Steve Spencer's Blog - Python

Generating your IoT Hub Shared Access Signature for your ESP 8266 using Azure Functions

Tue, 21 Jan 2020 21:09:02 +0000

In my last 2 posts I showed how you can connect your ESP 8266 to the IoT hub to receive messages from the hub and also to send messages. One of the issue I had was generating the Shared Access Signature (SAS) which is required to connect to the IoT hub. I was unable to generate this on the device so I decided to use Azure Functions. The code required is straight forward and can be found here: https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-security#security-tokens

To create an Azure Function, go to the Azure management portal click the menu icon in the top left and select “Create a Resource”

Search for “Function”

and select “Function App” and click Create

Complete the form

And click Review and Create to accept the defaults or click next and work through the wizard if you want to change from the default values.

Click create to kick of the deployment of your new Azure Function. Once the deployment is complete navigate to the Function by clicking “Go To Resource”. You now need to create your function.

Click the + sign next to “Functions”. I used the In-portal editor as it was the easiest to use at the time as I already had most of the code copied from the site mentioned above.

Click In-Portal, then Continue and choose the Webhook + API template and click Create

Your function is now ready for editing. It will have some default code in there to give you an idea how to start

We’re going to use the previous SAS code in here and modify it to accept a json payload with the parameters you need for the SAS to be created.

The json we’ll use is as follows:

{
     "resourceUri":"[Your IOT Hub Name].azure-devices.net/devices/[Your DeviceId]",
     "expiryInSeconds":86400,
     "key":"[SAS Key from IoT hub]"
}

You can get you SAS key from the IoT hub in the Azure Portal in the devices section. Click on the device

Then copy the Primary or Secondary key.

Back to the function. In the editor Paste the following code:

C# function

#r "Newtonsoft.Json"
using System;
using System.Net;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Primitives;
using Newtonsoft.Json;
using System.Globalization;
using System.Net.Http;
using System.Security.Cryptography;
using System.Text;
public static async Task<IActionResult> Run(HttpRequest req, ILogger log)
{
     log.LogInformation("C# HTTP trigger function processed a request.");
     string token = "";
     try
     {
          string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
          dynamic data = JsonConvert.DeserializeObject(requestBody);
          int expiryInSeconds = (int)data?.expiryInSeconds;
          string resourceUri = data?.resourceUri;
          string key = data?.key;
          string policyName = data?.policyName;
          TimeSpan fromEpochStart = DateTime.UtcNow - new DateTime(1970, 1, 1);
          string expiry = Convert.ToString((int)fromEpochStart.TotalSeconds + expiryInSeconds);
          string stringToSign = WebUtility.UrlEncode(resourceUri) + "\n" + expiry;
          HMACSHA256 hmac = new HMACSHA256(Convert.FromBase64String(key));
          string signature = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(stringToSign)));
          token = String.Format(CultureInfo.InvariantCulture, "SharedAccessSignature sr={0}&sig={1}&se={2}", WebUtility.UrlEncode(resourceUri), WebUtility.UrlEncode(signature), expiry);
          if (!String.IsNullOrEmpty(policyName))
          {
               token += "&skn=" + policyName;
          }
     }
     catch(Exception ex)
     {
          return (ActionResult)new OkObjectResult($"{ex.Message}");
     }
     return (ActionResult)new OkObjectResult($"{token}");
}

Click Save and Run and make sure that there are no compilation errors. To use the function you need to post the json payload to the following address:

https://[your Function Name].azurewebsites.net/api/HttpTrigger1?code=[your function access key]

To retrieve your function access key, click Manage and copy your key from the Function Keys section

We’re now ready to use this in micropython on your ESP 8266. I created a function to retrieve the SAS

def getsas(hubname, deviceid, key):

import urequests

import ujson

dict = {}

dict["resourceUri"] = hubname+'.azure-devices.net/devices/'+deviceid

dict["key"] = key

dict["expiryInSeconds"]=86400

payload = ujson.dumps(dict)

response = urequests.post('https://[your function name].azurewebsites.net/api/HttpTrigger1?code=[your function access key]', data=payload)

return response.text

In my connectMQTT() function from the first post I replaced the hard coded SAS string with a call to the getsas function. The function returns a SAS which is valid for 24 hours so you will need to retrieve a new SAS once 24 hours has elapsed.

I can now run my ESP 8266 code without modifying it to give it a new SAS each time I want to use it. I always forgot and wondered why it never worked the next time I used it. I can now both send and receive data from/to the ESP 8266 and also generate a SAS to access the IoT hub. The next step is to use the data received by the hub in an application and send action messages back to the ESP 8266 if changes are made. I look forward to letting you know how I got on with that in a future post.

Sending data from the ESP 8266 to the Azure IoT hub using MQTT and MicroPython

Mon, 23 Dec 2019 07:43:39 +0000

In my previous post I showed you how to connect your ESP 8266 to the Azure IoT hub and be able to receive messages from the IoT hub to turn on a LED. In this post I'll show you how to send data to the IoT hub. For this I need to use a sensor that I will read at regular intervals and then send the data back to the IoT hub. I picked a temperature and humidity sensor I had from the kit of sensors I bought

This sensor is compatible with the DHT MicroPython library. I order to connect to the IoT hub use the same connect code that is in my previous post. The difference with sending is you need a end point for MQTT to send you temperature and humidity data to. The topic to send to is as follows:

devices/<your deviceId>/messages/events/

So using the same device id as in the last post then my send topic would be devices/esp8266/messages/events/

To send a message to the IoT hub use the publish method. This needs the topic plus the message you want to send. I concatenated the temperature and humidity and separated them with a comma for simplicity

import dht
import time
sensor = dht.DHT11(machine.Pin(16))
mqtt=connectMQTT()
sendTopic = 'devices/<your deviceId>/messages/events/'
while True:
    sensor.measure()
    mqtt.publish(sendTopic,str(sensor.temperature())+','+str(sensor.humidity()),True)
    time.sleep(1)

The code above is all that is required to read the sensor every second and send the data to the IoT hub.

In Visual Studio Code with the Azure IoT Hub Toolkit extension installed, you can monitor the messages that are sent to your IoT hub. In the devices view, right click on the device that has sent the data and select “Start Monitoring Built-in Event Endpoint”

This then displays the messages that are received by your IoT hub in the output window

You can see in the body of the received message the temperature and humidity values that were sent.

I still need to sort out generating the Shared Access Signature and also programmatically access the data I send to the IoT hub. I hope to have blog posts for these soon.

Connecting the ESP 8266 to Azure IoT Hub using MQTT and MicroPython

Sat, 23 Nov 2019 11:34:00 +0000

Recently was introduced to the ESP 8266 processor which is a low cost IoT device with built in Wi-Fi, costing around £3 - £4 for a development board. The thing that interested me (apart from price) was the device is Arduino compatible and will also run MicroPython. The version I purchased from Amazon was the NodeMcu variant with built in power and serial port via a microUsb port, so it makes an ideal board to start with as there are no additional components required.

This board however did not have MicroPython installed and that required a firmware change. The instructions were fairly straight forward and I followed this tutorial.

After installing MicroPython you can connect to the device using a terminal emulator via the USB serial port. Check in Device Manager to find the COM port number and the default baud rate is 115200. I used the Arduino Serial Monitor tool. In the terminal emulator you can press enter and you should get back the python REPL prompt. If not then you have the COM port or Baud rate wrong.

You can write you python directly into here but its easier to write the python in you PC then run it on the device. For this I use ampy

In Command Prompt install ampy using:

pip install adafruit-ampy

This allows you to connect to your device. Close the terminal emulator to free up the COM port then type the following to list the files on your device:

ampy --port COM4 --baud 115200 ls

The MicroPython Quick Ref will summarise how to access the GPIO ports etc but in order to connect to the IoT hub you will need to configure the Wi-Fi on the device. This can be done using the network module.

So create a new text file on your PC and write the code to connect to your Wi-Fi. To test this you can use ampy to run the python on the device:

ampy --port COM4 --baud 115200 run networking.py

Its a good idea to use print statements to help debug as once the run has complete the output will be reflected back in your Command Prompt.

Now you are connected to Wi-Fi we can start to look at connecting to the IoT hub. I am assuming that you already have your IoT hub set up. We now need to configure you new device. Navigate to the IoT hub in your Azure Portal. In Explorers click IoT Devices, then New

Enter your device id, the name your device will be known as. All your devices need a name that is unique to your IoT hub. Then click Save. This will auto generate the keys needed to generate the shared access signature needed to access the IoT hub later.

Once created you may need to click refresh in the devices list to see you new device. Click the device and copy the primary key, you will ned this for later to generate the Shared Access Signature used in the connection string. In order to generate a new Shared Access Token you can use Visual Studio Code with the Azure IoT Hub Toolkit extension installed. This puts a list of devices and endpoints in the explorer view and allows you to create a new Shared Access Token. find your device in the Devices list, Right click and select Generate SAS Token For Device

You will be prompted to enter the number of hours the token is valid for and the new SAS token will appear in the output window:

SharedAccessSignature sr=[your iothub name].azure-devices.net%2Fdevices%2Fesp8266&sig=bSpX6UMM5hdUKXHfTagZF7cNKDwKnp7I3Oi9LWTZpXI%3D&se=1574590568

The shared access signature is made up of the full address of your device, a time stamp indicating how long the signature is valid for and the whole thing is signed. You can take this an use it to test your access to IoT hub, so make sure you make the time long enough to allow you to test. The ESP8266 doesn't have a clock that can be used to generate the correct time so you will need to create the SAS off board. I’m going to use an Azure function with the code here to generate it.

Back to Python now. In order to connect to the IoT hub you will need to use the MQTT protocol. MicroPython uses umqtt.simple.

There are a few things required before you can connect.

Firstly the Shared Access Signature that you created above.

Next you will need to get the DigiCert Baltimore Root certificate that Iot Hub uses for SSL. This can be found here. Copy the text from -----BEGIN CERTIFICATE----- to -----END CERTIFICATE-----, including both the Begin and End lines. Remove the quotes and replace the \r\n with real new line in your text editor then save the file as something like baltimore.cer.

Next you will need a ClientId. For IoT hub the ClientId is the name of your device in IoT Hub. In this example it is esp8266

Next you will new a Username. For IoT hub, this is the full cname of your IoT Hub with your client id and a version. e.g. [your iothub name].azure-devices.net/esp8266//?api-version=2018-06-30

The following code should allow you to connect to the IoT Hub:

def connectMQTT():
     from umqtt.simple import MQTTClient

    CERT_PATH = "baltimore.cer"
     print('getting cert')
     with open(CERT_PATH, 'r') as f:
         cert = f.read()
     print('got cert')
     sslparams = {'cert':cert}

   CLIENT_ID='esp8266'
     Username='yourIotHub.azure-devices.net/esp8266/?api-version=2018-06-30'
     Password='SharedAccessSignature sr=yourIotHub.azure-devices.net%2Fdevices%2Fesp8266&sig=bSpX6UMM5hdUKXHfTagZF7cNKDwKnp7I3Oi9LWTZpXI%3D&se=1574590568'


    mqtt=MQTTClient(client_id=CLIENT_ID,server='yourIotHub.azure-devices.net',port=8883,user=Username,password=Password, keepalive=4000, ssl=True, ssl_params=sslparams)

     mqtt.set_callback(lightLed)
     mqtt.connect(False)

    mqtt.subscribe('devices/esp8266/messages/devicebound/#')
     flashled(4,0.1, blueled)

    return mqtt

set_callback requires a function which will be called when there is a device message sent from the IoT Hub. Mine just turns a Led on or off

def lightLed(topic, msg):
     if msg== b'on':
         statusled.on()
     else:
         statusled.off()

connect(False) means that the topic this device subscribes to will persist after the device disconnects.

I’ve also configured the device to connect to its bound topics so that any message sent to the device will call the callback function.

Now we need to have a process loop so that we can receive the messages. The ESP8266 does not seem to run async code so we need to call the wait_msq function to get any message back from the IoT hub

mqtt=connectMQTT()
print('connected...')
while True:
mqtt.wait_msg()

save your python as networking.py (and make sure that all the code you wrote initially to connect to Wi-Fi is included) then run ampy again:

ampy --port COM4 --baud 115200 run networking.py

Your device should run now. I’ve used the Led flash to show me progress for connecting to Wi-Fi then connecting to IoT Hub and also through to receiving a message. There is a blue LED on the board which I’ve been using as well as a standard LED which is turned on/off based upon the device message received from the IoT Hub. The blue LED is GPIO 2.

In order to send a message from the IoT hub to your device then you can do this from the Azure Portal in the devices view. Click on the device then click Message To Device

Enter the Message Body (on or off) and click Send Message

Alternatively you can do this in Visual Studio Code by right clicking the device and selecting Send C2D Message To Device and enter the message in the box that pops up

In my example the Led lights when I enter on and turns off when I enter off. ampy is likely to timeout during this process, but that’s ok as the board will still be running. As we’ve put the message retrieval inside a loop then the board will continue to run. To stop it running you will need to reset the board by pressing the reset button.

My next step is to sort out automatically generating the Shared Access Signature and then I’ll look at sending data to the IoT Hub