genCert.go

package main

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"flag"
	"fmt"
	"log"
	"math/big"
	"net"
	"os"
	"strings"
	"time"
)

var (
	host = flag.String("host", "", "Comma-separated hostnames and IPs to generate a certificate for")

	validFrom = flag.String("start-date", "", "Creation date formatted as Jan 1 15:04:05 2011")

	validFor = flag.Duration("duration", 365*24*time.Hour, "Duration that certificate is valid for")

	isCA = flag.Bool("ca", false, "whether this cert should be its own Certificate Authority")

	rsaBits = flag.Int("rsa-bits", 2048, "Size of RSA key to generate. Ignored if --ecdsa-curve is set")

	ecdsaCurve = flag.String("ecdsa-curve", "", "ECDSA curve to use to generate a key. Valid values are P224, P256 (recommended), P384, P521")
)

func publicKey(priv interface{}) interface{} {

	switch k := priv.(type) {

	case *rsa.PrivateKey:

		return &k.PublicKey

	case *ecdsa.PrivateKey:

		return &k.PublicKey

	default:

		return nil

	}

}

func pemBlockForKey(priv interface{}) *pem.Block {

	switch k := priv.(type) {

	case *rsa.PrivateKey:

		return &pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(k)}

	case *ecdsa.PrivateKey:

		b, err := x509.MarshalECPrivateKey(k)

		if err != nil {

			fmt.Fprintf(os.Stderr, "Unable to marshal ECDSA private key: %v", err)

			os.Exit(2)

		}

		return &pem.Block{Type: "EC PRIVATE KEY", Bytes: b}

	default:

		return nil

	}

}

func main() {

	flag.Parse()

	if len(*host) == 0 {

		log.Fatalf("Missing required --host parameter")

	}

	var priv interface{}

	var err error

	switch *ecdsaCurve {

	case "":

		priv, err = rsa.GenerateKey(rand.Reader, *rsaBits)

	case "P224":

		priv, err = ecdsa.GenerateKey(elliptic.P224(), rand.Reader)

	case "P256":

		priv, err = ecdsa.GenerateKey(elliptic.P256(), rand.Reader)

	case "P384":

		priv, err = ecdsa.GenerateKey(elliptic.P384(), rand.Reader)

	case "P521":

		priv, err = ecdsa.GenerateKey(elliptic.P521(), rand.Reader)

	default:

		fmt.Fprintf(os.Stderr, "Unrecognized elliptic curve: %q", *ecdsaCurve)

		os.Exit(1)

	}

	if err != nil {

		log.Fatalf("failed to generate private key: %s", err)

	}

	var notBefore time.Time

	if len(*validFrom) == 0 {

		notBefore = time.Now()

	} else {

		notBefore, err = time.Parse("Jan 2 15:04:05 2006", *validFrom)

		if err != nil {

			fmt.Fprintf(os.Stderr, "Failed to parse creation date: %s\n", err)

			os.Exit(1)

		}

	}

	notAfter := notBefore.Add(*validFor)

	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)

	serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)

	if err != nil {

		log.Fatalf("failed to generate serial number: %s", err)

	}

	template := x509.Certificate{

		SerialNumber: serialNumber,

		Subject: pkix.Name{

			Organization: []string{"Acme Co"},
		},

		NotBefore: notBefore,

		NotAfter: notAfter,

		KeyUsage: x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,

		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},

		BasicConstraintsValid: true,
	}

	hosts := strings.Split(*host, ",")

	for _, h := range hosts {

		if ip := net.ParseIP(h); ip != nil {

			template.IPAddresses = append(template.IPAddresses, ip)

		} else {

			template.DNSNames = append(template.DNSNames, h)

		}

	}

	if *isCA {

		template.IsCA = true

		template.KeyUsage |= x509.KeyUsageCertSign

	}

	derBytes, err := x509.CreateCertificate(rand.Reader, &template, &template, publicKey(priv), priv)

	if err != nil {

		log.Fatalf("Failed to create certificate: %s", err)

	}

	certOut, err := os.Create("cert.pem")

	if err != nil {

		log.Fatalf("failed to open cert.pem for writing: %s", err)

	}

	if err := pem.Encode(certOut, &pem.Block{Type: "CERTIFICATE", Bytes: derBytes}); err != nil {

		log.Fatalf("failed to write data to cert.pem: %s", err)

	}

	if err := certOut.Close(); err != nil {

		log.Fatalf("error closing cert.pem: %s", err)

	}

	log.Print("wrote cert.pem\n")

	keyOut, err := os.OpenFile("key.pem", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)

	if err != nil {

		log.Print("failed to open key.pem for writing:", err)

		return

	}

	if err := pem.Encode(keyOut, pemBlockForKey(priv)); err != nil {

		log.Fatalf("failed to write data to key.pem: %s", err)

	}

	if err := keyOut.Close(); err != nil {

		log.Fatalf("error closing key.pem: %s", err)

	}

	log.Print("wrote key.pem\n")

}