package ecies
import (
"crypto/cipher"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/hmac"
"crypto/subtle"
"fmt"
"hash"
"io"
"math/big"
)
var (
ErrImport = fmt.Errorf("ecies: failed to import key")
ErrInvalidCurve = fmt.Errorf("ecies: invalid elliptic curve")
ErrInvalidParams = fmt.Errorf("ecies: invalid ECIES parameters")
ErrInvalidPublicKey = fmt.Errorf("ecies: invalid public key")
ErrSharedKeyIsPointAtInfinity = fmt.Errorf("ecies: shared key is point at infinity")
ErrSharedKeyTooBig = fmt.Errorf("ecies: shared key params are too big")
)
// PublicKey is a representation of an elliptic curve public key.
type PublicKey struct {
X *big.Int
Y *big.Int
elliptic.Curve
Params *ECIESParams
}
// Export an ECIES public key as an ECDSA public key.
func (pub *PublicKey) ExportECDSA() *ecdsa.PublicKey {
return &ecdsa.PublicKey{pub.Curve, pub.X, pub.Y}
}
// Import an ECDSA public key as an ECIES public key.
func ImportECDSAPublic(pub *ecdsa.PublicKey) *PublicKey {
return &PublicKey{
X: pub.X,
Y: pub.Y,
Curve: pub.Curve,
Params: ParamsFromCurve(pub.Curve),
}
}
// PrivateKey is a representation of an elliptic curve private key.
type PrivateKey struct {
PublicKey
D *big.Int
}
// Export an ECIES private key as an ECDSA private key.
func (prv *PrivateKey) ExportECDSA() *ecdsa.PrivateKey {
pub := &prv.PublicKey
pubECDSA := pub.ExportECDSA()
return &ecdsa.PrivateKey{*pubECDSA, prv.D}
}
// Import an ECDSA private key as an ECIES private key.
func ImportECDSA(prv *ecdsa.PrivateKey) *PrivateKey {
pub := ImportECDSAPublic(&prv.PublicKey)
return &PrivateKey{*pub, prv.D}
}
// Generate an elliptic curve public / private keypair. If params is nil,
// the recommended default paramters for the key will be chosen.
func GenerateKey(rand io.Reader, curve elliptic.Curve, params *ECIESParams) (prv *PrivateKey, err error) {
pb, x, y, err := elliptic.GenerateKey(curve, rand)
if err != nil {
return
}
prv = new(PrivateKey)
prv.PublicKey.X = x
prv.PublicKey.Y = y
prv.PublicKey.Curve = curve
prv.D = new(big.Int).SetBytes(pb)
if params == nil {
params = ParamsFromCurve(curve)
}
prv.PublicKey.Params = params
return
}
// MaxSharedKeyLength returns the maximum length of the shared key the
// public key can produce.
func MaxSharedKeyLength(pub *PublicKey) int {
return (pub.Curve.Params().BitSize + 7) / 8
}
// ECDH key agreement method used to establish secret keys for encryption.
func (prv *PrivateKey) GenerateShared(pub *PublicKey, skLen, macLen int) (sk []byte, err error) {
if prv.PublicKey.Curve != pub.Curve {
return nil, ErrInvalidCurve
}
if skLen+macLen > MaxSharedKeyLength(pub) {
return nil, ErrSharedKeyTooBig
}
x, _ := pub.Curve.ScalarMult(pub.X, pub.Y, prv.D.Bytes())
if x == nil {
return nil, ErrSharedKeyIsPointAtInfinity
}
sk = make([]byte, skLen+macLen)
skBytes := x.Bytes()
copy(sk[len(sk)-len(skBytes):], skBytes)
return sk, nil
}
var (
ErrKeyDataTooLong = fmt.Errorf("ecies: can't supply requested key data")
ErrSharedTooLong = fmt.Errorf("ecies: shared secret is too long")
ErrInvalidMessage = fmt.Errorf("ecies: invalid message")
)
var (
big2To32 = new(big.Int).Exp(big.NewInt(2), big.NewInt(32), nil)
big2To32M1 = new(big.Int).Sub(big2To32, big.NewInt(1))
)
func incCounter(ctr []byte) {
if ctr[3]++; ctr[3] != 0 {
return
} else if ctr[2]++; ctr[2] != 0 {
return
} else if ctr[1]++; ctr[1] != 0 {
return
} else if ctr[0]++; ctr[0] != 0 {
return
}
return
}
// NIST SP 800-56 Concatenation Key Derivation Function (see section 5.8.1).
func concatKDF(hash hash.Hash, z, s1 []byte, kdLen int) (k []byte, err error) {
if s1 == nil {
s1 = make([]byte, 0)
}
reps := ((kdLen + 7) * 8) / (hash.BlockSize() * 8)
if big.NewInt(int64(reps)).Cmp(big2To32M1) > 0 {
fmt.Println(big2To32M1)
return nil, ErrKeyDataTooLong
}
counter := []byte{0, 0, 0, 1}
k = make([]byte, 0)
for i := 0; i <= reps; i++ {
hash.Write(counter)
hash.Write(z)
hash.Write(s1)
k = append(k, hash.Sum(nil)...)
hash.Reset()
incCounter(counter)
}
k = k[:kdLen]
return
}
// messageTag computes the MAC of a message (called the tag) as per
// SEC 1, 3.5.
func messageTag(hash func() hash.Hash, km, msg, shared []byte) []byte {
if shared == nil {
shared = make([]byte, 0)
}
mac := hmac.New(hash, km)
mac.Write(msg)
tag := mac.Sum(nil)
return tag
}
// Generate an initialisation vector for CTR mode.
func generateIV(params *ECIESParams, rand io.Reader) (iv []byte, err error) {
iv = make([]byte, params.BlockSize)
_, err = io.ReadFull(rand, iv)
return
}
// symEncrypt carries out CTR encryption using the block cipher specified in the
// parameters.
func symEncrypt(rand io.Reader, params *ECIESParams, key, m []byte) (ct []byte, err error) {
c, err := params.Cipher(key)
if err != nil {
return
}
iv, err := generateIV(params, rand)
if err != nil {
return
}
ctr := cipher.NewCTR(c, iv)
ct = make([]byte, len(m)+params.BlockSize)
copy(ct, iv)
ctr.XORKeyStream(ct[params.BlockSize:], m)
return
}
// symDecrypt carries out CTR decryption using the block cipher specified in
// the parameters
func symDecrypt(rand io.Reader, params *ECIESParams, key, ct []byte) (m []byte, err error) {
c, err := params.Cipher(key)
if err != nil {
return
}
ctr := cipher.NewCTR(c, ct[:params.BlockSize])
m = make([]byte, len(ct)-params.BlockSize)
ctr.XORKeyStream(m, ct[params.BlockSize:])
return
}
// Encrypt encrypts a message using ECIES as specified in SEC 1, 5.1. If
// the shared information parameters aren't being used, they should be
// nil.
func Encrypt(rand io.Reader, pub *PublicKey, m, s1, s2 []byte) (ct []byte, err error) {
params := pub.Params
if params == nil {
if params = ParamsFromCurve(pub.Curve); params == nil {
err = ErrUnsupportedECIESParameters
return
}
}
R, err := GenerateKey(rand, pub.Curve, params)
if err != nil {
return
}
hash := params.Hash()
z, err := R.GenerateShared(pub, params.KeyLen, params.KeyLen)
if err != nil {
return
}
K, err := concatKDF(hash, z, s1, params.KeyLen+params.KeyLen)
if err != nil {
return
}
Ke := K[:params.KeyLen]
Km := K[params.KeyLen:]
hash.Write(Km)
Km = hash.Sum(nil)
hash.Reset()
em, err := symEncrypt(rand, params, Ke, m)
if err != nil || len(em) <= params.BlockSize {
return
}
d := messageTag(params.Hash, Km, em, s2)
Rb := elliptic.Marshal(pub.Curve, R.PublicKey.X, R.PublicKey.Y)
ct = make([]byte, len(Rb)+len(em)+len(d))
copy(ct, Rb)
copy(ct[len(Rb):], em)
copy(ct[len(Rb)+len(em):], d)
return
}
// Decrypt decrypts an ECIES ciphertext.
func (prv *PrivateKey) Decrypt(rand io.Reader, c, s1, s2 []byte) (m []byte, err error) {
if c == nil || len(c) == 0 {
err = ErrInvalidMessage
return
}
params := prv.PublicKey.Params
if params == nil {
if params = ParamsFromCurve(prv.PublicKey.Curve); params == nil {
err = ErrUnsupportedECIESParameters
return
}
}
hash := params.Hash()
var (
rLen int
hLen int = hash.Size()
mStart int
mEnd int
)
switch c[0] {
case 2, 3, 4:
rLen = ((prv.PublicKey.Curve.Params().BitSize + 7) / 4)
if len(c) < (rLen + hLen + 1) {
err = ErrInvalidMessage
return
}
default:
err = ErrInvalidPublicKey
return
}
mStart = rLen
mEnd = len(c) - hLen
R := new(PublicKey)
R.Curve = prv.PublicKey.Curve
R.X, R.Y = elliptic.Unmarshal(R.Curve, c[:rLen])
if R.X == nil {
err = ErrInvalidPublicKey
return
}
if !R.Curve.IsOnCurve(R.X, R.Y) {
err = ErrInvalidCurve
return
}
z, err := prv.GenerateShared(R, params.KeyLen, params.KeyLen)
if err != nil {
return
}
K, err := concatKDF(hash, z, s1, params.KeyLen+params.KeyLen)
if err != nil {
return
}
Ke := K[:params.KeyLen]
Km := K[params.KeyLen:]
hash.Write(Km)
Km = hash.Sum(nil)
hash.Reset()
d := messageTag(params.Hash, Km, c[mStart:mEnd], s2)
if subtle.ConstantTimeCompare(c[mEnd:], d) != 1 {
err = ErrInvalidMessage
return
}
m, err = symDecrypt(rand, params, Ke, c[mStart:mEnd])
return
}
