init

1 files changed, 326 insertions, 0 deletions

diff --git a/ecies.go b/ecies.go
new file mode 100644
index 000000000..0e2403d47
--- /dev/null
+++ b/ecies.go
@@ -0,0 +1,326 @@
+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")
+	ErrSharedKeyTooBig  = fmt.Errorf("ecies: shared key is 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 {
+		err = ErrInvalidCurve
+		return
+	}
+	x, _ := pub.Curve.ScalarMult(pub.X, pub.Y, prv.D.Bytes())
+	if x == nil || (x.BitLen()+7)/8 < (skLen+macLen) {
+		err = ErrSharedKeyTooBig
+		return
+	}
+	sk = x.Bytes()[:skLen+macLen]
+	return
+}
+
+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
+	}
+
+	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
+}