go-ethereum/crypto/ecies/ecies.go

// Copyright (c) 2013 Kyle Isom <kyle@tyrfingr.is>
// Copyright (c) 2012 The Go Authors. All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//    * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//    * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//    * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

package ecies

import (
	"bytes"
	"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{Curve: pub.Curve, X: pub.X, Y: 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{PublicKey: *pubECDSA, D: 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 {
	mac := hmac.New(hash, km)
	mac.Write(msg)
	mac.Write(shared)
	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
	}
	/*
		In SEC 1 Version 2.0 section 3.8 the IV value is not specified in for XOR CTR.
		It is specified, however, that it should not be transmitted as part of the ciphertext.
		This means it cannot be random, as the other party would not know the value.
		Therefore we set it to the zeroed value defined for AES in CTR mode.
	*/
	iv := bytes.Repeat([]byte{0}, 16)
	ctr := cipher.NewCTR(c, iv)

	ct = make([]byte, len(m))
	ctr.XORKeyStream(ct, 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
	}

	iv := bytes.Repeat([]byte{0}, 16)
	ctr := cipher.NewCTR(c, iv)

	m = make([]byte, len(ct))
	ctr.XORKeyStream(m, ct)
	return
}

// Encrypt encrypts a message using ECIES as specified in SEC 1, 5.1.
//
// s1 and s2 contain shared information that is not part of the resulting
// ciphertext. s1 is fed into key derivation, s2 is fed into the MAC. 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:]

	em, err := symEncrypt(rand, params, Ke, m)
	if err != nil {
		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:]

	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
}