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package main
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import (
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"crypto/ecdsa"
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"crypto/ed25519"
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"crypto/elliptic"
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"crypto/rand"
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"crypto/rsa"
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"crypto/x509"
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"crypto/x509/pkix"
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"encoding/pem"
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"flag"
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"log"
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"math/big"
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"net"
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"os"
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"strings"
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"time"
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)
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var (
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host = flag.String("host", "", "Comma-separated hostnames and IPs to generate a certificate for")
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validFrom = flag.String("start-date", "", "Creation date formatted as Jan 1 15:04:05 2011")
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validFor = flag.Duration("duration", 365*24*time.Hour, "Duration that certificate is valid for")
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isCA = flag.Bool("ca", false, "whether this cert should be its own Certificate Authority")
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rsaBits = flag.Int("rsa-bits", 2048, "Size of RSA key to generate. Ignored if --ecdsa-curve is set")
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ecdsaCurve = flag.String("ecdsa-curve", "", "ECDSA curve to use to generate a key. Valid values are P224, P256 (recommended), P384, P521")
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ed25519Key = flag.Bool("ed25519", false, "Generate an Ed25519 key")
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)
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func publicKey(priv any) any {
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switch k := priv.(type) {
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case *rsa.PrivateKey:
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return &k.PublicKey
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case *ecdsa.PrivateKey:
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return &k.PublicKey
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case ed25519.PrivateKey:
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return k.Public().(ed25519.PublicKey)
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default:
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return nil
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}
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}
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func main() {
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flag.Parse()
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if len(*host) == 0 {
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log.Fatalf("Missing required --host parameter")
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}
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var priv any
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var err error
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switch *ecdsaCurve {
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case "":
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if *ed25519Key {
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_, priv, err = ed25519.GenerateKey(rand.Reader)
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} else {
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priv, err = rsa.GenerateKey(rand.Reader, *rsaBits)
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}
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case "P224":
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priv, err = ecdsa.GenerateKey(elliptic.P224(), rand.Reader)
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case "P256":
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priv, err = ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
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case "P384":
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priv, err = ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
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case "P521":
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priv, err = ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
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default:
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log.Fatalf("Unrecognized elliptic curve: %q", *ecdsaCurve)
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}
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if err != nil {
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log.Fatalf("Failed to generate private key: %v", err)
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}
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keyUsage := x509.KeyUsageDigitalSignature
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if _, isRSA := priv.(*rsa.PrivateKey); isRSA {
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keyUsage |= x509.KeyUsageKeyEncipherment
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}
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var notBefore time.Time
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if len(*validFrom) == 0 {
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notBefore = time.Now()
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} else {
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notBefore, err = time.Parse("Jan 2 15:04:05 2006", *validFrom)
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if err != nil {
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log.Fatalf("Failed to parse creation date: %v", err)
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}
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}
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notAfter := notBefore.Add(*validFor)
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serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
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serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)
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if err != nil {
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log.Fatalf("Failed to generate serial number: %v", err)
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}
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template := x509.Certificate{
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SerialNumber: serialNumber,
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Subject: pkix.Name{
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Organization: []string{"Acme Co"},
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},
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NotBefore: notBefore,
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NotAfter: notAfter,
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KeyUsage: keyUsage,
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ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
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BasicConstraintsValid: true,
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}
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hosts := strings.Split(*host, ",")
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for _, h := range hosts {
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if ip := net.ParseIP(h); ip != nil {
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template.IPAddresses = append(template.IPAddresses, ip)
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} else {
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template.DNSNames = append(template.DNSNames, h)
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}
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}
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if *isCA {
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template.IsCA = true
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template.KeyUsage |= x509.KeyUsageCertSign
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}
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derBytes, err := x509.CreateCertificate(rand.Reader, &template, &template, publicKey(priv), priv)
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if err != nil {
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log.Fatalf("Failed to create certificate: %v", err)
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}
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certOut, err := os.Create("cert.pem")
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if err != nil {
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log.Fatalf("Failed to open cert.pem for writing: %v", err)
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}
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if err := pem.Encode(certOut, &pem.Block{Type: "CERTIFICATE", Bytes: derBytes}); err != nil {
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log.Fatalf("Failed to write data to cert.pem: %v", err)
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}
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if err := certOut.Close(); err != nil {
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log.Fatalf("Error closing cert.pem: %v", err)
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}
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log.Print("wrote cert.pem\n")
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keyOut, err := os.OpenFile("key.pem", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
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if err != nil {
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log.Fatalf("Failed to open key.pem for writing: %v", err)
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}
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privBytes, err := x509.MarshalPKCS8PrivateKey(priv)
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if err != nil {
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log.Fatalf("Unable to marshal private key: %v", err)
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}
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if err := pem.Encode(keyOut, &pem.Block{Type: "PRIVATE KEY", Bytes: privBytes}); err != nil {
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log.Fatalf("Failed to write data to key.pem: %v", err)
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}
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if err := keyOut.Close(); err != nil {
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log.Fatalf("Error closing key.pem: %v", err)
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}
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log.Print("wrote key.pem\n")
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}
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