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

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.

Hypoplasia

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.

Telegenic

You can always extract the public key from a private key. If you only want to write the public key you can use x509.MarshalPKIXPublicKey – Cortico

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")
    }
}

Polysyllabic

Slight problem with your verification at the end: the condition should be || not && – Waters

The public key header/footer in ExportRsaPublicKeyAsPemStr needs to be modified. Fix: Type: "PUBLIC KEY", – Antivenin

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.

Cochlea

Answer 4

math/big.Int implements the [encoding/gob.GobEncoder] interface, the [encoding.TextMarshaler] interface and the [encoding/json.Marshaler] interface. I would say you could use these encoders/decoders.

Here is a verification code you may use,

func TestRsaSaveLoad(t *testing.T) {
    pk0, err := rsa.GenerateKey(rand.Reader, 4096)
    if err != nil {
        t.Fatal(err.Error())
    }

    bytes, err := json.Marshal(pk0)
    if err != nil {
        t.Fatal(err.Error())
    }

    pk1 := &rsa.PrivateKey{}
    err = json.Unmarshal(bytes, &pk1)
    if err != nil {
        t.Fatal(err.Error())
    }

    err = pk1.Validate()
    if err != nil {
        t.Fatal(err.Error())
    }

    if !pk0.Equal(pk1) {
        t.Fatal("keys mismatch")
    }
}

Gertrudis