How to store ECDSA private key in Go

Question

I am using the ecdsa.GenerateKey method to generate a private/public key pair in Go. I would like to store the private key in a file on the users computer, and load it whenever the program starts. There is a method elliptic.Marshal that marshals the public key, but nothing for the private key. Should I simply roll my own, or is there a recommended way to store the private key?

Answer 1

Here is a code sample that demonstrates encoding and decoding of keys in Go. It helps to know that you need to connect couple of steps. Crypto algorithm is the fist step, in this case ECDSA key. Then you need standard encoding, x509 is most commontly used standard. Finally you need a file format, PEM is again commonly used one. This is currently most commonly used combination, but feel free to substitute any other algoriths or encoding.

func encode(privateKey *ecdsa.PrivateKey, publicKey *ecdsa.PublicKey) (string, string) {
    x509Encoded, _ := x509.MarshalECPrivateKey(privateKey)
    pemEncoded := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: x509Encoded})

    x509EncodedPub, _ := x509.MarshalPKIXPublicKey(publicKey)
    pemEncodedPub := pem.EncodeToMemory(&pem.Block{Type: "PUBLIC KEY", Bytes: x509EncodedPub})

    return string(pemEncoded), string(pemEncodedPub)
}

func decode(pemEncoded string, pemEncodedPub string) (*ecdsa.PrivateKey, *ecdsa.PublicKey) {
    block, _ := pem.Decode([]byte(pemEncoded))
    x509Encoded := block.Bytes
    privateKey, _ := x509.ParseECPrivateKey(x509Encoded)

    blockPub, _ := pem.Decode([]byte(pemEncodedPub))
    x509EncodedPub := blockPub.Bytes
    genericPublicKey, _ := x509.ParsePKIXPublicKey(x509EncodedPub)
    publicKey := genericPublicKey.(*ecdsa.PublicKey)

    return privateKey, publicKey
}

func test() {
    privateKey, _ := ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
    publicKey := &privateKey.PublicKey

    encPriv, encPub := encode(privateKey, publicKey)

    fmt.Println(encPriv)
    fmt.Println(encPub)

    priv2, pub2 := decode(encPriv, encPub)

    if !reflect.DeepEqual(privateKey, priv2) {
        fmt.Println("Private keys do not match.")
    }
    if !reflect.DeepEqual(publicKey, pub2) {
        fmt.Println("Public keys do not match.")
    }
}

Answer 2

I believe the standard format for those keys is to use the X.509 ASN.1 DER representation. See http://golang.org/pkg/crypto/x509/#MarshalECPrivateKey and http://golang.org/pkg/crypto/x509/#ParseECPrivateKey.