Connecting to Azure IoT Hub with Certificate Based Authentication

The certificate-based authentication for Azure IoT works like this:

• A root certificate used to sign device certificates will be created and added into Azure IoTHub
• For each IoT device we want to authenticate, we will create a separate client certificate, signed by the same certificate authority.
• At the time of the authentication, our IoT device will present the client certificate as the proof of identity.
• Azure IoT Hub will verify the identity based on the root certificate and the device name.

This document details how to create the certificate authority, a root certificate, and a device certificate along with adding the required certificates to both the Azure IoT Hub and Azure Injector configurations.

As a first step, we need to generate the certificate hierarchy. 

Create the following folder structure on your local drive to hold the various certificates in the hierarchy that we will be generating:

iotcerts/
├── ca/
└── certs/
    ├── device/

These are the steps that need to be completed for the certificate hierarchy:

• Generate Root CA

• Generate Device certificate signed with the Root CA’s private key

Generate Root Certificate

1. Generate a private key file (ca.key) for the Root CA using the command below. You may choose to enter a passphrase to be associated with the ca.key file as well.

   Make note of this passphrase if you set one for the Root CA private key file (ca.key) as it will be used multiple times.

   openssl genrsa -des3 -out ca/ca.key 4096

2. Generate a self-signed certificate (ca.crt) for the Root CA using the command below. This command generates a new self-signed X.509 certificate named "ca.crt" valid for 3650 days (10 years) using the RSA private key "ca.key". You will be required to enter the pass phrase associated with the private key file "ca.key". 

   openssl req -new -x509 -key ca/ca.key -days 3650 -out ca/ca.crt

   There are a number of fields associated with the creation of the certificate. Fill them out with your relevant details.
   Example CA Creation

   $ openssl req -new -x509 -key ca/ca.key -days 3650 -out ca/ca.crt
   Enter pass phrase for ca/ca.key:
   You are about to be asked to enter information that will be incorporated
   into your certificate request.
   What you are about to enter is what is called a Distinguished Name or a DN.
   There are quite a few fields but you can leave some blank
   For some fields there will be a default value,
   If you enter '.', the field will be left blank.
   -----
   Country Name (2 letter code) [AU]:US
   State or Province Name (full name) [Some-State]:KS
   Locality Name (eg, city) []:Stilwell
   Organization Name (eg, company) [Internet Widgits Pty Ltd]:Cirrus Link Solutions
   Organizational Unit Name (eg, section) []:Support
   Common Name (e.g. server FQDN or YOUR name) []:CLS Example Root CA  
   Email Address []:
   $

3. Azure will need the certificates in PEM format. Convert the CRT format to PEM format using the command below.

   openssl x509 -in ca/ca.crt -out ca/ca.pem -outform PEM

You should have the following files created: 

iotcerts/
├── ca/
   ├── ca.crt
   ├── ca.pem
   ├── ca.key

Generate Device Certificate

1. Generate private key in PSCK8 format (CertDevice.key) for the device using the command below.

   openssl genrsa -out certs/device/CertDevice.key 4096

2. Generate a Certificate Signing Request (CSR) for the device using the command below. This command generates a new CSR named "CertDevice.csr’ using the RSA private key "CertDevice.key".

   openssl req -new -key certs/device/CertDevice.key -out certs/device/CertDevice.csr

   There are a number of fields associated with the creation of the certificate. Fill them out with your relevant details.
   Example Device CSR Creation

   $ openssl req -new -key certs/device/CertDevice.key -out certs/device/CertDevice.csr
   You are about to be asked to enter information that will be incorporated
   into your certificate request.
   What you are about to enter is what is called a Distinguished Name or a DN.
   There are quite a few fields but you can leave some blank
   For some fields there will be a default value,
   If you enter '.', the field will be left blank.
   -----
   Country Name (2 letter code) [AU]:US
   State or Province Name (full name) [Some-State]:KS
   Locality Name (eg, city) []:Stilwell
   Organization Name (eg, company) [Internet Widgits Pty Ltd]:Cirrus Link Solutions
   Organizational Unit Name (eg, section) []:Support
   Common Name (e.g. server FQDN or YOUR name) []:Device01
   Email Address []:
    
   Please enter the following 'extra' attributes
   to be sent with your certificate request
   A challenge password []:
   An optional company name []: $

   The Common Name or FQDN must match the name of the logical Device Id in IoT Hub. In our example the Device Id is Device01

3. Sign the Device CSR with the Device CA using the command below. This command will sign the CSR "CertDevice.csr" with the Root CA certificate ‘ca.crt’ and Root CA's RSA private key ‘ca.key’, creating a new X.509 certificate named ‘CertDevice.crt’ valid for 365 days (1 year). You will be required to enter the passphrase associated with the private key file "ca.key".

   openssl x509 -req -in certs/device/CertDevice.csr -CA ca/ca.crt -CAkey ca/ca.key -CAcreateserial -out certs/device/CertDevice.crt -days 365

4. Azure will need the certificate in PEM format. Convert the CRT format to PEM format using the command below:

   openssl x509 -in certs/device/CertDevice.crt -out certs/device/CertDevice.pem -outform PEM

You should now have the following files created:

iotcerts/
├── ca/
│   ├── ca.crt
│   ├── ca.pem
│   ├── ca.key
└── certs/
    ├── device/
       ├── CertDevice.crt
       ├── CertDevice.pem
       ├── CertDevice.csr
       └── CertDevice.key

Add Root Certificate to IoT Hub

• On the IoT Hub resource Overview page, click “Certificates” menu on the left blade, and click the “Add” button.
• Give a certificate name (eg. MyOrg RootCertificate) and import the ca.pem file from your iotcerts/ca folder. Check the “Set certificate status to verified on upload” checkbox and click Save.
  

Add device certificates to Azure Injector IoT Hub configuration

Navigate to the Azure Injector > Settings > Azure IoT Hubs > Certificates and add the certificates as shown below:

Update the Azure Injector > Settings > Azure IoT Hubs > Settings configuration to use the certificates as shown below. Note : the certificates created do not use a Password

Verify the connection is established as shown by the Status on the Azure Injector > Settings > Azure IoT Hubs view