Generate DER encoding from known ECDSA private key in Golang

der.go

package main

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/x509"
	"flag"
	"fmt"
	"io/ioutil"
	"math/big"
	"path/filepath"
)

func main() {
	seed := flag.String("s", "", "seed to generate private key")

	flag.Parse()

	var priv *ecdsa.PrivateKey
	if *seed != "" {
		k := new(big.Int)
		k.SetString(*seed, 16)

		priv = new(ecdsa.PrivateKey)
		curve := elliptic.P256()
		priv.PublicKey.Curve = curve
		priv.D = k
		priv.PublicKey.X, priv.PublicKey.Y = curve.ScalarBaseMult(k.Bytes())
	} else {
		priv, _ = ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	}

	fmt.Printf("%#v\n", priv)
	pubkey_bytes, _ := x509.MarshalPKIXPublicKey(&priv.PublicKey)
	ioutil.WriteFile(
		filepath.Join("./", "go.der"),
		pubkey_bytes,
		0644,
	)
}