AWS IoT - TLS Client Certificate Authentication

All AWS settings can be configured either manually through the AWS web interface or programatically through AWS’ boto 3 module in Python. If you would like to use scripts to programatically set up your client certificate, you will need to do the following steps.

First, follow these instructions on the boto3 page to set up the boto3 module for Python: http://boto3.readthedocs.io/en/latest/guide/quickstart.html

The boto3 module authenticates with AWS based on a IAM Access ID and Secret Key. The boto3 tutorial will ask you to setup an IAM user, here are some instructions on how to do so:

1. From the AWS console, choose the IAM service.
2. Go to Users and select Add User
3. Choose a username and check the Programmatic access box
4. For simplicity, choose Attach existing policies directly and select AdministratorAccess
5. If you wish to better manage your IAM credentials, feel free to customize your Access Policy.
6. Click Review and then Create User
7. Save the Access ID and Secret Key and follow the boto3 guide.

Signing CSR with AWS’ Certificate Authority

Using AWS’ Certificate Authority is the easier option in terms of setup and allows you to use a trustworthy Certificate Authority that Amazon uses its services. The following steps show you how to sign your Zymkey’s private key with Amazon’s CA and get a valid device certificate:

Manually:

1. Sign into your AWS console. Here is a link to the console sign-in page: https://aws.amazon.com/console/
2. From the console, select the AWS IoT Core service
3. Under the secure tab, click on Certificates and click the blue Create a certificate bar
4. Choose the Create with CSR option
5. When the File Selection menu pops up point to your CSR file. It shoud be zymkey.csr by default. Choose Upload file. We created this file on your IoT device, you may need to transfer it to where your logged into AWS.
6. Download the Certificate file on the next screen and save as zymkey.crt. If you are not logged into AWS from your IoT device, transfer the file to your IoT device. We will use this later.
7. You also need to download the root CA for AWS IoT, by clicking Download. On the page that opens, scroll down to the section CA certificates for server authentication, click ECC 256 bit key and click on the adjacent link Amazon Root CA 3. Copy the public key into a file named AWS_CA.pem file. If you are not logged into AWS from your IoT device, transfer AWS_CA.pem to your IoT device. We will use this later.
8. Return to the Create Certificate AWS IoT page and click Activate

Programatically: Using the boto3 client, this python script will read the csr file zymkey.csr, give it to AWS to sign with their Certificate Authority, and create a signed certificate zymkey.crt in the directory where this program is run. Furthermore the certificate will be automatically registered and activated with AWS IoT and will be ready for use.

import boto3

iot_client = boto3.client('iot')
with open('zymkey.csr', 'r') as zymkey_csr_file:
	zymkey_csr = zymkey_csr_file.read()

zymkey_cert = iot_client.create_certificate_from_csr(
	certificateSigningRequest=zymkey_csr,
	setAsActive=True
)

with open('zymkey.crt', 'w') as zymkey_cert_file:
	zymkey_cert_file.write(zymkey_cert['certificatePem'])

Save the above script into a file called aws_sign_csr.py and run with the following command:

python aws_sign_csr.py

Now that your Certificate has been signed and activated by AWS IoT, it can be used to establish a TLS connection with the AWS IoT servers. Skip down to Testing the TLS Connection section to continue.

AWS IoT also allows you do you use own Certificate Authority as long as you register it with them. This constitutes proving you own the private key to the CA by signing a verification CSR that includes a registration code.

If you already have a Certificate Authority that you would like to use, you can ignore the next part. Here we create an example Certificate Authority using OpenSSL.

Creating an example CA with OpenSSL

#!/bin/bash
set -e
mkdir CA_files
cd CA_files

openssl ecparam -genkey -name prime256v1 -out zk_ca.key
OPENSSL_CONF=/etc/ssl/openssl.cnf openssl req \
  -x509 -new -SHA256 -nodes -key zk_ca.key \
  -days 3650 -out zk_ca.crt \
  -subj "/C=US/ST=California/L=Santa Barbara/O=Zymkey/CN=zymkey-verify.zymbit.com.dev"

cp zk_ca.crt zk_ca.pem

Copy the above lines into a script called mk_ca.sh. You can then run the script in the command line by being in the same directory with the following command:

bash mk_ca.sh

The script will create a directory called CA_files and a couple of files: zk_ca.key: Private key for the created CA, will be supplied to OpenSSL for signing CSRs. zk_ca.pem: PEM formatted certificate for the CA zk_ca.crt: Same file as zk_ca.pem

Signing CSR with Certificate Authority

Next we will be signing the Zymkey CSR with your chosen Certificate Authority.

#!/bin/bash
set -e

SCRIPT_NAME=$(basename $0)

[ -z $2 ] && echo "${SCRIPT_NAME} <csr filename> <crt filename>" 1>&2 && exit 1

csr=$1
crt=$2
openssl x509 -req -SHA256 -days 3650 \
  -CA CA_files/zk_ca.crt -CAkey CA_files/zk_ca.key -CAcreateserial \
  -in ${csr} -out ${crt}

Copy the above lines into a script called sign_csr.sh. The first argument is the relative or absolute path of your csr file, such as zymkey.csr. The second argument is what you want to name the certificate file of the signed cert. Change the -CA and -CAkey file path, can be relative or absolute, if you are using your own CA. You can then run the script in the command line by being in the same directory with the following command:

bash sign_csr.sh zymkey.csr zymkey.crt

Registering Your Certificate Authority with AWS

You now have a valid certificate, zymkey.crt signed by the Certificate Authority of your choice. If you are using your own Certificate Authority, you have to register your Certificate Authority with Amazon’s IoT service so that AWS IoT will accept certificates signed by that Certificate Authority.

Manually:

1. From the AWS IoT console select Secure, then click CAs and then click Register
2. Click Register CA
3. Follow Steps 1 through 3 on the next screen to create a verification certificate.
4. When signing the verification certificate with your CA in Step 4 run the following command:

openssl x509 -req -in verificationCert.csr -CA CA_files/zk_ca.pem -CAkey CA_files/zk_ca.key -CAcreateserial -out verificationCert.crt -days 500 -sha256

Note that if you use a different CA and not the demo one we generated, change the **-CA** and **-CAkey** paths appropriately.

5. Step 5: click Select CA certificate and point to the correct .pem file. If you use the OpenSSL generated SSL point to CA_files/zk_ca.pem
6. Step 6: click Select verification certificate and point to verificationCert.crt which was created in Step 4.
7. Select Activate CA certificate and Enable auto-registration of device certificates
8. Click Register CA certificate button.

Programatically:

The following python script will automatically create a verification cert with a registration code and automatically activate your Certificate Authority. While it may look a bit intimidating, all you have to worry about is the very last line, where you can change to point to your CA files.

import OpenSSL
import boto3
import os

def gen_AWS_verification_csr(registrationCode):
	key = OpenSSL.crypto.PKey()
	key.generate_key(OpenSSL.crypto.TYPE_RSA, 2048)
	req = OpenSSL.crypto.X509Req()
	req.get_subject().CN = registrationCode
	req.set_pubkey(key)
	req.sign(key, "sha256")
	return OpenSSL.crypto.dump_certificate_request(OpenSSL.crypto.FILETYPE_PEM, req)

def sign_CSR_with_CA(verification_csr, CA_cert_path, CA_key_path):
	ca_cert = OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_PEM, open(CA_cert_path).read())
	ca_key = OpenSSL.crypto.load_privatekey(OpenSSL.crypto.FILETYPE_PEM, open(CA_key_path).read())
	req = OpenSSL.crypto.load_certificate_request(OpenSSL.crypto.FILETYPE_PEM, verification_csr)
	cert = OpenSSL.crypto.X509()
	cert.set_subject(req.get_subject())
	cert.set_serial_number(1)
	cert.gmtime_adj_notBefore(0)
	cert.gmtime_adj_notAfter(24 * 60 * 60)
	cert.set_issuer(ca_cert.get_subject())
	cert.set_pubkey(req.get_pubkey())
	cert.sign(ca_key, "sha256")
	return OpenSSL.crypto.dump_certificate(OpenSSL.crypto.FILETYPE_PEM, cert)

def register_CA_AWS(CA_cert_path, CA_key_path):
	client = boto3.client('iot')

	response = client.get_registration_code()
	registration_key = response['registrationCode']

	verification_pem = gen_AWS_verification_csr(registrationCode=registration_key)
	verification_cert = sign_CSR_with_CA(verification_csr=verification_pem, CA_cert_path=CA_cert_path, CA_key_path=CA_key_path)

	response = client.register_ca_certificate(
		caCertificate=open(CA_cert_path).read(),
		verificationCertificate=verification_cert,
		setAsActive=True,
		allowAutoRegistration=True
	)

	return response

register_CA_AWS(CA_cert_path='CA_files/zk_ca.crt', CA_key_path='CA_files/zk_ca.key')

Copy the above lines into a file called activate_aws_ca.py and run with the following command:

python activate_aws_ca.py

Manually:

1. From the AWS IoT Console click Secure, then Certificates and then click the blue Create button
2. Under Use My Certificate click the Get Started button
3. If you registered your own CA, choose the CA you registered on the Select a CA screen, then click Next
4. Click Select certificates and navigate to the certificate that was signed by your CA. Its default name is zymkey.crt
5. Make sure to check the Activate all circle on the certificate box, and finally click the blue Register certificates button.

Programatically:

The same thing can be done in Python. Just change the last line to point to your CA_Path and Cert_Path, the paths can be either relative or absolute. For example if the two certificate files: zk_ca.crt and zymkey.crt are in the same directory as the Python script, you don’t have to change anything from the following:

import boto3
import OpenSSL

def activate_cert_AWS(CA_path, Cert_path):
	boto3client = boto3.client('iot')
	with open(CA_path) as CA_file:
               CA_Pem = CA_file.read()
        with open(Cert_path) as Cert_file:
                Cert_Pem = Cert_file.read()
        return boto3client.register_certificate(
		certificatePem=Cert_Pem,
		caCertificatePem=CA_Pem,
		setAsActive=True,
	)

activate_cert_AWS(CA_path='CA_files/zk_ca.crt', Cert_path='zymkey.crt')

Copy the above lines into a file called activate_aws_cert.py and run with the following command: Change the CA_Path and Cert_path if necessary.

python activate_aws_cert.py

The first thing to do is to look for your AWS endpoint:

1. From the AWS IoT console screen, click on Settings in the left hand bar.

2. In the Device data endpoint section, copy the Endpoint.

3. Replace endpoint.iot.region.amazonaws.com with the Endpoint you just copied in the following command. Now run the command, making sure to do it in the same directory where you keep your signed certificate, zymkey.crt and your CA cert/pem file, AWS_CA.pem, or CA_files/zk_ca.pem:

   #replace endpoint iot region with the copied endpoint
   ```bash
   curl --tlsv1.2 --cacert AWS_CA.pem --cert zymkey.crt --key nonzymkey.key --engine zymkey_ssl --key-type ENG -v -X POST -d "{ \"hello\": \"world\"}" "https://endpoint.iot.region.amazonaws.com:8443/topics/hello/world"
   ```
   

Optionally, you can set an environment variable specifying the slot to use. For the HSM6, you will need to specify the variable prior to running the curl command:

export ZK_SSL_SLOT=0

You should see a successful TLS connection, but receive a 403 Forbidden Exception from AWS. This is because the certificate you registered, zymkey.crt doesn’t have the appropriate permissions to publish a message to the topic hello/world. We can fix this by adding a policy and attaching it to the certificate.

Here’s what the successful TLS connection looks like:

Here’s the 403 exception you should receive:

Here we will attach a Policy to your Zymkey certificate that allows it to publish data to any topic on AWS IoT.

1. From the AWS IoT console click on Secure and then Policies. Click the blue Create a policy button.

2. Give your Policy an appropriate name.

3. Under Action, write the following:

    iot:Connect, iot:Publish
   

4. For Resource ARN write:

    *
   

5. Check the Allow box, and click Create

6. Now, click the Certificates tab on the left and click on the … option on the top right corner of your certificate. Select Attach Policy

7. Attach the appropriate policy and you are done.

Now the previous command should work and {“hello”: “world”} should be published to the hello/world topic on your AWS IoT endpoint.

1. On the AWS IoT console and the left hand bar, click on the Test option, and then the MQTT test client option.

2. In the Subscribe to a topic tab, in the Subscription topic box, type in hello/world. Click the Subscribe button

3. Test your TLS connection with the following CURL command pointing to the CA cert/pem file and your Zymkey certificate:

    #replace endpoint.iot.region with the appropriate values
   

curl --tlsv1.2 --cacert CA_files/zk_ca.pem --cert zymkey.crt --key nonzymkey.key --engine zymkey_ssl --key-type ENG -v -X POST -d "{ \"hello\": \"world\"}" "https://endpoint.iot.region.amazonaws.com:8443/topics/hello/world"

If it works, your command line should have indication of successful TLS connection and “hello”: “world” should show up in your subscribed topic.