How to write *PrivateKey type variable to a file in golang?

Question

I have generated a RSA private key (using crypto/rsa) and corresponding public key in go. Now I want to write them to two files (each one to a file), but WriteString and Write functions are specific to string and []byte variable. Also instructions like String(privateKey) generate error. How I can write these keys to the files?

Answer 1

You would need pem.Encode. An example can be seen in maplepie/rsa#savePrivateKey():

func (p *PemKey) savePrivateKey(privateKey *rsa.PrivateKey, filename string) error {
    raw := x509.MarshalPKCS1PrivateKey(privateKey)
    block := &pem.Block{
        Type:  "RSA PRIVATE KEY",
        Bytes: raw,
    }
    file, err := os.Create(filename)
    if err != nil {
        return err
    }
    err = pem.Encode(file, block)
    if err != nil {
        return err
    }
    return nil
}

Same idea for public key:

func (p *PemKey) savePublicKey(publicKey *rsa.PublicKey, filename string) error {
    raw, err := x509.MarshalPKIXPublicKey(publicKey)
    if err != nil {
        return err
    }
    block := &pem.Block{
        Type:  "PUBLIC KEY",
        Bytes: raw,
    }
    file, err := os.Create(filename)
    if err != nil {
        return err
    }
    err = pem.Encode(file, block)
    if err != nil {
        return err
    }
    return nil
}