plugeth/crypto/ecies/ecies.go
Luke Champine 462ddce5b2
crypto/ecies: improve concatKDF (#20836)
This removes a bunch of weird code around the counter overflow check in
concatKDF and makes it actually work for different hash output sizes.

The overflow check worked as follows: concatKDF applies the hash function N
times, where N is roundup(kdLen, hashsize) / hashsize. N should not
overflow 32 bits because that would lead to a repetition in the KDF output.

A couple issues with the overflow check:

- It used the hash.BlockSize, which is wrong because the
  block size is about the input of the hash function. Luckily, all standard
  hash functions have a block size that's greater than the output size, so
  concatKDF didn't crash, it just generated too much key material.
- The check used big.Int to compare against 2^32-1.
- The calculation could still overflow before reaching the check.

The new code in concatKDF doesn't check for overflow. Instead, there is a
new check on ECIESParams which ensures that params.KeyLen is < 512. This
removes any possibility of overflow.

There are a couple of miscellaneous improvements bundled in with this
change:

- The key buffer is pre-allocated instead of appending the hash output
  to an initially empty slice.
- The code that uses concatKDF to derive keys is now shared between Encrypt
  and Decrypt.
- There was a redundant invocation of IsOnCurve in Decrypt. This is now removed
  because elliptic.Unmarshal already checks whether the input is a valid curve
  point since Go 1.5.

Co-authored-by: Felix Lange <fjl@twurst.com>
2020-04-03 11:57:24 +02:00

318 lines
8.8 KiB
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 (
"crypto/cipher"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/hmac"
"crypto/subtle"
"encoding/binary"
"fmt"
"hash"
"io"
"math/big"
)
var (
ErrImport = fmt.Errorf("ecies: failed to import key")
ErrInvalidCurve = fmt.Errorf("ecies: invalid elliptic curve")
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 parameters 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 (
ErrSharedTooLong = fmt.Errorf("ecies: shared secret is too long")
ErrInvalidMessage = fmt.Errorf("ecies: invalid message")
)
// NIST SP 800-56 Concatenation Key Derivation Function (see section 5.8.1).
func concatKDF(hash hash.Hash, z, s1 []byte, kdLen int) []byte {
counterBytes := make([]byte, 4)
k := make([]byte, 0, roundup(kdLen, hash.Size()))
for counter := uint32(1); len(k) < kdLen; counter++ {
binary.BigEndian.PutUint32(counterBytes, counter)
hash.Reset()
hash.Write(counterBytes)
hash.Write(z)
hash.Write(s1)
k = hash.Sum(k)
}
return k[:kdLen]
}
// roundup rounds size up to the next multiple of blocksize.
func roundup(size, blocksize int) int {
return size + blocksize - (size % blocksize)
}
// deriveKeys creates the encryption and MAC keys using concatKDF.
func deriveKeys(hash hash.Hash, z, s1 []byte, keyLen int) (Ke, Km []byte) {
K := concatKDF(hash, z, s1, 2*keyLen)
Ke = K[:keyLen]
Km = K[keyLen:]
hash.Reset()
hash.Write(Km)
Km = hash.Sum(Km[:0])
return Ke, Km
}
// 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
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(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.
//
// 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, err := pubkeyParams(pub)
if err != nil {
return nil, err
}
R, err := GenerateKey(rand, pub.Curve, params)
if err != nil {
return nil, err
}
z, err := R.GenerateShared(pub, params.KeyLen, params.KeyLen)
if err != nil {
return nil, err
}
hash := params.Hash()
Ke, Km := deriveKeys(hash, z, s1, params.KeyLen)
em, err := symEncrypt(rand, params, Ke, m)
if err != nil || len(em) <= params.BlockSize {
return nil, err
}
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 ct, nil
}
// Decrypt decrypts an ECIES ciphertext.
func (prv *PrivateKey) Decrypt(c, s1, s2 []byte) (m []byte, err error) {
if len(c) == 0 {
return nil, ErrInvalidMessage
}
params, err := pubkeyParams(&prv.PublicKey)
if err != nil {
return nil, err
}
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) {
return nil, ErrInvalidMessage
}
default:
return nil, ErrInvalidPublicKey
}
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 {
return nil, ErrInvalidPublicKey
}
z, err := prv.GenerateShared(R, params.KeyLen, params.KeyLen)
if err != nil {
return nil, err
}
Ke, Km := deriveKeys(hash, z, s1, params.KeyLen)
d := messageTag(params.Hash, Km, c[mStart:mEnd], s2)
if subtle.ConstantTimeCompare(c[mEnd:], d) != 1 {
return nil, ErrInvalidMessage
}
return symDecrypt(params, Ke, c[mStart:mEnd])
}