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cbbf6cd Nov 22, 2015
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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Generate a self-signed X.509 certificate for a TLS server. Outputs to
// 'cert.pem' and 'key.pem' and will overwrite existing files.
package main
import (
var (
emailAddress = flag.String("email-address", "", "The email address of the user you wish to create the 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, P384, P521")
func publicKey(priv interface{}) interface{} {
switch k := priv.(type) {
case *rsa.PrivateKey:
return &k.PublicKey
case *ecdsa.PrivateKey:
return &k.PublicKey
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)
return &pem.Block{Type: "EC PRIVATE KEY", Bytes: b}
return nil
func main() {
if len(*emailAddress) == 0 {
log.Fatalf("Missing required --email-address 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)
fmt.Fprintf(os.Stderr, "Unrecognized elliptic curve: %q", *ecdsaCurve)
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)
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,
template.DNSNames = append(template.DNSNames, "localhost")
template.EmailAddresses = append(template.EmailAddresses, *emailAddress)
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)
pem.Encode(certOut, &pem.Block{Type: "CERTIFICATE", Bytes: derBytes})
log.Print("written 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)
pem.Encode(keyOut, pemBlockForKey(priv))
log.Print("written key.pem\n")