// 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 Golang LICENSE file.

package main

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"encoding/asn1"
	"errors"
)

// ecPrivateKey reflects an ASN.1 Elliptic Curve Private Key Structure.
// References:
//   RFC5915
//   SEC1 - http://www.secg.org/download/aid-780/sec1-v2.pdf
// Per RFC5915 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"`
}

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}
)

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 MarshalECPrivateKey(key *ecdsa.PrivateKey) ([]byte, error) {
	oid, ok := oidFromNamedCurve(key.Curve)
	if !ok {
		return nil, errors.New("x509: unknown elliptic curve")
	}
	return asn1.Marshal(ecPrivateKey{
		Version:       1,
		PrivateKey:    key.D.Bytes(),
		NamedCurveOID: oid,
		PublicKey:     asn1.BitString{Bytes: elliptic.Marshal(key.Curve, key.X, key.Y)},
	})
}