Go で RSA キー ペアを生成し、別のファイルに書き込む方法は?

import (
    "crypto/rand"
    "crypto/rsa"
    "crypto/x509"
    "encoding/pem"
    "io/ioutil"
)

filename := "key"
bitSize := 4096

// Generate RSA keypair
key, err := rsa.GenerateKey(rand.Reader, bitSize)

pub := key.Public()

// Encode private key to PKCS#1 ASN.1 PEM
keyPEM := pem.EncodeToMemory(
    &pem.Block{
        Type:  "RSA PRIVATE KEY",
        Bytes: x509.MarshalPKCS1PrivateKey(key),
    },
)

// Encode public key to PKCS#1 ASN.1 PEM
pubPEM := pem.EncodeToMemory(
    &pem.Block{
        Type:  "RSA PUBLIC KEY",
        Bytes: x509.MarshalPKCS1PublicKey(pub.(*rsa.PublicKey)),
    },
)

// Write private key to file
if err := ioutil.WriteFile(filename+".rsa", keyPEM, 0700); err != nil {
    panic(err)
}

// Write public key to file
if err := ioutil.WriteFile(filename+".rsa.pub", pubPEM, 0755); err != nil {
    panic(err)
}