Use a Cloud HSM key for TLS offloading with NGINX

This guide provides instructions for setting up NGINX to use a Cloud HSM key for TLS offloading on Debian 11 (Bullseye). You might need to modify these commands to work with your OS or Linux distribution.

Note: TLS termination with Nginx is only supported for Elliptic Curve Digital Signature Algorithm (ECDSA) keys.

Use cases

Using a Cloud HSM key with NGINX for TLS offloading helps address the following enterprise security needs:

• You want your NGINX web server to offload TLS cryptographic operations to Cloud HSM.
• You don't want to store your certificate’s private key on the Compute Engine instance’s local file system that is hosting your web application.
• You need to meet regulatory requirements where public facing applications need their certificates to be protected by an HSM that has FIPS 140-2 Level 3 certification.
• You want to use NGINX to create a reverse proxy with TLS termination to protect your web application.

Before you begin

Before continuing, complete the steps in Using a Cloud HSM key with OpenSSL.

Once OpenSSL setup is complete, ensure that a recent version of nginx is installed:

sudo apt-get update
sudo apt-get install libengine-pkcs11-openssl opensc nginx

Note: nginx 1.18.0-6.1 and newer supports the PKCS #11 URI Scheme (RFC 7512). You might need to upgrade your previously installed nginx package.

Security configuration recommendations

Secure your instance that is hosting NGINX with the following recommendations:

1. Follow the instructions for creating and enabling service accounts for instances to host NGINX.

   1. Assign the following roles:
      • roles/cloudkms.signerVerifier
      • roles/cloudkms.viewer

2. Configure organization policies as follows to limit external IPs and creation of service account keys.

   • constraints/compute.vmExternalIpAccess
   • constraints/iam.disableServiceAccountKeyCreation

3. Create a custom subnet that enables private Google access.

4. Configure firewall rules.

   • Create IAP firewall rules for SSH only.

5. Create a Linux VM and configure it as follows:

   • Select the correct service account that you created earlier.
   • Select the network that you created earlier.
     • Add appropriate labels for any firewall rules.
     • Ensure the subnet has the "external IP" field set to none.

6. Grant your identity the IAP-Secured Tunnel User (roles/iap.tunnelResourceAccessor) role on the instance.

   • Learn more by reading IAP configuration for compute.

Create and configure a Cloud KMS-hosted signing key

The next sections detail steps needed to create and configure a Cloud KMS-hosted signing key.

Create a Cloud KMS-hosted signing key

Create a Cloud KMS EC-P256-SHA256 signing key in your Google Cloud project, in the key ring that you previously configured for OpenSSL:

gcloud kms keys create NGINX_KEY \
  --keyring "KEY_RING" --project "PROJECT_ID" \
  --location "LOCATION" --purpose "asymmetric-signing" \
  --default-algorithm "ec-sign-p256-sha256" --protection-level "hsm"

Note: Make sure that your Compute Engine service account has the correct Identity and Access Management (IAM) permissions on the key ring to be able to use it.

SSH into your VM using IAP

SSH into your VM using IAP with the following command:

gcloud compute ssh INSTANCE \
  --zone ZONE --tunnel-through-iap

If you run into an issue, confirm that you used the --tunnel-through-iap flag. Also, confirm that you have the IAP-Secured Tunnel User (roles/iap.tunnelResourceAccessor) role on the instance for the identity authenticated with gcloud CLI.

Create a certificate with OpenSSL

For a production environment, create a certificate signing request (CSR). Learn more by reading the example to generate a CSR. Provide the CSR to your certificate authority (CA) so that they can create a certificate for you. Use the certificate provided by your CA in the subsequent sections.

For example purposes, you can generate a self-signed certificate with the Cloud KMS-hosted signing key. To do so, OpenSSL lets you use PKCS #11 URIs instead of a regular path, identifying the key by its label (for Cloud KMS keys, the label is the CryptoKey name).

openssl req -new -x509 -days 3650 -subj '/CN=CERTIFICATE_NAME/' \
  DIGEST_FLAG -engine pkcs11 -keyform engine \
  -key PKCS_KEY_TYPE=KEY_IDENTIFIER > CA_CERT

Replace the following:

• CERTIFICATE_NAME: a name for the certificate.
• DIGEST_FLAG: the digest algorithm used by the asymmetric signing key. Use -sha256, -sha384, or -sha512 depending on the key.
• PKCS_KEY_TYPE: the type of identifier used to identify the key. To use the latest key version, use pkcs11:object with the key's name. To use a specific key version, use pkcs11:id with the full resource ID of the key version.
• KEY_IDENTIFIER: an identifier for the key. If you're using pkcs11:object, use the key's name—for example, NGINX_KEY. If you're using pkcs11:id, use the full resource ID of the key or key version—for example, projects/PROJECT_ID/locations/LOCATION/keyRings/KEY_RING/cryptoKeys/NGINX_KEY/cryptoKeyVersions/KEY_VERSION.
• CA_CERT: the path where you want to save the certificate file.

If the command fails, PKCS11_MODULE_PATH may have been set incorrectly, or you might not have the correct permissions to use the Cloud KMS signing key.

You should now have a certificate that looks like this:

-----BEGIN CERTIFICATE-----
...
...
...
-----END CERTIFICATE-----

Install your certificate for NGINX

Run the following commands to create a location to place your public certificate:

sudo mkdir /etc/ssl/nginx
sudo mv CA_CERT /etc/ssl/nginx

Configure your environment to use the PKCS #11 library

The next sections detail steps needed to prepare and test your environment.

Prepare library configurations for NGINX

Allow NGINX to log its PKCS #11 engine operations with the library with the following:

sudo mkdir /var/log/kmsp11
sudo chown www-data /var/log/kmsp11

Create an empty library configuration file with the appropriate permissions for NGINX.

sudo touch /etc/nginx/pkcs11-config.yaml
sudo chmod 744 /etc/nginx/pkcs11-config.yaml

Edit the empty config file and add the needed configuration as shown in the following snippet:

# cat /etc/nginx/pkcs11-config.yaml
---
tokens:
  - key_ring: "projects/PROJECT_ID/locations/LOCATION/keyRings/KEY_RING"
log_directory: "/var/log/kmsp11"

Test your OpenSSL configuration

Run the following command:

openssl engine -tt -c -v pkcs11

You should see output similar to the following:

(pkcs11) pkcs11 engine
 [RSA, rsaEncryption, id-ecPublicKey]
     [ available ]
     SO_PATH, MODULE_PATH, PIN, VERBOSE, QUIET, INIT_ARGS, FORCE_LOGIN

Configure NGINX to use Cloud HSM

Allow TLS offloading by editing a few NGINX files. First, edit the /etc/nginx/nginx.conf file in two places to add a few directives to configure NGINX to use PKCS #11.

After the event block and before the http block, add the following directives:

ssl_engine pkcs11;
env KMS_PKCS11_CONFIG=/etc/nginx/pkcs11-config.yaml;

In the same /etc/nginx/nginx.conf file, configure SSL directives to use your certificate and its private key in Cloud HSM. In the http block add the following attributes:

ssl_certificate "/etc/ssl/nginx/CA_CERT";
ssl_certificate_key "engine:pkcs11:PKCS_KEY_TYPE=KEY_IDENTIFIER";
ssl_protocols TLSv1.2 TLSv1.3; # Consider changing the default to only TLS1.2 or newer

# Consider defining the `ssl_ciphers` to use ciphers approved by your security teams and handle
# appropriate client compatibility requirements.

Your /etc/nginx/nginx.conf file should look like the following:

user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

events {
        worker_connections 768;
        # multi_accept on;
}

ssl_engine pkcs11;
env KMS_PKCS11_CONFIG=/etc/nginx/pkcs11-config.yaml;

http {

        #...
        #...

        # SSL configuration
        ssl_certificate "/etc/ssl/nginx/CA_CERT";
        ssl_certificate_key "engine:pkcs11:pkcs11:object=NGINX_KEY";
        ssl_protocols TLSv1.2 TLSv1.3; # Dropping SSLv3, ref: POODLE
        # ssl_ciphers YOUR_CIPHERS
        ssl_prefer_server_ciphers on;

        #...
        #...

}

Configure NGINX to listen for TLS traffic

Edit the /etc/nginx/sites-enabled/default file to listen for TLS traffic. Uncomment the SSL configuration in the server block. The resulting change should look like the following example:

server {
        listen 80 default_server;
        listen [::]:80 default_server;

        # SSL configuration
        listen 443 ssl default_server;
        listen [::]:443 ssl default_server;

        # ...
        # ...
}

Provide environment variables to the NGINX service

Run the following command:

sudo systemctl edit nginx.service

In the resulting editor, add the following lines and replace the LIBPATH with the value for the location where you installed libkmsp11.so:

[Service]
Environment="GRPC_ENABLE_FORK_SUPPORT=1"
Environment="KMS_PKCS11_CONFIG=/etc/nginx/pkcs11-config.yaml"
Environment="PKCS11_MODULE_PATH=LIBPATH/libkmsp11-1.0-linux-amd64/libkmsp11.so"

After you configure these values, you will need to run the following command to make them available:

sudo systemctl daemon-reload

Restart NGINX with TLS Offloading

Run the following command so that NGINX restarts and uses the updated configuration:

sudo systemctl start nginx

Note: If you see any errors upon restart, check your /var/log/nginx/error.log and /var/log/kmsp11/ for additional details.

Test NGINX uses TLS offloading to your Cloud HSM

Use the openssl s_client to test connection to your NGINX server by running the following command:

openssl s_client -connect localhost:443

The client should complete SSL handshake and pause. The client is awaiting input from you as shown following:

# completes SSL handshake
# ...
# ...
# ...
    Verify return code: 18 (self signed certificate)
# ...
    Max Early Data: 0
---
read R BLOCK

# When the client pauses, it’s waiting for instructions.
# Have the client get the index.html file in the root path (“/”), by typing the following:

GET /

# Press enter.
# You should now see the default NGINX index.html file.

Your audit logs should now show operations to your NGINX_KEY key. To view the logs, navigate to Cloud Logging in your cloud console. In the project you've been using, add the following filter:

resource.type="cloudkms_cryptokeyversion"

After running the query, you should see asymmetric key operations to your NGINX_KEY key.

Optional configurations

You may need to create an external passthrough Network Load Balancer to expose your NGINX server with an external IP.

If you need to use NGINX as a reverse proxy with load balancing, consider updating the NGINX configuration file. Learn more about configuring NGINX as a reverse proxy by reading All-Active HA for NGINX Plus on the Google Cloud Platform.

Next steps

You now have configured your NGINX server to use TLS offloading to Cloud HSM.