MQTT integration

NervesKeys aren't only for use with NervesHub. If a service allows users to provide their own certificate authority, it should be possible to also use the NervesKey. Nearly all services use TLS or DTLS so these instructions provide guidance for using the Erlang/OTP SSL application. The following diagram shows the other compoents that are involved with establishing and authenticating MQTT connections over TLS:

In particular, the NervesKey integration with Erlang makes use of the Engine API to redirect private key operations to the cryptographic module. While nerves_key_pkcs11 provides a PKCS#11 implementation, be aware that it is minimal and only supports the operations necessary for establishing TLS connections. Programs using the NervesKey should still use the nerves_key library for provisioning and everything else.

To use the Erlang/OTP SSL application's Engine API, you will need to do the following:

1. Initialize the engine to start the nerves_key_pkcs11 integration

2. Replace the locations where you would have suppliced a private key with an Elixir map that has a reference to the engine and other information

The nerves_key_pkcs11library has a helper functions for both tasks.

The following example code fragment shows how to start the Tortoise MQTT client API:

{:ok, engine} = NervesKey.PKCS11.load_engine()
{:ok, i2c} = ATECC508A.Transport.I2C.init([])
​
signer_cert = X509.Certificate.to_der(NervesKey.signer_cert(i2c, :primary))
cert = X509.Certificate.to_der(NervesKey.device_cert(i2c, :primary))
key = NervesKey.PKCS11.private_key(engine, {:i2c, 1})
cacerts = [signer_cert | aws_root_certs()]
​
Tortoise.Supervisor.start_child(
  client_id: manufacturing_serial_number(),
  handler: {MQTTHandler, []},
  server: {
    Tortoise.Transport.SSL,
    verify: :verify_peer,
    host: mqtt_host(),
    port: mqtt_port(),
    alpn_advertised_protocols: alpn_advertised_protocols(),
    server_name_indication: server_name_indication(),
    cert: cert,
    key: key,
    cacerts: cacerts,
    versions: [:"tlsv1.2"]
  },
  subscriptions: topics()
 )