func DERToPrivateKey(der []byte) (key interface{}, err error) {
	if key, err = x509.ParsePKCS1PrivateKey(der); err == nil {
		return key, nil
	}
	if key, err = x509.ParsePKCS8PrivateKey(der); err == nil {
		switch key.(type) {
		case *rsa.PrivateKey, *ecdsa.PrivateKey:
			return
		default:
			return nil, errors.New("Found unknown private key type in PKCS#8 wrapping")
		}
	}
	if key, err = x509.ParseECPrivateKey(der); err == nil {
		return
	}
	return nil, errors.New("Invalid key type. The DER must contain an rsa.PrivareKey or ecdsa.PrivateKey")
}
DERToPrivateKey unmarshals a der to private key
package utils
import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/asn1"
	"encoding/pem"
	"errors"
	"fmt"
)
type pkcs8Info struct {
	Version             int
	PrivateKeyAlgorithm []asn1.ObjectIdentifier
	PrivateKey          []byte
}
type ecPrivateKey struct {
	Version       int
	PrivateKey    []byte
	NamedCurveOID asn1.ObjectIdentifier `asn1:"optional,explicit,tag:0"`
	PublicKey     asn1.BitString        `asn1:"optional,explicit,tag:1"`
}
var (
	oidNamedCurveP224 = asn1.ObjectIdentifier{1, 3, 132, 0, 33}
	oidNamedCurveP256 = asn1.ObjectIdentifier{1, 2, 840, 10045, 3, 1, 7}
	oidNamedCurveP384 = asn1.ObjectIdentifier{1, 3, 132, 0, 34}
	oidNamedCurveP521 = asn1.ObjectIdentifier{1, 3, 132, 0, 35}
)
var oidPublicKeyECDSA = asn1.ObjectIdentifier{1, 2, 840, 10045, 2, 1}
func oidFromNamedCurve(curve elliptic.Curve) (asn1.ObjectIdentifier, bool) {
	switch curve {
	case elliptic.P224():
		return oidNamedCurveP224, true
	case elliptic.P256():
		return oidNamedCurveP256, true
	case elliptic.P384():
		return oidNamedCurveP384, true
	case elliptic.P521():
		return oidNamedCurveP521, true
	}
	return nil, false
}
func PrivateKeyToDER(privateKey *ecdsa.PrivateKey) ([]byte, error) {
	if privateKey == nil {
		return nil, errors.New("Invalid ecdsa private key. It must be different from nil.")
	}
	return x509.MarshalECPrivateKey(privateKey)
}
func PrivateKeyToPEM(privateKey interface{}, pwd []byte) ([]byte, error) {
	if len(pwd) != 0 {
		return PrivateKeyToEncryptedPEM(privateKey, pwd)
	}
	if privateKey == nil {
		return nil, errors.New("Invalid key. It must be different from nil.")
	}
	switch k := privateKey.(type) {
	case *ecdsa.PrivateKey:
		if k == nil {
			return nil, errors.New("Invalid ecdsa private key. It must be different from nil.")
		}
		oidNamedCurve, ok := oidFromNamedCurve(k.Curve)
		if !ok {
			return nil, errors.New("unknown elliptic curve")
		}
		privateKeyBytes := k.D.Bytes()
		paddedPrivateKey := make([]byte, (k.Curve.Params().N.BitLen()+7)/8)
		copy(paddedPrivateKey[len(paddedPrivateKey)-len(privateKeyBytes):], privateKeyBytes)
		asn1Bytes, err := asn1.Marshal(ecPrivateKey{
			Version:    1,
			PrivateKey: paddedPrivateKey,
			PublicKey:  asn1.BitString{Bytes: elliptic.Marshal(k.Curve, k.X, k.Y)},
		})
		if err != nil {
			return nil, fmt.Errorf("error marshaling EC key to asn1 [%s]", err)
		}
		var pkcs8Key pkcs8Info
		pkcs8Key.Version = 0
		pkcs8Key.PrivateKeyAlgorithm = make([]asn1.ObjectIdentifier, 2)
		pkcs8Key.PrivateKeyAlgorithm[0] = oidPublicKeyECDSA
		pkcs8Key.PrivateKeyAlgorithm[1] = oidNamedCurve
		pkcs8Key.PrivateKey = asn1Bytes
		pkcs8Bytes, err := asn1.Marshal(pkcs8Key)
		if err != nil {
			return nil, fmt.Errorf("error marshaling EC key to asn1 [%s]", err)
		}
		return pem.EncodeToMemory(
			&pem.Block{
				Type:  "PRIVATE KEY",
				Bytes: pkcs8Bytes,
			},
		), nil
	case *rsa.PrivateKey:
		if k == nil {
			return nil, errors.New("Invalid rsa private key. It must be different from nil.")
		}
		raw := x509.MarshalPKCS1PrivateKey(k)
		return pem.EncodeToMemory(
			&pem.Block{
				Type:  "RSA PRIVATE KEY",
				Bytes: raw,
			},
		), nil
	default:
		return nil, errors.New("Invalid key type. It must be *ecdsa.PrivateKey or *rsa.PrivateKey")
	}
}
func PrivateKeyToEncryptedPEM(privateKey interface{}, pwd []byte) ([]byte, error) {
	if privateKey == nil {
		return nil, errors.New("Invalid private key. It must be different from nil.")
	}
	switch k := privateKey.(type) {
	case *ecdsa.PrivateKey:
		if k == nil {
			return nil, errors.New("Invalid ecdsa private key. It must be different from nil.")
		}
		raw, err := x509.MarshalECPrivateKey(k)
		if err != nil {
			return nil, err
		}
		block, err := x509.EncryptPEMBlock(
			rand.Reader,
			"PRIVATE KEY",
			raw,
			pwd,
			x509.PEMCipherAES256)
		if err != nil {
			return nil, err
		}
		return pem.EncodeToMemory(block), nil
	default:
		return nil, errors.New("Invalid key type. It must be *ecdsa.PrivateKey")
	}
}