE);
+    update_block_crc(&state->SecureElement, sizeof(state->SecureElement));
+
+    // Reset all other configuration parameters. Note that we intentionally do
+    // not recompute the CRC32 checksums here (except for MacGroup2) since we
+    // don't want the state to be reloaded upon reboot. We want the LoRaMac to
+    // initialize itself from defaults.
     memset(&state->MacGroup1, 0, sizeof(state->MacGroup1));
     memset(&state->MacGroup2, 0, sizeof(state->MacGroup2));
     memset(&state->RegionGroup1, 0, sizeof(state->RegionGroup1));
@@ -1002,11 +1009,12 @@ int lrw_set_region(unsigned int region)
 
     // Save all reset parameters in non-volatile memory.
     state_changed(
-        LORAMAC_NVM_NOTIFY_FLAG_CRYPTO        |
-        LORAMAC_NVM_NOTIFY_FLAG_MAC_GROUP1    |
-        LORAMAC_NVM_NOTIFY_FLAG_MAC_GROUP2    |
-        LORAMAC_NVM_NOTIFY_FLAG_REGION_GROUP1 |
-        LORAMAC_NVM_NOTIFY_FLAG_REGION_GROUP2 |
+        LORAMAC_NVM_NOTIFY_FLAG_CRYPTO         |
+        LORAMAC_NVM_NOTIFY_FLAG_SECURE_ELEMENT |
+        LORAMAC_NVM_NOTIFY_FLAG_MAC_GROUP1     |
+        LORAMAC_NVM_NOTIFY_FLAG_MAC_GROUP2     |
+        LORAMAC_NVM_NOTIFY_FLAG_REGION_GROUP1  |
+        LORAMAC_NVM_NOTIFY_FLAG_REGION_GROUP2  |
         LORAMAC_NVM_NOTIFY_FLAG_CLASS_B);
 
     return LORAMAC_STATUS_OK;