Initial commit of RSA CLI tool, still very much WIP

cmd/keygen/keygen.go

@@ -0,0 +1,72 @@
+package keygen
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+	"src.lwithers.me.uk/go/rsa/pkg/pemfile"
+)
+
+var (
+	bits  int
+	pkcs1 bool
+)
+
+// Register the "keygen" subcommand.
+func Register(root *cobra.Command) {
+	cmd := &cobra.Command{
+		Use:   "keygen",
+		Short: "Generate a new private key and save to file",
+		Run:   Keygen,
+		Args:  cobra.ExactArgs(1),
+	}
+
+	cmd.Flags().IntVarP(&bits, "bits", "b", 3072, "Key size in bits")
+	cmd.Flags().BoolVarP(&pkcs1, "pkcs1", "", false, "Write key as PKCS#1 rather than PKCS#8")
+
+	root.AddCommand(cmd)
+}
+
+// Keygen will generate a new RSA private key and save it to a file.
+func Keygen(cmd *cobra.Command, args []string) {
+	filename := args[0]
+
+	key, err := rsa.GenerateKey(rand.Reader, bits)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to generate new key (%d bits): %v\n", bits, err)
+		os.Exit(1)
+	}
+
+	// prepare DER-form data
+	var der []byte
+	if pkcs1 {
+		der = x509.MarshalPKCS1PrivateKey(key)
+	} else {
+		der, err = x509.MarshalPKCS8PrivateKey(key)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Failed to marshal PKCS#8 private key: %v\n", err)
+			os.Exit(1)
+		}
+	}
+
+	// prepare PEM-form data
+	typ := pemfile.TypePKCS8PrivateKey
+	if pkcs1 {
+		typ = pemfile.TypePKCS1PrivateKey
+	}
+	x509raw := pem.EncodeToMemory(&pem.Block{
+		Type:  typ,
+		Bytes: der,
+	})
+
+	// write to file
+	if err := os.WriteFile(filename, x509raw, 0600); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}