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

  • There will be a root certificate signed by a certificate authority.
  • The root certificate will be uploaded to the Azure IoT Hub.
  • 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 a certificate authority, a root certificate, and the client certificate(s).


The command line tools openssl and keytool are used.

Install the OpenSSL command line tool and add the OpenSSL PATH in the Windows environment variables if necessary.

Keytool is part of the standard java distribution and is located in the bin sub-directory of your jdk installation directory. Chariot includes a java distribution under the <chariot_install_dir>/lib/runtime/jdk11.0.12_7/bin folder. Add the keytool PATH in the Windows environment variables if necessary.

You will need to restart your any open command window to pick up this configuration change.

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 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 []:
$

You should have the following files created: 

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


Depending on the version of openSSL that you are using, you may see additional .srl files created which contain the signed certificate's unique serial number. These files are not used directly by the modules and not included in the certificate hierachy displayed above. 

Generate Device Certificate

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

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

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

    openssl req -new -key certs/device/device.key -out certs/device/device.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/device.key -out certs/device/device.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 "device.csr" with the Root CA certificate ‘ca.crt’ and Root CA's RSA private key ‘ca.key’, creating a new X.509 certificate named ‘device.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/device.csr -CA ca/ca.crt -CAkey ca/ca.key -CAcreateserial -out certs/device/device.crt -days 365


A unique client certificates will be required for each logical Device Id.

If you have multiple devices, you will need to expand your folder structure adding an appropriate folder for each device and run the Generate Client Certificate steps for each device amending the commands above appropriately. 


Import Server Configuration to IoT Hub

On the IoT Hub resource Overview page, click “Certificates” menu on the left blade, and click the “Add” button.


In the “Certificates” blade that appears, give a certficate name (eg. MyOrgIoTHubRootCertificate).

Browse to the IotCerts/rootca folder on your computer and import the rootca.pem. Check the “Set certificate status to verified on upload” checkbox.


Click Save.

Create Logical Device myiotdevice1
On the resource Overview page, click “Devices” menu on the left blade, and click “Add Device” button.


In the “Create a device” page that appears, give myiotdevice1 as the Device Id.

Note that this name must match the commonName/FQDN we used when generating the client (device) certificate above.

Select Authentication type X.509 CA Signed. Keep “Connect this device to an IoT Hub” as Enabled.


it 

Click Save.



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