Better key distribution topic extraction

Author: PaulZC

Firmware/RTK_Surveyor/menuPP.ino

@@ -398,7 +398,17 @@ bool pointperfectProvisionDevice()
 
                     strcpy(settings.pointPerfectClientID, (const char *)((*jsonZtp)["clientId"]));
                     strcpy(settings.pointPerfectBrokerHost, (const char *)((*jsonZtp)["brokerHost"]));
-                    strcpy(settings.pointPerfectLBandTopic, (const char *)((*jsonZtp)["subscriptions"][0]["path"]));
+
+                    // Note: from the ZTP documentation:
+                    // ["subscriptions"][0] will contain the key distribution topic
+                    // But, assuming the key distribution topic is always ["subscriptions"][0] is potentially brittle
+                    // It is safer to check the "description" contains "key distribution topic"
+                    int subscription =
+                        findZtpJSONEntry("subscriptions", "description", "key distribution topic", jsonZtp);
+                    if (subscription >= 0)
+                        strncpy(settings.pointPerfectLBandTopic,
+                                (const char *)((*jsonZtp)["subscriptions"][subscription]["path"]),
+                                sizeof(settings.pointPerfectLBandTopic));
 
                     strcpy(settings.pointPerfectCurrentKey,
                            (const char *)((*jsonZtp)["dynamickeys"]["current"]["value"]));
@@ -451,6 +461,26 @@ bool pointperfectProvisionDevice()
 #endif // COMPILE_WIFI
 }
 
+// Find thing3 in (*jsonZtp)[thing1][n][thing2]. Return n on success. Return -1 on error / not found.
+int findZtpJSONEntry(const char *thing1, const char *thing2, const char *thing3, DynamicJsonDocument *jsonZtp)
+{
+    if (!jsonZtp)
+        return (-1);
+
+    int i = (*jsonZtp)[thing1].size();
+
+    if (i == 0)
+        return (-1);
+
+    for (int j = 0; j < i; j++)
+        if (strstr((const char *)(*jsonZtp)[thing1][j][thing2], thing3) != nullptr)
+        {
+            return j;
+        }
+
+    return (-1);
+}
+
 // Check certificate and privatekey for valid formatting
 // Return false if improperly formatted
 bool checkCertificates()
@@ -618,8 +648,6 @@ bool pointperfectUpdateKeys()
             // Successful connection
             systemPrintln("MQTT connected");
 
-            // Originally the provisioning process reported the '/pp/key/Lb' channel which fails to respond with
-            // keys. Looks like they fixed it to /pp/ubx/0236/Lb.
             mqttClient.subscribe(settings.pointPerfectLBandTopic);
         }
         else

Firmware/RTK_Surveyor/settings.h

@@ -961,7 +961,7 @@ typedef struct
     bool autoKeyRenewal = true; // Attempt to get keys if we get under 28 days from the expiration date
     char pointPerfectClientID[50] = "";
     char pointPerfectBrokerHost[50] = ""; // pp.services.u-blox.com
-    char pointPerfectLBandTopic[20] = ""; // /pp/key/Lb
+    char pointPerfectLBandTopic[20] = ""; // /pp/ubx/0236/Lb
 
     char pointPerfectCurrentKey[33] = ""; // 32 hexadecimal digits = 128 bits = 16 Bytes
     uint64_t pointPerfectCurrentKeyDuration = 0;