// Copyright 2018 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
package scwallet
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"crypto/sha256"
"crypto/sha512"
"fmt"
"github.com/ethereum/go-ethereum/crypto"
pcsc "github.com/gballet/go-libpcsclite"
"github.com/wsddn/go-ecdh"
"golang.org/x/crypto/pbkdf2"
"golang.org/x/text/unicode/norm"
)
const (
maxPayloadSize = 223
pairP1FirstStep = 0
pairP1LastStep = 1
scSecretLength = 32
scBlockSize = 16
insOpenSecureChannel = 0x10
insMutuallyAuthenticate = 0x11
insPair = 0x12
insUnpair = 0x13
pairingSalt = "Keycard Pairing Password Salt"
)
// SecureChannelSession enables secure communication with a hardware wallet.
type SecureChannelSession struct {
card *pcsc.Card // A handle to the smartcard for communication
secret []byte // A shared secret generated from our ECDSA keys
publicKey []byte // Our own ephemeral public key
PairingKey []byte // A permanent shared secret for a pairing, if present
sessionEncKey []byte // The current session encryption key
sessionMacKey []byte // The current session MAC key
iv []byte // The current IV
PairingIndex uint8 // The pairing index
}
// NewSecureChannelSession creates a new secure channel for the given card and public key.
func NewSecureChannelSession(card *pcsc.Card, keyData []byte) (*SecureChannelSession, error) {
// Generate an ECDSA keypair for ourselves
gen := ecdh.NewEllipticECDH(crypto.S256())
private, public, err := gen.GenerateKey(rand.Reader)
if err != nil {
return nil, err
}
cardPublic, ok := gen.Unmarshal(keyData)
if !ok {
return nil, fmt.Errorf("could not unmarshal public key from card")
}
secret, err := gen.GenerateSharedSecret(private, cardPublic)
if err != nil {
return nil, err
}
return &SecureChannelSession{
card: card,
secret: secret,
publicKey: gen.Marshal(public),
}, nil
}
// Pair establishes a new pairing with the smartcard.
func (s *SecureChannelSession) Pair(pairingPassword []byte) error {
secretHash := pbkdf2.Key(norm.NFKD.Bytes(pairingPassword), norm.NFKD.Bytes([]byte(pairingSalt)), 50000, 32, sha256.New)
challenge := make([]byte, 32)
if _, err := rand.Read(challenge); err != nil {
return err
}
response, err := s.pair(pairP1FirstStep, challenge)
if err != nil {
return err
}
md := sha256.New()
md.Write(secretHash[:])
md.Write(challenge)
expectedCryptogram := md.Sum(nil)
cardCryptogram := response.Data[:32]
cardChallenge := response.Data[32:64]
if !bytes.Equal(expectedCryptogram, cardCryptogram) {
return fmt.Errorf("invalid card cryptogram %v != %v", expectedCryptogram, cardCryptogram)
}
md.Reset()
md.Write(secretHash[:])
md.Write(cardChallenge)
response, err = s.pair(pairP1LastStep, md.Sum(nil))
if err != nil {
return err
}
md.Reset()
md.Write(secretHash[:])
md.Write(response.Data[1:])
s.PairingKey = md.Sum(nil)
s.PairingIndex = response.Data[0]
return nil
}
// Unpair disestablishes an existing pairing.
func (s *SecureChannelSession) Unpair() error {
if s.PairingKey == nil {
return fmt.Errorf("cannot unpair: not paired")
}
_, err := s.transmitEncrypted(claSCWallet, insUnpair, s.PairingIndex, 0, []byte{})
if err != nil {
return err
}
s.PairingKey = nil
// Close channel
s.iv = nil
return nil
}
// Open initializes the secure channel.
func (s *SecureChannelSession) Open() error {
if s.iv != nil {
return fmt.Errorf("session already opened")
}
response, err := s.open()
if err != nil {
return err
}
// Generate the encryption/mac key by hashing our shared secret,
// pairing key, and the first bytes returned from the Open APDU.
md := sha512.New()
md.Write(s.secret)
md.Write(s.PairingKey)
md.Write(response.Data[:scSecretLength])
keyData := md.Sum(nil)
s.sessionEncKey = keyData[:scSecretLength]
s.sessionMacKey = keyData[scSecretLength : scSecretLength*2]
// The IV is the last bytes returned from the Open APDU.
s.iv = response.Data[scSecretLength:]
return s.mutuallyAuthenticate()
}
// mutuallyAuthenticate is an internal method to authenticate both ends of the
// connection.
func (s *SecureChannelSession) mutuallyAuthenticate() error {
data := make([]byte, scSecretLength)
if _, err := rand.Read(data); err != nil {
return err
}
response, err := s.transmitEncrypted(claSCWallet, insMutuallyAuthenticate, 0, 0, data)
if err != nil {
return err
}
if response.Sw1 != 0x90 || response.Sw2 != 0x00 {
return fmt.Errorf("got unexpected response from MUTUALLY_AUTHENTICATE: 0x%x%x", response.Sw1, response.Sw2)
}
if len(response.Data) != scSecretLength {
return fmt.Errorf("response from MUTUALLY_AUTHENTICATE was %d bytes, expected %d", len(response.Data), scSecretLength)
}
return nil
}
// open is an internal method that sends an open APDU.
func (s *SecureChannelSession) open() (*responseAPDU, error) {
return transmit(s.card, &commandAPDU{
Cla: claSCWallet,
Ins: insOpenSecureChannel,
P1: s.PairingIndex,
P2: 0,
Data: s.publicKey,
Le: 0,
})
}
// pair is an internal method that sends a pair APDU.
func (s *SecureChannelSession) pair(p1 uint8, data []byte) (*responseAPDU, error) {
return transmit(s.card, &commandAPDU{
Cla: claSCWallet,
Ins: insPair,
P1: p1,
P2: 0,
Data: data,
Le: 0,
})
}
// transmitEncrypted sends an encrypted message, and decrypts and returns the response.
func (s *SecureChannelSession) transmitEncrypted(cla, ins, p1, p2 byte, data []byte) (*responseAPDU, error) {
if s.iv == nil {
return nil, fmt.Errorf("channel not open")
}
data, err := s.encryptAPDU(data)
if err != nil {
return nil, err
}
meta := [16]byte{cla, ins, p1, p2, byte(len(data) + scBlockSize)}
if err = s.updateIV(meta[:], data); err != nil {
return nil, err
}
fulldata := make([]byte, len(s.iv)+len(data))
copy(fulldata, s.iv)
copy(fulldata[len(s.iv):], data)
response, err := transmit(s.card, &commandAPDU{
Cla: cla,
Ins: ins,
P1: p1,
P2: p2,
Data: fulldata,
})
if err != nil {
return nil, err
}
rmeta := [16]byte{byte(len(response.Data))}
rmac := response.Data[:len(s.iv)]
rdata := response.Data[len(s.iv):]
plainData, err := s.decryptAPDU(rdata)
if err != nil {
return nil, err
}
if err = s.updateIV(rmeta[:], rdata); err != nil {
return nil, err
}
if !bytes.Equal(s.iv, rmac) {
return nil, fmt.Errorf("invalid MAC in response")
}
rapdu := &responseAPDU{}
rapdu.deserialize(plainData)
if rapdu.Sw1 != sw1Ok {
return nil, fmt.Errorf("unexpected response status Cla=0x%x, Ins=0x%x, Sw=0x%x%x", cla, ins, rapdu.Sw1, rapdu.Sw2)
}
return rapdu, nil
}
// encryptAPDU is an internal method that serializes and encrypts an APDU.
func (s *SecureChannelSession) encryptAPDU(data []byte) ([]byte, error) {
if len(data) > maxPayloadSize {
return nil, fmt.Errorf("payload of %d bytes exceeds maximum of %d", len(data), maxPayloadSize)
}
data = pad(data, 0x80)
ret := make([]byte, len(data))
a, err := aes.NewCipher(s.sessionEncKey)
if err != nil {
return nil, err
}
crypter := cipher.NewCBCEncrypter(a, s.iv)
crypter.CryptBlocks(ret, data)
return ret, nil
}
// pad applies message padding to a 16 byte boundary.
func pad(data []byte, terminator byte) []byte {
padded := make([]byte, (len(data)/16+1)*16)
copy(padded, data)
padded[len(data)] = terminator
return padded
}
// decryptAPDU is an internal method that decrypts and deserializes an APDU.
func (s *SecureChannelSession) decryptAPDU(data []byte) ([]byte, error) {
a, err := aes.NewCipher(s.sessionEncKey)
if err != nil {
return nil, err
}
ret := make([]byte, len(data))
crypter := cipher.NewCBCDecrypter(a, s.iv)
crypter.CryptBlocks(ret, data)
return unpad(ret, 0x80)
}
// unpad strips padding from a message.
func unpad(data []byte, terminator byte) ([]byte, error) {
for i := 1; i <= 16; i++ {
switch data[len(data)-i] {
case 0:
continue
case terminator:
return data[:len(data)-i], nil
default:
return nil, fmt.Errorf("expected end of padding, got %d", data[len(data)-i])
}
}
return nil, fmt.Errorf("expected end of padding, got 0")
}
// updateIV is an internal method that updates the initialization vector after
// each message exchanged.
func (s *SecureChannelSession) updateIV(meta, data []byte) error {
data = pad(data, 0)
a, err := aes.NewCipher(s.sessionMacKey)
if err != nil {
return err
}
crypter := cipher.NewCBCEncrypter(a, make([]byte, 16))
crypter.CryptBlocks(meta, meta)
crypter.CryptBlocks(data, data)
// The first 16 bytes of the last block is the MAC
s.iv = data[len(data)-32 : len(data)-16]
return nil
}