openvpn certificate creation with golang

openvpn-cert-generate.go

package main

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"log"
	"math/big"
	"os"
	"time"
)

func main() {
	ca := &x509.Certificate{
		SerialNumber: big.NewInt(2023),
		Subject: pkix.Name{
			Organization:  []string{"Company, INC."},
			Country:       []string{"TR"},
			Province:      []string{"Istanbul"},
			Locality:      []string{"Kadikoy"},
			StreetAddress: []string{"Bağdat Cad. No: 292/16"},
			PostalCode:    []string{"34728"},
			CommonName:    "openvpn-ca-cert",
		},
		NotBefore:             time.Now().UTC(),
		NotAfter:              time.Now().AddDate(10, 0, 0),
		IsCA:                  true,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth},
		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
		BasicConstraintsValid: true,
	}

	// create ca private and public key
	caPrivKey, err := rsa.GenerateKey(rand.Reader, 4096)
	if err != nil {
		log.Fatalf("%s", err)
	}

	// create the CA
	caBytes, err := x509.CreateCertificate(rand.Reader, ca, ca, &caPrivKey.PublicKey, caPrivKey)
	if err != nil {
		log.Fatalf("%s", err)
	}

	caKeyPem := pem.EncodeToMemory(&pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(caPrivKey),
	})

	caCertPem := pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: caBytes,
	})

	err = os.WriteFile("ca.crt", caCertPem, 0600)
	if err != nil {
		log.Fatalf("ca cert err %s", err)
	}
	err = os.WriteFile("ca.key", caKeyPem, 0600)
	if err != nil {
		log.Fatalf("ca pem err %s", err)
	}

	//? Create cert for server side
	serverCert := &x509.Certificate{
		SerialNumber: big.NewInt(2023),
		Subject: pkix.Name{
			Organization:  []string{"Company, INC."},
			Country:       []string{"TR"},
			Province:      []string{"Mugla"},
			Locality:      []string{"Marmaris"},
			StreetAddress: []string{"Barbaros"},
			PostalCode:    []string{"48700"},
			CommonName:    "openvpn-server-cert",
		},
		NotBefore:    time.Now().UTC(),
		NotAfter:     time.Now().AddDate(10, 0, 0).UTC(),
		SubjectKeyId: []byte{1, 2, 3, 4, 6},
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth},
		KeyUsage:     x509.KeyUsageDigitalSignature,
	}

	serverCertPrivKey, err := rsa.GenerateKey(rand.Reader, 4096)
	if err != nil {
		log.Fatalf("%s", err)
	}

	serverCertBytes, err := x509.CreateCertificate(rand.Reader, serverCert, ca, &serverCertPrivKey.PublicKey, caPrivKey)
	if err != nil {
		log.Fatalf("%s", err)
	}

	serverKeyPem := pem.EncodeToMemory(&pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(serverCertPrivKey),
	})

	serverCertPem := pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: serverCertBytes,
	})

	err = os.WriteFile("server.key", serverKeyPem, 0600)
	if err != nil {
		log.Fatalf("ca err %s", err)
	}
	err = os.WriteFile("server.crt", serverCertPem, 0600)
	if err != nil {
		log.Fatalf("ca err %s", err)
	}

	//? Create cert for client side
	clientCert := &x509.Certificate{
		SerialNumber: big.NewInt(2023),
		Subject: pkix.Name{
			Organization:  []string{"Company, INC."},
			Country:       []string{"TR"},
			Province:      []string{"Istanbul"},
			Locality:      []string{"Besiktas"},
			StreetAddress: []string{"Ciragan cad"},
			PostalCode:    []string{"34349"},
			CommonName:    "ahmet.ozer@localhost",
		},
		NotBefore:    time.Now().UTC(),
		NotAfter:     time.Now().AddDate(10, 0, 0).UTC(),
		SubjectKeyId: []byte{1, 2, 3, 4, 6},
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth},
		KeyUsage:     x509.KeyUsageDigitalSignature,
		Issuer:       ca.Issuer,
	}

	clientCertPrivKey, err := rsa.GenerateKey(rand.Reader, 4096)
	if err != nil {
		log.Fatalf("%s", err)
	}

	clientCertBytes, err := x509.CreateCertificate(rand.Reader, clientCert, ca, &clientCertPrivKey.PublicKey, caPrivKey)
	if err != nil {
		log.Fatalf("%s", err)
	}

	clientKeyPem := pem.EncodeToMemory(&pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(clientCertPrivKey),
	})

	clientCertPem := pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: clientCertBytes,
	})

	err = os.WriteFile("client.key", clientKeyPem, 0600)
	if err != nil {
		log.Fatalf("client key err %s", err)
	}
	err = os.WriteFile("client.crt", clientCertPem, 0600)
	if err != nil {
		log.Fatalf("client cert err %s", err)
	}
}