Save and load crypto/rsa PrivateKey to and from the disk - go

Question

I'm using crypto/rsa, and trying to find a way to properly save and load a key. Is there a correct way to create a []byte from an rsa.PrivateKey. If so, is there a way to properly do so for an rsa.PublicKey?
Thank you all very much.

Answer 1

You need some sort of format to marshal the key into. One format supported by the Go standard library can be found here: http://golang.org/pkg/crypto/x509/#MarshalPKCS1PrivateKey
func MarshalPKCS1PrivateKey(key *rsa.PrivateKey) []byte
The inverse function is http://golang.org/pkg/crypto/x509/#ParsePKCS1PrivateKey.
func ParsePKCS1PrivateKey(der []byte) (key *rsa.PrivateKey, err error)
However, it is relatively standard to encode the marshaled key into a PEM file.
pemdata := pem.EncodeToMemory(
&pem.Block{
Type: "RSA PRIVATE KEY",
Bytes: x509.MarshalPKCS1PrivateKey(key),
},
)
You can find a full example here.

Answer 2

Here's code snippet that shows the import and export of both public and private keys. It's based on the other answers which were super helpful, as well as copy-pasta from the official docs.
package main
import (
"crypto/rand"
"crypto/rsa"
"crypto/x509"
"encoding/pem"
"errors"
"fmt"
)
func GenerateRsaKeyPair() (*rsa.PrivateKey, *rsa.PublicKey) {
privkey, _ := rsa.GenerateKey(rand.Reader, 4096)
return privkey, &privkey.PublicKey
}
func ExportRsaPrivateKeyAsPemStr(privkey *rsa.PrivateKey) string {
privkey_bytes := x509.MarshalPKCS1PrivateKey(privkey)
privkey_pem := pem.EncodeToMemory(
&pem.Block{
Type: "RSA PRIVATE KEY",
Bytes: privkey_bytes,
},
)
return string(privkey_pem)
}
func ParseRsaPrivateKeyFromPemStr(privPEM string) (*rsa.PrivateKey, error) {
block, _ := pem.Decode([]byte(privPEM))
if block == nil {
return nil, errors.New("failed to parse PEM block containing the key")
}
priv, err := x509.ParsePKCS1PrivateKey(block.Bytes)
if err != nil {
return nil, err
}
return priv, nil
}
func ExportRsaPublicKeyAsPemStr(pubkey *rsa.PublicKey) (string, error) {
pubkey_bytes, err := x509.MarshalPKIXPublicKey(pubkey)
if err != nil {
return "", err
}
pubkey_pem := pem.EncodeToMemory(
&pem.Block{
Type: "RSA PUBLIC KEY",
Bytes: pubkey_bytes,
},
)
return string(pubkey_pem), nil
}
func ParseRsaPublicKeyFromPemStr(pubPEM string) (*rsa.PublicKey, error) {
block, _ := pem.Decode([]byte(pubPEM))
if block == nil {
return nil, errors.New("failed to parse PEM block containing the key")
}
pub, err := x509.ParsePKIXPublicKey(block.Bytes)
if err != nil {
return nil, err
}
switch pub := pub.(type) {
case *rsa.PublicKey:
return pub, nil
default:
break // fall through
}
return nil, errors.New("Key type is not RSA")
}
func main() {
// Create the keys
priv, pub := GenerateRsaKeyPair()
// Export the keys to pem string
priv_pem := ExportRsaPrivateKeyAsPemStr(priv)
pub_pem, _ := ExportRsaPublicKeyAsPemStr(pub)
// Import the keys from pem string
priv_parsed, _ := ParseRsaPrivateKeyFromPemStr(priv_pem)
pub_parsed, _ := ParseRsaPublicKeyFromPemStr(pub_pem)
// Export the newly imported keys
priv_parsed_pem := ExportRsaPrivateKeyAsPemStr(priv_parsed)
pub_parsed_pem, _ := ExportRsaPublicKeyAsPemStr(pub_parsed)
fmt.Println(priv_parsed_pem)
fmt.Println(pub_parsed_pem)
// Check that the exported/imported keys match the original keys
if priv_pem != priv_parsed_pem || pub_pem != pub_parsed_pem {
fmt.Println("Failure: Export and Import did not result in same Keys")
} else {
fmt.Println("Success")
}
}

Answer 3

Since the public key part of your question wasn't answered and I just ran into the same problem and solved it, here it is:
Note the & in front of the Argument to MarshalPKIXPublicKey
Priv := rsa.GenerateKey(rand.Reader, 4096)
pubASN1, err := x509.MarshalPKIXPublicKey(&Priv.PublicKey)
if err != nil {
// do something about it
}
pubBytes := pem.EncodeToMemory(&pem.Block{
Type: "RSA PUBLIC KEY",
Bytes: pubASN1,
})
ioutil.WriteFile("key.pub", pubBytes, 0644)
Relevant reads:
MarshalPKIXPublicKey(pub interface{}) ([]byte, error) godoc
EncodeToMemory(b *Block) []byte godoc
Block godoc
PS: MarshalPKIXPublicKey also accepts ECDSA keys, ajust the pem header appropriately.