Setting up mutual authentication can be a little daunting, especially when the docs for a library you’re using don’t always have a good example. Top it off with having to make your own certificates, and the whole process can be a real PITA! To make it easier, we’re going to be using a tool from the great people at Square, certstrap. If you’ve ever used the easyrsa utility bundled with OpenVPN, it will feel very familiar as it makes generating your own PKI much simpler than manually using OpenSSL.

Generating a root certificate authority

The client and server will both trust the same, private root certificate. We’re generating this manually for this example but you could alternatively use an existing PKI, for example from a MS Windows Server Domain Controller.

Find a release of certstrap for your operating system from their releases page. Once you’ve downloaded the binary, rename it to something more convenient and make it executable (if applicable).

wget https://github.com/square/certstrap/releases/download/v1.1.1/certstrap-v1.1.1-linux-amd64
mv certstrap-v1.1.1-linux-amd64 certstrap
chmod +x certstrap

Generate the certificate authority with a name that makes sense for your use case:

$ ./certstrap init --organization "Widgets Inc" \
                   --common-name "Snazzy Microservices"

Enter passphrase (empty for no passphrase):

Enter same passphrase again:

Created out/Snazzy_Microservices.key
Created out/Snazzy_Microservices.crt
Created out/Snazzy_Microservices.crl

# Want to set more information, choose an expiration date or key size? See...
./certstrap init -h

As you can see above, we’ve now generated the main certificate that we’ll be trusting on both the client and the server (out/Snazzy_Microservices.crt).

Generating a server certificate

The hostname of the server’s certificate will be validated upon connection so ensure that the common name and DNS name match the hostname of your service. Generating a server certificate for your services is as easy as:

$ ./certstrap request-cert --common-name "login.services.widgets.inc" \
                           --domain "login.services.widgets.inc"

Enter passphrase (empty for no passphrase):

Enter same passphrase again:

Created out/login.services.widgets.inc.key
Created out/login.services.widgets.inc.csr

This will create the private key and certificate signing request, but not the certificate itself. We can sign the service’s certificate as follows:

$ ./certstrap sign --CA Snazzy_Microservices "login.services.widgets.inc"

Created out/login.services.widgets.inc.crt from out/login.services.widgets.inc.csr signed by out/Snazzy_Microservices.key

Setting up the GRPC server

In development, you’ll likely be using something like this:

var server = new grpc.Server();
server.addProtoService(hello_proto.Greeter.service,
                       {sayHello: sayHello, sayHelloAgain: sayHelloAgain});
server.bind('0.0.0.0:50051', grpc.ServerCredentials.createInsecure());
server.start();

With SSL, the server.bind line becomes:

server.bind('0.0.0.0:50051', grpc.ServerCredentials.createSsl({
      rootCerts: fs.readFileSync(path.join(process.cwd, "server-certs", "Snazzy_Microservices.crt")),
      keyCertPairs: {
            privateKey: fs.readFileSync(path.join(process.cwd, "server-certs", "login.services.widgets.inc.key")),
            certChain: fs.readFileSync(path.join(process.cwd, "server-certs", "login.services.widgets.inc.crt"))
      },
      checkClientCertificate: true
}));

As you’ll probably want to keep the existing setup for testing, we can choose a setup based on the current process.env.NODE_ENV:

if (process.env.NODE_ENV === "production") {
      server.bind('0.0.0.0:50051', grpc.ServerCredentials.createSsl({
            // ... as above
      }));
} else {
      server.bind('0.0.0.0:50051', grpc.ServerCredentials.createInsecure());
}

Additionally, you may want to name your files differently so that they’re consistent between services, e.g. ca.crt, service.crt and service.key.

Setting up the GRPC client

Like the server, we also need to generate a certificate for the client. This time, we don’t need a DNS name and can use any common name we like:

$ ./certstrap request-cert --common-name "client-1010101"

Enter passphrase (empty for no passphrase):

Enter same passphrase again:

Created out/client-1010101.key
Created out/client-1010101.csr

$ ./certstrap sign --CA Snazzy_Microservices "client-1010101"

Created out/client-1010101.crt from out/client-1010101.csr signed by out/Snazzy_Microservices.key

As with the server, we must provide the certificate authority, client cert and private key:

if (process.env.NODE_ENV === "production") {
      client = new hello_proto.Greeter('localhost:50051', grpc.credentials.createSsl(
      fs.readFileSync(path.join(process.cwd(), "client-certs", "Snazzy_Microservices.crt")),
      fs.readFileSync(path.join(process.cwd(), "client-certs", "client-1010101.key")),
      fs.readFileSync(path.join(process.cwd(), "client-certs", "client-1010101.crt"))
      ));
} else {
      client = new hello_proto.Greeter('localhost:50051', grpc.credentials.createInsecure());
}

Full example

You can see a full example of the client and server setup on GitHub.

Troubleshooting

If at any stage the above doesn’t work, try turning on verbose logging:

export GRPC_TRACE=all
export GRPC_VERBOSITY=DEBUG

node server

Other considerations

To distribute certificates and keys to hosts, you could either bake them into a virtual machine image, pull them down from a central store (e.g. S3 with encrypted objects and restrictive permissions) or store the keys in your source code repository and decrypt them at runtime.

As you may have noticed above, we’ve not specified any expiration dates and we have no method of revoking certificates. If this is a concern for you, you could set a short expiration date on client / server certificates and frequently rotate them or load balance your GRPC servers behind another server that includes revocation checks (e.g. nginx).