githaven/vendor/github.com/ProtonMail/go-crypto/openpgp/packet/private_key.go

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// Copyright 2011 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 packet
import (
"bytes"
"crypto"
"crypto/cipher"
"crypto/dsa"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/rand"
"crypto/rsa"
"crypto/sha1"
"fmt"
"io"
"io/ioutil"
"math/big"
"strconv"
"time"
"github.com/ProtonMail/go-crypto/openpgp/internal/ecc"
"golang.org/x/crypto/curve25519"
"github.com/ProtonMail/go-crypto/openpgp/ecdh"
"github.com/ProtonMail/go-crypto/openpgp/elgamal"
"github.com/ProtonMail/go-crypto/openpgp/errors"
"github.com/ProtonMail/go-crypto/openpgp/internal/encoding"
"github.com/ProtonMail/go-crypto/openpgp/s2k"
"golang.org/x/crypto/ed25519"
)
// PrivateKey represents a possibly encrypted private key. See RFC 4880,
// section 5.5.3.
type PrivateKey struct {
PublicKey
Encrypted bool // if true then the private key is unavailable until Decrypt has been called.
encryptedData []byte
cipher CipherFunction
s2k func(out, in []byte)
// An *{rsa|dsa|elgamal|ecdh|ecdsa|ed25519}.PrivateKey or
// crypto.Signer/crypto.Decrypter (Decryptor RSA only).
PrivateKey interface{}
sha1Checksum bool
iv []byte
// Type of encryption of the S2K packet
// Allowed values are 0 (Not encrypted), 254 (SHA1), or
// 255 (2-byte checksum)
s2kType S2KType
// Full parameters of the S2K packet
s2kParams *s2k.Params
}
//S2KType s2k packet type
type S2KType uint8
const (
// S2KNON unencrypt
S2KNON S2KType = 0
// S2KSHA1 sha1 sum check
S2KSHA1 S2KType = 254
// S2KCHECKSUM sum check
S2KCHECKSUM S2KType = 255
)
func NewRSAPrivateKey(creationTime time.Time, priv *rsa.PrivateKey) *PrivateKey {
pk := new(PrivateKey)
pk.PublicKey = *NewRSAPublicKey(creationTime, &priv.PublicKey)
pk.PrivateKey = priv
return pk
}
func NewDSAPrivateKey(creationTime time.Time, priv *dsa.PrivateKey) *PrivateKey {
pk := new(PrivateKey)
pk.PublicKey = *NewDSAPublicKey(creationTime, &priv.PublicKey)
pk.PrivateKey = priv
return pk
}
func NewElGamalPrivateKey(creationTime time.Time, priv *elgamal.PrivateKey) *PrivateKey {
pk := new(PrivateKey)
pk.PublicKey = *NewElGamalPublicKey(creationTime, &priv.PublicKey)
pk.PrivateKey = priv
return pk
}
func NewECDSAPrivateKey(creationTime time.Time, priv *ecdsa.PrivateKey) *PrivateKey {
pk := new(PrivateKey)
pk.PublicKey = *NewECDSAPublicKey(creationTime, &priv.PublicKey)
pk.PrivateKey = priv
return pk
}
func NewEdDSAPrivateKey(creationTime time.Time, priv *ed25519.PrivateKey) *PrivateKey {
pk := new(PrivateKey)
pub := priv.Public().(ed25519.PublicKey)
pk.PublicKey = *NewEdDSAPublicKey(creationTime, &pub)
pk.PrivateKey = priv
return pk
}
func NewECDHPrivateKey(creationTime time.Time, priv *ecdh.PrivateKey) *PrivateKey {
pk := new(PrivateKey)
pk.PublicKey = *NewECDHPublicKey(creationTime, &priv.PublicKey)
pk.PrivateKey = priv
return pk
}
// NewSignerPrivateKey creates a PrivateKey from a crypto.Signer that
// implements RSA, ECDSA or EdDSA.
func NewSignerPrivateKey(creationTime time.Time, signer crypto.Signer) *PrivateKey {
pk := new(PrivateKey)
// In general, the public Keys should be used as pointers. We still
// type-switch on the values, for backwards-compatibility.
switch pubkey := signer.Public().(type) {
case *rsa.PublicKey:
pk.PublicKey = *NewRSAPublicKey(creationTime, pubkey)
case rsa.PublicKey:
pk.PublicKey = *NewRSAPublicKey(creationTime, &pubkey)
case *ecdsa.PublicKey:
pk.PublicKey = *NewECDSAPublicKey(creationTime, pubkey)
case ecdsa.PublicKey:
pk.PublicKey = *NewECDSAPublicKey(creationTime, &pubkey)
case *ed25519.PublicKey:
pk.PublicKey = *NewEdDSAPublicKey(creationTime, pubkey)
case ed25519.PublicKey:
pk.PublicKey = *NewEdDSAPublicKey(creationTime, &pubkey)
default:
panic("openpgp: unknown crypto.Signer type in NewSignerPrivateKey")
}
pk.PrivateKey = signer
return pk
}
// NewDecrypterPrivateKey creates a PrivateKey from a *{rsa|elgamal|ecdh}.PrivateKey.
func NewDecrypterPrivateKey(creationTime time.Time, decrypter interface{}) *PrivateKey {
pk := new(PrivateKey)
switch priv := decrypter.(type) {
case *rsa.PrivateKey:
pk.PublicKey = *NewRSAPublicKey(creationTime, &priv.PublicKey)
case *elgamal.PrivateKey:
pk.PublicKey = *NewElGamalPublicKey(creationTime, &priv.PublicKey)
case *ecdh.PrivateKey:
pk.PublicKey = *NewECDHPublicKey(creationTime, &priv.PublicKey)
default:
panic("openpgp: unknown decrypter type in NewDecrypterPrivateKey")
}
pk.PrivateKey = decrypter
return pk
}
func (pk *PrivateKey) parse(r io.Reader) (err error) {
err = (&pk.PublicKey).parse(r)
if err != nil {
return
}
v5 := pk.PublicKey.Version == 5
var buf [1]byte
_, err = readFull(r, buf[:])
if err != nil {
return
}
pk.s2kType = S2KType(buf[0])
var optCount [1]byte
if v5 {
if _, err = readFull(r, optCount[:]); err != nil {
return
}
}
switch pk.s2kType {
case S2KNON:
pk.s2k = nil
pk.Encrypted = false
case S2KSHA1, S2KCHECKSUM:
if v5 && pk.s2kType == S2KCHECKSUM {
return errors.StructuralError("wrong s2k identifier for version 5")
}
_, err = readFull(r, buf[:])
if err != nil {
return
}
pk.cipher = CipherFunction(buf[0])
pk.s2kParams, err = s2k.ParseIntoParams(r)
if err != nil {
return
}
if pk.s2kParams.Dummy() {
return
}
pk.s2k, err = pk.s2kParams.Function()
if err != nil {
return
}
pk.Encrypted = true
if pk.s2kType == S2KSHA1 {
pk.sha1Checksum = true
}
default:
return errors.UnsupportedError("deprecated s2k function in private key")
}
if pk.Encrypted {
blockSize := pk.cipher.blockSize()
if blockSize == 0 {
return errors.UnsupportedError("unsupported cipher in private key: " + strconv.Itoa(int(pk.cipher)))
}
pk.iv = make([]byte, blockSize)
_, err = readFull(r, pk.iv)
if err != nil {
return
}
}
var privateKeyData []byte
if v5 {
var n [4]byte /* secret material four octet count */
_, err = readFull(r, n[:])
if err != nil {
return
}
count := uint32(uint32(n[0])<<24 | uint32(n[1])<<16 | uint32(n[2])<<8 | uint32(n[3]))
if !pk.Encrypted {
count = count + 2 /* two octet checksum */
}
privateKeyData = make([]byte, count)
_, err = readFull(r, privateKeyData)
if err != nil {
return
}
} else {
privateKeyData, err = ioutil.ReadAll(r)
if err != nil {
return
}
}
if !pk.Encrypted {
return pk.parsePrivateKey(privateKeyData)
}
pk.encryptedData = privateKeyData
return
}
// Dummy returns true if the private key is a dummy key. This is a GNU extension.
func (pk *PrivateKey) Dummy() bool {
return pk.s2kParams.Dummy()
}
func mod64kHash(d []byte) uint16 {
var h uint16
for _, b := range d {
h += uint16(b)
}
return h
}
func (pk *PrivateKey) Serialize(w io.Writer) (err error) {
contents := bytes.NewBuffer(nil)
err = pk.PublicKey.serializeWithoutHeaders(contents)
if err != nil {
return
}
if _, err = contents.Write([]byte{uint8(pk.s2kType)}); err != nil {
return
}
optional := bytes.NewBuffer(nil)
if pk.Encrypted || pk.Dummy() {
optional.Write([]byte{uint8(pk.cipher)})
if err := pk.s2kParams.Serialize(optional); err != nil {
return err
}
if pk.Encrypted {
optional.Write(pk.iv)
}
}
if pk.Version == 5 {
contents.Write([]byte{uint8(optional.Len())})
}
io.Copy(contents, optional)
if !pk.Dummy() {
l := 0
var priv []byte
if !pk.Encrypted {
buf := bytes.NewBuffer(nil)
err = pk.serializePrivateKey(buf)
if err != nil {
return err
}
l = buf.Len()
if pk.sha1Checksum {
h := sha1.New()
h.Write(buf.Bytes())
buf.Write(h.Sum(nil))
} else {
checksum := mod64kHash(buf.Bytes())
buf.Write([]byte{byte(checksum >> 8), byte(checksum)})
}
priv = buf.Bytes()
} else {
priv, l = pk.encryptedData, len(pk.encryptedData)
}
if pk.Version == 5 {
contents.Write([]byte{byte(l >> 24), byte(l >> 16), byte(l >> 8), byte(l)})
}
contents.Write(priv)
}
ptype := packetTypePrivateKey
if pk.IsSubkey {
ptype = packetTypePrivateSubkey
}
err = serializeHeader(w, ptype, contents.Len())
if err != nil {
return
}
_, err = io.Copy(w, contents)
if err != nil {
return
}
return
}
func serializeRSAPrivateKey(w io.Writer, priv *rsa.PrivateKey) error {
if _, err := w.Write(new(encoding.MPI).SetBig(priv.D).EncodedBytes()); err != nil {
return err
}
if _, err := w.Write(new(encoding.MPI).SetBig(priv.Primes[1]).EncodedBytes()); err != nil {
return err
}
if _, err := w.Write(new(encoding.MPI).SetBig(priv.Primes[0]).EncodedBytes()); err != nil {
return err
}
_, err := w.Write(new(encoding.MPI).SetBig(priv.Precomputed.Qinv).EncodedBytes())
return err
}
func serializeDSAPrivateKey(w io.Writer, priv *dsa.PrivateKey) error {
_, err := w.Write(new(encoding.MPI).SetBig(priv.X).EncodedBytes())
return err
}
func serializeElGamalPrivateKey(w io.Writer, priv *elgamal.PrivateKey) error {
_, err := w.Write(new(encoding.MPI).SetBig(priv.X).EncodedBytes())
return err
}
func serializeECDSAPrivateKey(w io.Writer, priv *ecdsa.PrivateKey) error {
_, err := w.Write(new(encoding.MPI).SetBig(priv.D).EncodedBytes())
return err
}
func serializeEdDSAPrivateKey(w io.Writer, priv *ed25519.PrivateKey) error {
keySize := ed25519.PrivateKeySize - ed25519.PublicKeySize
_, err := w.Write(encoding.NewMPI((*priv)[:keySize]).EncodedBytes())
return err
}
func serializeECDHPrivateKey(w io.Writer, priv *ecdh.PrivateKey) error {
_, err := w.Write(encoding.NewMPI(priv.D).EncodedBytes())
return err
}
// Decrypt decrypts an encrypted private key using a passphrase.
func (pk *PrivateKey) Decrypt(passphrase []byte) error {
if pk.Dummy() {
return errors.ErrDummyPrivateKey("dummy key found")
}
if !pk.Encrypted {
return nil
}
key := make([]byte, pk.cipher.KeySize())
pk.s2k(key, passphrase)
block := pk.cipher.new(key)
cfb := cipher.NewCFBDecrypter(block, pk.iv)
data := make([]byte, len(pk.encryptedData))
cfb.XORKeyStream(data, pk.encryptedData)
if pk.sha1Checksum {
if len(data) < sha1.Size {
return errors.StructuralError("truncated private key data")
}
h := sha1.New()
h.Write(data[:len(data)-sha1.Size])
sum := h.Sum(nil)
if !bytes.Equal(sum, data[len(data)-sha1.Size:]) {
return errors.StructuralError("private key checksum failure")
}
data = data[:len(data)-sha1.Size]
} else {
if len(data) < 2 {
return errors.StructuralError("truncated private key data")
}
var sum uint16
for i := 0; i < len(data)-2; i++ {
sum += uint16(data[i])
}
if data[len(data)-2] != uint8(sum>>8) ||
data[len(data)-1] != uint8(sum) {
return errors.StructuralError("private key checksum failure")
}
data = data[:len(data)-2]
}
err := pk.parsePrivateKey(data)
if _, ok := err.(errors.KeyInvalidError); ok {
return errors.KeyInvalidError("invalid key parameters")
}
if err != nil {
return err
}
// Mark key as unencrypted
pk.s2kType = S2KNON
pk.s2k = nil
pk.Encrypted = false
pk.encryptedData = nil
return nil
}
// Encrypt encrypts an unencrypted private key using a passphrase.
func (pk *PrivateKey) Encrypt(passphrase []byte) error {
priv := bytes.NewBuffer(nil)
err := pk.serializePrivateKey(priv)
if err != nil {
return err
}
//Default config of private key encryption
pk.cipher = CipherAES256
s2kConfig := &s2k.Config{
S2KMode: 3, //Iterated
S2KCount: 65536,
Hash: crypto.SHA256,
}
pk.s2kParams, err = s2k.Generate(rand.Reader, s2kConfig)
if err != nil {
return err
}
privateKeyBytes := priv.Bytes()
key := make([]byte, pk.cipher.KeySize())
pk.sha1Checksum = true
pk.s2k, err = pk.s2kParams.Function()
if err != nil {
return err
}
pk.s2k(key, passphrase)
block := pk.cipher.new(key)
pk.iv = make([]byte, pk.cipher.blockSize())
_, err = rand.Read(pk.iv)
if err != nil {
return err
}
cfb := cipher.NewCFBEncrypter(block, pk.iv)
if pk.sha1Checksum {
pk.s2kType = S2KSHA1
h := sha1.New()
h.Write(privateKeyBytes)
sum := h.Sum(nil)
privateKeyBytes = append(privateKeyBytes, sum...)
} else {
pk.s2kType = S2KCHECKSUM
var sum uint16
for _, b := range privateKeyBytes {
sum += uint16(b)
}
priv.Write([]byte{uint8(sum >> 8), uint8(sum)})
}
pk.encryptedData = make([]byte, len(privateKeyBytes))
cfb.XORKeyStream(pk.encryptedData, privateKeyBytes)
pk.Encrypted = true
pk.PrivateKey = nil
return err
}
func (pk *PrivateKey) serializePrivateKey(w io.Writer) (err error) {
switch priv := pk.PrivateKey.(type) {
case *rsa.PrivateKey:
err = serializeRSAPrivateKey(w, priv)
case *dsa.PrivateKey:
err = serializeDSAPrivateKey(w, priv)
case *elgamal.PrivateKey:
err = serializeElGamalPrivateKey(w, priv)
case *ecdsa.PrivateKey:
err = serializeECDSAPrivateKey(w, priv)
case *ed25519.PrivateKey:
err = serializeEdDSAPrivateKey(w, priv)
case *ecdh.PrivateKey:
err = serializeECDHPrivateKey(w, priv)
default:
err = errors.InvalidArgumentError("unknown private key type")
}
return
}
func (pk *PrivateKey) parsePrivateKey(data []byte) (err error) {
switch pk.PublicKey.PubKeyAlgo {
case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly, PubKeyAlgoRSAEncryptOnly:
return pk.parseRSAPrivateKey(data)
case PubKeyAlgoDSA:
return pk.parseDSAPrivateKey(data)
case PubKeyAlgoElGamal:
return pk.parseElGamalPrivateKey(data)
case PubKeyAlgoECDSA:
return pk.parseECDSAPrivateKey(data)
case PubKeyAlgoECDH:
return pk.parseECDHPrivateKey(data)
case PubKeyAlgoEdDSA:
return pk.parseEdDSAPrivateKey(data)
}
panic("impossible")
}
func (pk *PrivateKey) parseRSAPrivateKey(data []byte) (err error) {
rsaPub := pk.PublicKey.PublicKey.(*rsa.PublicKey)
rsaPriv := new(rsa.PrivateKey)
rsaPriv.PublicKey = *rsaPub
buf := bytes.NewBuffer(data)
d := new(encoding.MPI)
if _, err := d.ReadFrom(buf); err != nil {
return err
}
p := new(encoding.MPI)
if _, err := p.ReadFrom(buf); err != nil {
return err
}
q := new(encoding.MPI)
if _, err := q.ReadFrom(buf); err != nil {
return err
}
rsaPriv.D = new(big.Int).SetBytes(d.Bytes())
rsaPriv.Primes = make([]*big.Int, 2)
rsaPriv.Primes[0] = new(big.Int).SetBytes(p.Bytes())
rsaPriv.Primes[1] = new(big.Int).SetBytes(q.Bytes())
if err := rsaPriv.Validate(); err != nil {
return errors.KeyInvalidError(err.Error())
}
rsaPriv.Precompute()
pk.PrivateKey = rsaPriv
return nil
}
func (pk *PrivateKey) parseDSAPrivateKey(data []byte) (err error) {
dsaPub := pk.PublicKey.PublicKey.(*dsa.PublicKey)
dsaPriv := new(dsa.PrivateKey)
dsaPriv.PublicKey = *dsaPub
buf := bytes.NewBuffer(data)
x := new(encoding.MPI)
if _, err := x.ReadFrom(buf); err != nil {
return err
}
dsaPriv.X = new(big.Int).SetBytes(x.Bytes())
if err := validateDSAParameters(dsaPriv); err != nil {
return err
}
pk.PrivateKey = dsaPriv
return nil
}
func (pk *PrivateKey) parseElGamalPrivateKey(data []byte) (err error) {
pub := pk.PublicKey.PublicKey.(*elgamal.PublicKey)
priv := new(elgamal.PrivateKey)
priv.PublicKey = *pub
buf := bytes.NewBuffer(data)
x := new(encoding.MPI)
if _, err := x.ReadFrom(buf); err != nil {
return err
}
priv.X = new(big.Int).SetBytes(x.Bytes())
if err := validateElGamalParameters(priv); err != nil {
return err
}
pk.PrivateKey = priv
return nil
}
func (pk *PrivateKey) parseECDSAPrivateKey(data []byte) (err error) {
ecdsaPub := pk.PublicKey.PublicKey.(*ecdsa.PublicKey)
ecdsaPriv := new(ecdsa.PrivateKey)
ecdsaPriv.PublicKey = *ecdsaPub
buf := bytes.NewBuffer(data)
d := new(encoding.MPI)
if _, err := d.ReadFrom(buf); err != nil {
return err
}
ecdsaPriv.D = new(big.Int).SetBytes(d.Bytes())
if err := validateECDSAParameters(ecdsaPriv); err != nil {
return err
}
pk.PrivateKey = ecdsaPriv
return nil
}
func (pk *PrivateKey) parseECDHPrivateKey(data []byte) (err error) {
ecdhPub := pk.PublicKey.PublicKey.(*ecdh.PublicKey)
ecdhPriv := new(ecdh.PrivateKey)
ecdhPriv.PublicKey = *ecdhPub
buf := bytes.NewBuffer(data)
d := new(encoding.MPI)
if _, err := d.ReadFrom(buf); err != nil {
return err
}
ecdhPriv.D = d.Bytes()
if err := validateECDHParameters(ecdhPriv); err != nil {
return err
}
pk.PrivateKey = ecdhPriv
return nil
}
func (pk *PrivateKey) parseEdDSAPrivateKey(data []byte) (err error) {
eddsaPub := pk.PublicKey.PublicKey.(*ed25519.PublicKey)
eddsaPriv := make(ed25519.PrivateKey, ed25519.PrivateKeySize)
buf := bytes.NewBuffer(data)
d := new(encoding.MPI)
if _, err := d.ReadFrom(buf); err != nil {
return err
}
priv := d.Bytes()
copy(eddsaPriv[32-len(priv):32], priv)
copy(eddsaPriv[32:], (*eddsaPub)[:])
if err := validateEdDSAParameters(&eddsaPriv); err != nil {
return err
}
pk.PrivateKey = &eddsaPriv
return nil
}
func validateECDSAParameters(priv *ecdsa.PrivateKey) error {
return validateCommonECC(priv.Curve, priv.D.Bytes(), priv.X, priv.Y)
}
func validateECDHParameters(priv *ecdh.PrivateKey) error {
if priv.CurveType != ecc.Curve25519 {
return validateCommonECC(priv.Curve, priv.D, priv.X, priv.Y)
}
// Handle Curve25519
Q := priv.X.Bytes()[1:]
var d [32]byte
// Copy reversed d
l := len(priv.D)
for i := 0; i < l; i++ {
d[i] = priv.D[l-i-1]
}
var expectedQ [32]byte
curve25519.ScalarBaseMult(&expectedQ, &d)
if !bytes.Equal(Q, expectedQ[:]) {
return errors.KeyInvalidError("ECDH curve25519: invalid point")
}
return nil
}
func validateCommonECC(curve elliptic.Curve, d []byte, X, Y *big.Int) error {
// the public point should not be at infinity (0,0)
zero := new(big.Int)
if X.Cmp(zero) == 0 && Y.Cmp(zero) == 0 {
return errors.KeyInvalidError(fmt.Sprintf("ecc (%s): infinity point", curve.Params().Name))
}
// re-derive the public point Q' = (X,Y) = dG
// to compare to declared Q in public key
expectedX, expectedY := curve.ScalarBaseMult(d)
if X.Cmp(expectedX) != 0 || Y.Cmp(expectedY) != 0 {
return errors.KeyInvalidError(fmt.Sprintf("ecc (%s): invalid point", curve.Params().Name))
}
return nil
}
func validateEdDSAParameters(priv *ed25519.PrivateKey) error {
// In EdDSA, the serialized public point is stored as part of private key (together with the seed),
// hence we can re-derive the key from the seed
seed := priv.Seed()
expectedPriv := ed25519.NewKeyFromSeed(seed)
if !bytes.Equal(*priv, expectedPriv) {
return errors.KeyInvalidError("eddsa: invalid point")
}
return nil
}
func validateDSAParameters(priv *dsa.PrivateKey) error {
p := priv.P // group prime
q := priv.Q // subgroup order
g := priv.G // g has order q mod p
x := priv.X // secret
y := priv.Y // y == g**x mod p
one := big.NewInt(1)
// expect g, y >= 2 and g < p
if g.Cmp(one) <= 0 || y.Cmp(one) <= 0 || g.Cmp(p) > 0 {
return errors.KeyInvalidError("dsa: invalid group")
}
// expect p > q
if p.Cmp(q) <= 0 {
return errors.KeyInvalidError("dsa: invalid group prime")
}
// q should be large enough and divide p-1
pSub1 := new(big.Int).Sub(p, one)
if q.BitLen() < 150 || new(big.Int).Mod(pSub1, q).Cmp(big.NewInt(0)) != 0 {
return errors.KeyInvalidError("dsa: invalid order")
}
// confirm that g has order q mod p
if !q.ProbablyPrime(32) || new(big.Int).Exp(g, q, p).Cmp(one) != 0 {
return errors.KeyInvalidError("dsa: invalid order")
}
// check y
if new(big.Int).Exp(g, x, p).Cmp(y) != 0 {
return errors.KeyInvalidError("dsa: mismatching values")
}
return nil
}
func validateElGamalParameters(priv *elgamal.PrivateKey) error {
p := priv.P // group prime
g := priv.G // g has order p-1 mod p
x := priv.X // secret
y := priv.Y // y == g**x mod p
one := big.NewInt(1)
// Expect g, y >= 2 and g < p
if g.Cmp(one) <= 0 || y.Cmp(one) <= 0 || g.Cmp(p) > 0 {
return errors.KeyInvalidError("elgamal: invalid group")
}
if p.BitLen() < 1024 {
return errors.KeyInvalidError("elgamal: group order too small")
}
pSub1 := new(big.Int).Sub(p, one)
if new(big.Int).Exp(g, pSub1, p).Cmp(one) != 0 {
return errors.KeyInvalidError("elgamal: invalid group")
}
// Since p-1 is not prime, g might have a smaller order that divides p-1.
// We cannot confirm the exact order of g, but we make sure it is not too small.
gExpI := new(big.Int).Set(g)
i := 1
threshold := 2 << 17 // we want order > threshold
for i < threshold {
i++ // we check every order to make sure key validation is not easily bypassed by guessing y'
gExpI.Mod(new(big.Int).Mul(gExpI, g), p)
if gExpI.Cmp(one) == 0 {
return errors.KeyInvalidError("elgamal: order too small")
}
}
// Check y
if new(big.Int).Exp(g, x, p).Cmp(y) != 0 {
return errors.KeyInvalidError("elgamal: mismatching values")
}
return nil
}