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// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package x509
import (
const ecPrivKeyVersion = 1
// ecPrivateKey reflects an ASN.1 Elliptic Curve Private Key Structure.
// References:
// RFC 5915
// SEC1 -
// Per RFC 5915 the NamedCurveOID is marked as ASN.1 OPTIONAL, however in
// most cases it is not.
type ecPrivateKey struct {
Version int
PrivateKey []byte
NamedCurveOID asn1.ObjectIdentifier `asn1:"optional,explicit,tag:0"`
PublicKey asn1.BitString `asn1:"optional,explicit,tag:1"`
// ParseECPrivateKey parses an ASN.1 Elliptic Curve Private Key Structure.
func ParseECPrivateKey(der []byte) (*ecdsa.PrivateKey, error) {
return parseECPrivateKey(nil, der)
// MarshalECPrivateKey marshals an EC private key into ASN.1, DER format.
func MarshalECPrivateKey(key *ecdsa.PrivateKey) ([]byte, error) {
oid, ok := oidFromNamedCurve(key.Curve)
if !ok {
return nil, errors.New("x509: unknown elliptic curve")
privateKeyBytes := key.D.Bytes()
paddedPrivateKey := make([]byte, (key.Curve.Params().N.BitLen()+7)/8)
copy(paddedPrivateKey[len(paddedPrivateKey)-len(privateKeyBytes):], privateKeyBytes)
return asn1.Marshal(ecPrivateKey{
Version: 1,
PrivateKey: paddedPrivateKey,
NamedCurveOID: oid,
PublicKey: asn1.BitString{Bytes: elliptic.Marshal(key.Curve, key.X, key.Y)},
// parseECPrivateKey parses an ASN.1 Elliptic Curve Private Key Structure.
// The OID for the named curve may be provided from another source (such as
// the PKCS8 container) - if it is provided then use this instead of the OID
// that may exist in the EC private key structure.
func parseECPrivateKey(namedCurveOID *asn1.ObjectIdentifier, der []byte) (key *ecdsa.PrivateKey, err error) {
var privKey ecPrivateKey
if _, err := asn1.Unmarshal(der, &privKey); err != nil {
return nil, errors.New("x509: failed to parse EC private key: " + err.Error())
if privKey.Version != ecPrivKeyVersion {
return nil, fmt.Errorf("x509: unknown EC private key version %d", privKey.Version)
var curve elliptic.Curve
if namedCurveOID != nil {
curve = namedCurveFromOID(*namedCurveOID)
} else {
curve = namedCurveFromOID(privKey.NamedCurveOID)
if curve == nil {
return nil, errors.New("x509: unknown elliptic curve")
k := new(big.Int).SetBytes(privKey.PrivateKey)
curveOrder := curve.Params().N
if k.Cmp(curveOrder) >= 0 {
return nil, errors.New("x509: invalid elliptic curve private key value")
priv := new(ecdsa.PrivateKey)
priv.Curve = curve
priv.D = k
privateKey := make([]byte, (curveOrder.BitLen()+7)/8)
// Some private keys have leading zero padding. This is invalid
// according to [SEC1], but this code will ignore it.
for len(privKey.PrivateKey) > len(privateKey) {
if privKey.PrivateKey[0] != 0 {
return nil, errors.New("x509: invalid private key length")
privKey.PrivateKey = privKey.PrivateKey[1:]
// Some private keys remove all leading zeros, this is also invalid
// according to [SEC1] but since OpenSSL used to do this, we ignore
// this too.
copy(privateKey[len(privateKey)-len(privKey.PrivateKey):], privKey.PrivateKey)
priv.X, priv.Y = curve.ScalarBaseMult(privateKey)
return priv, nil