Add command AT$LOCKKEYS

This AT command can be used to prevent the application from reading
LoRaWAN security keys over the AT command interface. After this command
has been invoked, any attempt to read LoRaWAN security keys via the ATCI
will return an error (-50, access denied). This setting will remain in
effect until the next reset to factory defaults.

Note: This command provides only minimal accidental protection. If the
application has access to USART1, USART2, or SPI ports on the modem, it
could simply downgrade the modem's firmware to read the keys, or it
could switch into the STM32 bootloader mode to directly read the EEPROM.
LoRaWAN security keys are stored in the EEPROM in an unencrypted form.

This patch represents the first step towards securing LoRaWAN security
keys. Future patches may implement additional protection, e.g., store
the security keys in the EEPROM in an encrypted form.

Related to #121

src/cmd.c

@@ -42,7 +42,8 @@ typedef enum cmd_errno {
     ERR_UNSUPPORTED   = -17,  // Not supported in the current band
     ERR_DUTYCYCLE     = -18,  // Cannot transmit due to duty cycling
     ERR_NO_CHANNEL    = -19,  // Channel unavailable due to LBT or error
-    ERR_TOO_MANY      = -20   // Too many link check requests
+    ERR_TOO_MANY      = -20,  // Too many link check requests
+    ERR_ACCESS_DENIED = -50   // Read access to security keys is denied
 } cmd_errno_t;
 
 
@@ -413,6 +414,8 @@ static void set_joineui(atci_param_t *param)
 
 static void get_nwkskey(void)
 {
+    if (sysconf.lock_keys) abort(ERR_ACCESS_DENIED);
+
     atci_print("+OK=");
 
     // We operate in a backwards-compatible 1.0 mode here and in that mode, the
@@ -464,6 +467,8 @@ static void set_nwkskey(atci_param_t *param)
 
 static void get_appskey(void)
 {
+    if (sysconf.lock_keys) abort(ERR_ACCESS_DENIED);
+
     atci_print("+OK=");
     atci_print_buffer_as_hex(find_key(APP_S_KEY), SE_KEY_SIZE);
     EOL();
@@ -489,6 +494,8 @@ static void set_appskey(atci_param_t *param)
 
 static void get_appkey(void)
 {
+    if (sysconf.lock_keys) abort(ERR_ACCESS_DENIED);
+
     atci_print("+OK=");
     atci_print_buffer_as_hex(find_key(APP_KEY), SE_KEY_SIZE);
     EOL();
@@ -1675,6 +1682,8 @@ static void do_halt(atci_param_t *param)
 
 static void get_nwkkey(void)
 {
+    if (sysconf.lock_keys) abort(ERR_ACCESS_DENIED);
+
     atci_print("+OK=");
     atci_print_buffer_as_hex(find_key(NWK_KEY), SE_KEY_SIZE);
     EOL();
@@ -1700,6 +1709,8 @@ static void set_nwkkey(atci_param_t *param)
 
 static void get_fnwksintkey(void)
 {
+    if (sysconf.lock_keys) abort(ERR_ACCESS_DENIED);
+
     atci_print("+OK=");
     atci_print_buffer_as_hex(find_key(F_NWK_S_INT_KEY), SE_KEY_SIZE);
     EOL();
@@ -1725,6 +1736,8 @@ static void set_fnwksintkey(atci_param_t *param)
 
 static void get_snwksintkey(void)
 {
+    if (sysconf.lock_keys) abort(ERR_ACCESS_DENIED);
+
     atci_print("+OK=");
     atci_print_buffer_as_hex(find_key(S_NWK_S_INT_KEY), SE_KEY_SIZE);
     EOL();
@@ -1750,6 +1763,8 @@ static void set_snwksintkey(atci_param_t *param)
 
 static void get_nwksenckey(void)
 {
+    if (sysconf.lock_keys) abort(ERR_ACCESS_DENIED);
+
     atci_print("+OK=");
     atci_print_buffer_as_hex(find_key(NWK_S_ENC_KEY), SE_KEY_SIZE);
     EOL();
@@ -2083,6 +2098,15 @@ static void nvm_userdata(atci_param_t *param)
 }
 
 
+static void lock_keys(atci_param_t *param)
+{
+    (void)param;
+    sysconf.lock_keys = 1;
+    sysconf_modified = true;
+    OK_();
+}
+
+
 static const atci_command_t cmds[] = {
     {"+UART",        NULL,         set_uart,         get_uart,         NULL, "Configure UART interface"},
     {"+VER",         NULL,         NULL,             get_version_comp, NULL, "Firmware version and build time"},
@@ -2154,6 +2178,7 @@ static const atci_command_t cmds[] = {
     {"$CW",          cw,           NULL,             NULL,             NULL, "Start continuous carrier wave transmission"},
     {"$CM",          cm,           NULL,             NULL,             NULL, "Start continuous modulated FSK transmission"},
     {"$NVM",         nvm_userdata, NULL,             NULL,             NULL, "Manage data in NVM user registers"},
+    {"$LOCKKEYS",    lock_keys,    NULL,             NULL,             NULL, "Prevent read access to security keys from ATCI"},
     ATCI_COMMAND_CLAC,
     ATCI_COMMAND_HELP};
 

src/nvm.c

@@ -83,6 +83,7 @@ sysconf_t sysconf = {
     .default_port = 2,
     .data_format = 0,
     .sleep = 1,
+    .lock_keys = 0,
     .device_class = CLASS_A,
     .unconfirmed_retransmissions = 1,
     .confirmed_retransmissions = 8

src/nvm.h

@@ -29,19 +29,26 @@ typedef struct sysconf
      * indicates that the client submits payloads in binary form. The value 1
      * indicates that the client submits payloads in a hex form.
      */
-    uint8_t data_format;
+    uint8_t data_format : 1;
 
     /* This parameter controls whether the firmware enters low-power modes when
      * idle. Set to 0 to disable all low-power modes, set to 1 to enable
      * low-power modes.
      */
-    uint8_t sleep;
+    uint8_t sleep : 1;
 
     /* We need to keep LoRa device class here, in addition to the MIB, because
      * the MIB variable is reset to class A during Join. Having a separate copy
      * here allows us to restore the class after Join.
      */
-    uint8_t device_class;
+    uint8_t device_class : 2;
+
+    /* When this flag is set to 1, the AT command interface will prevent the
+     * application from reading the various LoRaWAN security keys. The
+     * corresponding AT commands will return an error. This flag can be only
+     * reset back to 0 with a factory reset.
+     */
+    uint8_t lock_keys : 1;
 
     /* The maximum number of retransmissions of unconfirmed uplink messages.
      * Receiving a downlink message from the network stops retransmissions.