Go program to add name constraints to a certificate

name_constrain.go

/* 
 * Adds name constraints to a certificate.  Useful if you need to
 * import your organization's private CA into your web browser, but
 * you only want to trust it for your organization's domains and not
 * the Internet at large.
 *
 * The certificate is re-signed by an ephemeral issuer with a random
 * key so you don't need access to the private key.  A random serial number
 * is placed in the Issuer DN so browsers don't attempt to verify the
 * signature when you import the certificate.
 *
 * Note that name-constrained roots only work in some web browsers.
 * So far, the certificates produced by this program have been tested
 * successfully in:
 *
 *  - Firefox 38 ESR on Linux
 *
 * A PEM-encoded certificate is read from stdin and the new PEM-encoded
 * certificate is written to stdout.  The domain names to which the
 * certificate should be constrained are specified as command line arguments.
 *
 * Example:
 *  name_constrain example.com example.org < example-root.crt > example-root-constrained.crt
 *
 * Written in 2015 by Andrew Ayer <agwa@andrewayer.name>
 *
 * To the extent possible under law, the author(s) have dedicated all
 * copyright and related and neighboring rights to this software to the
 * public domain worldwide. This software is distributed without any
 * warranty.
 *
 * You should have received a copy of the CC0 Public
 * Domain Dedication along with this software. If not, see
 * <http://creativecommons.org/publicdomain/zero/1.0/>.
 */

package main

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"fmt"
	"io/ioutil"
	"math/big"
	"os"
)

var maxSerialNumber = new(big.Int).Lsh(big.NewInt(1), 128)

func randomSerialNumber() string {
	n, err := rand.Int(rand.Reader, maxSerialNumber)
	if err != nil {
		panic("Random number generator failed: " + err.Error())
	}
	return n.String()
}

func main() {
	crtPem, err := ioutil.ReadAll(os.Stdin)
	if err != nil {
		fmt.Fprintln(os.Stderr, "Error reading from stdin:", err)
		os.Exit(1)
	}
	pemBlock, _ := pem.Decode(crtPem)
	if pemBlock == nil {
		fmt.Fprintln(os.Stderr, "Invalid PEM read from stdin")
		os.Exit(1)
	}
	crts, err := x509.ParseCertificates(pemBlock.Bytes)
	if err != nil {
		fmt.Fprintln(os.Stderr, "Error parsing certificate:", err)
		os.Exit(1)
	}

	crts[0].PermittedDNSDomainsCritical = true
	crts[0].PermittedDNSDomains = os.Args[1:]

	parent := &x509.Certificate{Subject: crts[0].Issuer}
	parent.Subject.OrganizationalUnit = append(parent.Subject.OrganizationalUnit, "Dummy issuer added by name_constrain.go")
	parent.Subject.SerialNumber = randomSerialNumber()

	key, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		fmt.Println("Error generating RSA key:", err)
		os.Exit(1)
	}

	newCrtDer, err := x509.CreateCertificate(rand.Reader, crts[0], parent, crts[0].PublicKey, key)
	if err != nil {
		fmt.Println("Error creating certificate:", err)
		os.Exit(1)
	}

	err = pem.Encode(os.Stdout, &pem.Block{Type: "CERTIFICATE", Bytes: newCrtDer})
	if err != nil {
		fmt.Println("Error writing certificate to stdout:", err)
		os.Exit(1)
	}
}