Generate RSA key pair in PEM format

genrsa.go

// Generate RSA key pair in PEM format.
package main

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"flag"
	"fmt"
	"os"
)

var (
	flagSize     = flag.Int("size", 2048, "bit size")
	flagPrivFile = flag.String("priv", "", "save private key to a file")
	flagPubFile  = flag.String("pub", "", "save public key to a file")
)

func main() {
	flag.Parse()

	priv, err := rsa.GenerateKey(rand.Reader, *flagSize)
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	err = priv.Validate()
	if err != nil {
		fmt.Println("Validation failed.", err)
		os.Exit(1)
	}

	privPem := pem.EncodeToMemory(&pem.Block{
		Type:    "RSA PRIVATE KEY",
		Headers: nil,
		Bytes:   x509.MarshalPKCS1PrivateKey(priv),
	})

	pubDer, err := x509.MarshalPKIXPublicKey(&priv.PublicKey)
	if err != nil {
		fmt.Println("Failed to get der format for PublicKey.", err)
		os.Exit(1)
	}
	pubPem := pem.EncodeToMemory(&pem.Block{
		Type:    "PUBLIC KEY",
		Headers: nil,
		Bytes:   pubDer,
	})

	if *flagPrivFile != "" {
		pv, err := os.OpenFile(*flagPrivFile, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
		if err != nil {
			fmt.Println(err)
			os.Exit(1)
		}
		defer pv.Close()
		pv.Write(privPem)
	} else {
		fmt.Printf("%s\n", privPem)
	}

	if *flagPubFile != "" {
		pb, err := os.OpenFile(*flagPubFile, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0664)
		if err != nil {
			fmt.Println(err)
			os.Exit(1)
		}
		defer pb.Close()
		pb.Write(pubPem)
	} else {
		fmt.Printf("%s\n", pubPem)
	}
}