mTLS using Golang Fiber

In this demo we're going to create mTLS using Go and Fiber. To create certificates that can be used for mutual authentication, what you need to have is just an OpenSSL program (or simplecert, or mkcert like in previous natsmtls1 example), create a CA (certificate authority), server certs, and client certs, something like this:

# generate CA Root
openssl req -newkey rsa:2048 -new -nodes -x509 -days 3650 -out ca.crt -keyout ca.key -subj "/C=SO/ST=Earth/L=MyLocation/O=MyOrganiz/OU=MyOrgUnit/CN=localhost"

# generate Server Certs
openssl genrsa -out server.key 2048
# generate server Cert Signing request
openssl req -new -key server.key -days 3650 -out server.csr -subj "/C=SO/ST=Earth/L=MyLocation/O=MyOrganiz/OU=MyOrgUnit/CN=localhost"
# sign with CA Root
openssl x509  -req -in server.csr -extfile <(printf "subjectAltName=DNS:localhost") -CA ca.crt -CAkey ca.key -days 3650 -sha256 -CAcreateserial -out server.crt

# generate Client Certs
openssl genrsa -out client.key 2048
# generate client Cert Signing request
openssl req -new -key client.key -days 3650 -out client.csr -subj "/C=SO/ST=Earth/L=MyLocation/O=$O/OU=$OU/CN=localhost"
# sign with CA Root
openssl x509  -req -in client.csr -extfile <(printf "subjectAltName=DNS:localhost") -CA ca.crt -CAkey ca.key -out client.crt -days 3650 -sha256 -CAcreateserial

You will get at least 2 files related to CA, 3 files related to server, and 3 files related to client, but what you really need is just CA public key, server private and public key (key pairs), and client private and public key (key pairs). If you need to generate another client or rollover server keys, you will still need CA's private key so don't erase it.

Next, now that you already have those 5 keys, you will need to load CA public key, and server key pair and use it on fiber, something like this:

caCertFile, _ := os.ReadFile(in.CaCrt)
caCertPool := x509.NewCertPool()
caCertPool.AppendCertsFromPEM(caCertFile)

serverCerts, _ := tls.LoadX509KeyPair(in.ServerCrt, in.ServerKey)

tlsConfig := &tls.Config{
    ClientCAs:        caCertPool,
    ClientAuth:       tls.RequireAndVerifyClientCert,
    MinVersion:       tls.VersionTLS12,
    CurvePreferences: []tls.CurveID{tls.CurveP521, tls.CurveP384, tls.CurveP256},
        CipherSuites: []uint16{
        tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
        tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
        tls.TLS_RSA_WITH_AES_256_GCM_SHA384,
        tls.TLS_RSA_WITH_AES_256_CBC_SHA,
         tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
        tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
     },
     Certificates: []tls.Certificate{serverCerts},
}

// attach the certs to TCP socket, and start Fiber server
app := fiber.New(fiber.Config{
    Immutable: true,
})
app.Get("/", func(c *fiber.Ctx) error {
    return c.String(`secured string`)
})
ln, _ := tls.Listen("tcp", `:1443`, tlsConfig)
app.Listener(ln)

next on the client side, you just need to load CA public key, client key pairs, something like this:

caCertFile, _ := os.ReadFile(in.CaCrt)
caCertPool := x509.NewCertPool()
caCertPool.AppendCertsFromPEM(caCertFile)
certificate, _ := tls.LoadX509KeyPair(in.ClientCrt, in.ClientKey)

httpClient := &http.Client{
    Timeout: time.Minute * 3,
    Transport: &http.Transport{
        TLSClientConfig: &tls.Config{
            RootCAs:      caCertPool,
            Certificates: []tls.Certificate{certificate},
        },
    },
}

r, _ := httpClient.Get(`https://localhost:1443`)

that's it, that's how you secure client-server communication between Go client and server with mTLS, this code can be found here.