1 // https://wiki.vuze.com/w/Message_Stream_Encryption
20 "github.com/anacrolix/missinggo"
21 "github.com/bradfitz/iter"
27 cryptoMethodPlaintext = 1
32 // Prime P according to the spec, and G, the generator.
34 // The rand.Int max arg for use in newPadLen()
36 // For use in initer's hashes
40 // Verification constant "VC" which is all zeroes in the bittorrent
45 // Tracks counts of received crypto_provides
46 cryptoProvidesCount = expvar.NewMap("mseCryptoProvides")
50 p.SetString("0xFFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A63A36210000000000090563", 0)
52 newPadLenMax.SetInt64(maxPadLen + 1)
55 func hash(parts ...[]byte) []byte {
57 for _, p := range parts {
69 func newEncrypt(initer bool, s []byte, skey []byte) (c *rc4.Cipher) {
70 c, err := rc4.NewCipher(hash([]byte(func() string {
80 var burnSrc, burnDst [1024]byte
81 c.XORKeyStream(burnDst[:], burnSrc[:])
85 type cipherReader struct {
90 func (cr *cipherReader) Read(b []byte) (n int, err error) {
91 be := make([]byte, len(b))
92 n, err = cr.r.Read(be)
93 cr.c.XORKeyStream(b[:n], be[:n])
97 func newCipherReader(c *rc4.Cipher, r io.Reader) io.Reader {
98 return &cipherReader{c, r}
101 type cipherWriter struct {
106 func (cr *cipherWriter) Write(b []byte) (n int, err error) {
107 be := make([]byte, len(b))
108 cr.c.XORKeyStream(be, b)
109 n, err = cr.w.Write(be)
111 // The cipher will have advanced beyond the callers stream position.
112 // We can't use the cipher anymore.
118 func readY(r io.Reader) (y big.Int, err error) {
120 _, err = io.ReadFull(r, b[:])
128 func newX() big.Int {
130 X.SetBytes(func() []byte {
132 _, err := rand.Read(b[:])
141 func paddedLeft(b []byte, _len int) []byte {
145 ret := make([]byte, _len)
146 if n := copy(ret[_len-len(b):], b); n != len(b) {
152 // Calculate, and send Y, our public key.
153 func (h *handshake) postY(x *big.Int) error {
156 return h.postWrite(paddedLeft(y.Bytes(), 96))
159 func (h *handshake) establishS() (err error) {
163 _, err = io.ReadFull(h.conn, b[:])
170 missinggo.CopyExact(&h.s, paddedLeft(S.Bytes(), 96))
174 func newPadLen() int64 {
175 i, err := rand.Int(rand.Reader, &newPadLenMax)
180 if ret < 0 || ret > maxPadLen {
186 type handshake struct {
192 ia []byte // Initial payload. Only used by the initiator.
205 func (h *handshake) finishWriting() {
208 h.writeCond.Broadcast()
219 func (h *handshake) writer() {
223 h.writerCond.Broadcast()
229 if len(h.writes) != 0 {
239 h.writes = h.writes[1:]
241 _, err := h.conn.Write(b)
251 func (h *handshake) postWrite(b []byte) error {
253 defer h.writeMu.Unlock()
254 if h.writeErr != nil {
257 h.writes = append(h.writes, b)
262 func xor(dst, src []byte) (ret []byte) {
267 ret = make([]byte, 0, max)
268 for i := range iter.N(max) {
269 ret = append(ret, dst[i]^src[i])
274 func marshal(w io.Writer, data ...interface{}) (err error) {
275 for _, data := range data {
276 err = binary.Write(w, binary.BigEndian, data)
284 func unmarshal(r io.Reader, data ...interface{}) (err error) {
285 for _, data := range data {
286 err = binary.Read(r, binary.BigEndian, data)
294 // Looking for b at the end of a.
295 func suffixMatchLen(a, b []byte) int {
299 // i is how much of b to try to match
300 for i := len(b); i > 0; i-- {
301 // j is how many chars we've compared
304 if b[i-1-j] != a[len(a)-1-j] {
314 func readUntil(r io.Reader, b []byte) error {
315 b1 := make([]byte, len(b))
318 _, err := io.ReadFull(r, b1[i:])
322 i = suffixMatchLen(b1, b)
326 if copy(b1, b1[len(b1)-i:]) != i {
333 type readWriter struct {
338 func (h *handshake) newEncrypt(initer bool) *rc4.Cipher {
339 return newEncrypt(initer, h.s[:], h.skey)
342 func (h *handshake) initerSteps() (ret io.ReadWriter, err error) {
343 h.postWrite(hash(req1, h.s[:]))
344 h.postWrite(xor(hash(req2, h.skey), hash(req3, h.s[:])))
345 buf := &bytes.Buffer{}
346 padLen := uint16(newPadLen())
347 err = marshal(buf, vc[:], uint32(cryptoMethodRC4), padLen, zeroPad[:padLen], uint16(len(h.ia)), h.ia)
351 e := h.newEncrypt(true)
352 be := make([]byte, buf.Len())
353 e.XORKeyStream(be, buf.Bytes())
355 bC := h.newEncrypt(false)
357 bC.XORKeyStream(eVC[:], vc[:])
358 // Read until the all zero VC. At this point we've only read the 96 byte
359 // public key, Y. There is potentially 512 byte padding, between us and
360 // the 8 byte verification constant.
361 err = readUntil(io.LimitReader(h.conn, 520), eVC[:])
364 err = errors.New("failed to synchronize on VC")
366 err = fmt.Errorf("error reading until VC: %s", err)
370 r := &cipherReader{bC, h.conn}
372 err = unmarshal(r, &method, &padLen)
376 if method != cryptoMethodRC4 {
377 err = fmt.Errorf("receiver chose unsupported method: %x", method)
380 _, err = io.CopyN(ioutil.Discard, r, int64(padLen))
384 ret = readWriter{r, &cipherWriter{e, h.conn}}
388 var ErrNoSecretKeyMatch = errors.New("no skey matched")
390 func (h *handshake) receiverSteps() (ret io.ReadWriter, err error) {
391 // There is up to 512 bytes of padding, then the 20 byte hash.
392 err = readUntil(io.LimitReader(h.conn, 532), hash(req1, h.s[:]))
395 err = errors.New("failed to synchronize on S hash")
400 _, err = io.ReadFull(h.conn, b[:])
404 err = ErrNoSecretKeyMatch
405 for _, skey := range h.skeys {
406 if bytes.Equal(xor(hash(req2, skey), hash(req3, h.s[:])), b[:]) {
415 r := newCipherReader(newEncrypt(true, h.s[:], h.skey), h.conn)
422 err = unmarshal(r, vc[:], &method, &padLen)
426 cryptoProvidesCount.Add(strconv.FormatUint(uint64(method), 16), 1)
427 if method&cryptoMethodRC4 == 0 {
428 err = errors.New("no supported crypto methods were provided")
431 _, err = io.CopyN(ioutil.Discard, r, int64(padLen))
438 h.ia = make([]byte, lenIA)
441 buf := &bytes.Buffer{}
442 w := cipherWriter{h.newEncrypt(false), buf}
443 padLen = uint16(newPadLen())
444 err = marshal(&w, &vc, uint32(cryptoMethodRC4), padLen, zeroPad[:padLen])
448 err = h.postWrite(buf.Bytes())
452 ret = readWriter{io.MultiReader(bytes.NewReader(h.ia), r), &cipherWriter{w.c, h.conn}}
456 func (h *handshake) Do() (ret io.ReadWriter, err error) {
457 h.writeCond.L = &h.writeMu
458 h.writerCond.L = &h.writerMu
468 err = fmt.Errorf("error while establishing secret: %s", err)
471 pad := make([]byte, newPadLen())
472 io.ReadFull(rand.Reader, pad)
473 err = h.postWrite(pad)
478 ret, err = h.initerSteps()
480 ret, err = h.receiverSteps()
485 func InitiateHandshake(rw io.ReadWriter, skey []byte, initialPayload []byte) (ret io.ReadWriter, err error) {
494 func ReceiveHandshake(rw io.ReadWriter, skeys [][]byte) (ret io.ReadWriter, err error) {