cmd/dkim-keygen: new utility

cmd/dkim-keygen/main.go

@@ -0,0 +1,90 @@
+package main
+
+import (
+	"crypto"
+	"crypto/ed25519"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/pem"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+)
+
+var (
+	keyType  string
+	nBits    int
+	filename string
+)
+
+func init() {
+	flag.StringVar(&keyType, "t", "rsa", "key type (rsa, ed25519)")
+	flag.IntVar(&nBits, "b", 3072, "number of bits in the key (only for RSA)")
+	flag.StringVar(&filename, "f", "dkim.priv", "private key filename")
+	flag.Parse()
+}
+
+func main() {
+	var (
+		privKey crypto.Signer
+		err     error
+	)
+	switch keyType {
+	case "rsa":
+		log.Printf("Generating a %v-bit RSA key", nBits)
+		privKey, err = rsa.GenerateKey(rand.Reader, nBits)
+	case "ed25519":
+		log.Printf("Generating an Ed25519 key")
+		_, privKey, err = ed25519.GenerateKey(rand.Reader)
+	default:
+		log.Fatalf("Unsupported key type %q", keyType)
+	}
+	if err != nil {
+		log.Fatalf("Failed to generate key: %v", err)
+	}
+
+	privBytes, err := x509.MarshalPKCS8PrivateKey(privKey)
+	if err != nil {
+		log.Fatalf("Failed to marshal private key: %v", err)
+	}
+
+	f, err := os.Create(filename)
+	if err != nil {
+		log.Fatalf("Failed to create key file: %v", err)
+	}
+	defer f.Close()
+
+	privBlock := pem.Block{
+		Type:  "PRIVATE KEY",
+		Bytes: privBytes,
+	}
+	if err := pem.Encode(f, &privBlock); err != nil {
+		log.Fatalf("Failed to write key PEM block: %v", err)
+	}
+	if err := f.Close(); err != nil {
+		log.Fatalf("Failed to close key file: %v", err)
+	}
+	log.Printf("Private key written to %q", filename)
+
+	var pubBytes []byte
+	switch pubKey := privKey.Public().(type) {
+	case *rsa.PublicKey:
+		pubBytes = x509.MarshalPKCS1PublicKey(pubKey)
+	case ed25519.PublicKey:
+		pubBytes = pubKey
+	default:
+		panic("unreachable")
+	}
+
+	params := []string{
+		"v=DKIM1",
+		"k=" + keyType,
+		"p=" + base64.StdEncoding.EncodeToString(pubBytes),
+	}
+	log.Println("Public key, to be stored in the TXT record \"<selector>._domainkey\":")
+	fmt.Println(strings.Join(params, "; "))
+}