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// Copyright 2017 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 ecdh implements ECDH encryption, suitable for OpenPGP,
// as specified in RFC 6637, section 8.
package ecdh
import (
"bytes"
"errors"
"io"
"github.com/ProtonMail/go-crypto/openpgp/aes/keywrap"
"github.com/ProtonMail/go-crypto/openpgp/internal/algorithm"
"github.com/ProtonMail/go-crypto/openpgp/internal/ecc"
)
type KDF struct {
Hash algorithm.Hash
Cipher algorithm.Cipher
}
type PublicKey struct {
curve ecc.ECDHCurve
Point []byte
KDF
}
type PrivateKey struct {
PublicKey
D []byte
}
func NewPublicKey(curve ecc.ECDHCurve, kdfHash algorithm.Hash, kdfCipher algorithm.Cipher) *PublicKey {
return &PublicKey{
curve: curve,
KDF: KDF{
Hash: kdfHash,
Cipher: kdfCipher,
},
}
}
func NewPrivateKey(key PublicKey) *PrivateKey {
return &PrivateKey{
PublicKey: key,
}
}
func (pk *PublicKey) GetCurve() ecc.ECDHCurve {
return pk.curve
}
func (pk *PublicKey) MarshalPoint() []byte {
return pk.curve.MarshalBytePoint(pk.Point)
}
func (pk *PublicKey) UnmarshalPoint(p []byte) error {
pk.Point = pk.curve.UnmarshalBytePoint(p)
if pk.Point == nil {
return errors.New("ecdh: failed to parse EC point")
}
return nil
}
func (sk *PrivateKey) MarshalByteSecret() []byte {
return sk.curve.MarshalByteSecret(sk.D)
}
func (sk *PrivateKey) UnmarshalByteSecret(d []byte) error {
sk.D = sk.curve.UnmarshalByteSecret(d)
if sk.D == nil {
return errors.New("ecdh: failed to parse scalar")
}
return nil
}
func GenerateKey(rand io.Reader, c ecc.ECDHCurve, kdf KDF) (priv *PrivateKey, err error) {
priv = new(PrivateKey)
priv.PublicKey.curve = c
priv.PublicKey.KDF = kdf
priv.PublicKey.Point, priv.D, err = c.GenerateECDH(rand)
return
}
func Encrypt(random io.Reader, pub *PublicKey, msg, curveOID, fingerprint []byte) (vsG, c []byte, err error) {
if len(msg) > 40 {
return nil, nil, errors.New("ecdh: message too long")
}
// the sender MAY use 21, 13, and 5 bytes of padding for AES-128,
// AES-192, and AES-256, respectively, to provide the same number of
// octets, 40 total, as an input to the key wrapping method.
padding := make([]byte, 40-len(msg))
for i := range padding {
padding[i] = byte(40 - len(msg))
}
m := append(msg, padding...)
ephemeral, zb, err := pub.curve.Encaps(random, pub.Point)
if err != nil {
return nil, nil, err
}
vsG = pub.curve.MarshalBytePoint(ephemeral)
z, err := buildKey(pub, zb, curveOID, fingerprint, false, false)
if err != nil {
return nil, nil, err
}
if c, err = keywrap.Wrap(z, m); err != nil {
return nil, nil, err
}
return vsG, c, nil
}
func Decrypt(priv *PrivateKey, vsG, c, curveOID, fingerprint []byte) (msg []byte, err error) {
var m []byte
zb, err := priv.PublicKey.curve.Decaps(priv.curve.UnmarshalBytePoint(vsG), priv.D)
// Try buildKey three times to workaround an old bug, see comments in buildKey.
for i := 0; i < 3; i++ {
var z []byte
// RFC6637 §8: "Compute Z = KDF( S, Z_len, Param );"
z, err = buildKey(&priv.PublicKey, zb, curveOID, fingerprint, i == 1, i == 2)
if err != nil {
return nil, err
}
// RFC6637 §8: "Compute C = AESKeyWrap( Z, c ) as per [RFC3394]"
m, err = keywrap.Unwrap(z, c)
if err == nil {
break
}
}
// Only return an error after we've tried all (required) variants of buildKey.
if err != nil {
return nil, err
}
// RFC6637 §8: "m = symm_alg_ID || session key || checksum || pkcs5_padding"
// The last byte should be the length of the padding, as per PKCS5; strip it off.
return m[:len(m)-int(m[len(m)-1])], nil
}
func buildKey(pub *PublicKey, zb []byte, curveOID, fingerprint []byte, stripLeading, stripTrailing bool) ([]byte, error) {
// Param = curve_OID_len || curve_OID || public_key_alg_ID || 03
// || 01 || KDF_hash_ID || KEK_alg_ID for AESKeyWrap
// || "Anonymous Sender " || recipient_fingerprint;
param := new(bytes.Buffer)
if _, err := param.Write(curveOID); err != nil {
return nil, err
}
algKDF := []byte{18, 3, 1, pub.KDF.Hash.Id(), pub.KDF.Cipher.Id()}
if _, err := param.Write(algKDF); err != nil {
return nil, err
}
if _, err := param.Write([]byte("Anonymous Sender ")); err != nil {
return nil, err
}
if _, err := param.Write(fingerprint[:]); err != nil {
return nil, err
}
// MB = Hash ( 00 || 00 || 00 || 01 || ZB || Param );
h := pub.KDF.Hash.New()
if _, err := h.Write([]byte{0x0, 0x0, 0x0, 0x1}); err != nil {
return nil, err
}
zbLen := len(zb)
i := 0
j := zbLen - 1
if stripLeading {
// Work around old go crypto bug where the leading zeros are missing.
for i < zbLen && zb[i] == 0 {
i++
}
}
if stripTrailing {
// Work around old OpenPGP.js bug where insignificant trailing zeros in
// this little-endian number are missing.
// (See https://github.com/openpgpjs/openpgpjs/pull/853.)
for j >= 0 && zb[j] == 0 {
j--
}
}
if _, err := h.Write(zb[i : j+1]); err != nil {
return nil, err
}
if _, err := h.Write(param.Bytes()); err != nil {
return nil, err
}
mb := h.Sum(nil)
return mb[:pub.KDF.Cipher.KeySize()], nil // return oBits leftmost bits of MB.
}
func Validate(priv *PrivateKey) error {
return priv.curve.ValidateECDH(priv.Point, priv.D)
}
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