1 // https://wiki.vuze.com/w/Message_Stream_Encryption
21 "github.com/anacrolix/missinggo/perf"
22 "github.com/bradfitz/iter"
28 CryptoMethodPlaintext CryptoMethod = 1 // After header obfuscation, drop into plaintext
29 CryptoMethodRC4 CryptoMethod = 2 // After header obfuscation, use RC4 for the rest of the stream
30 AllSupportedCrypto = CryptoMethodPlaintext | CryptoMethodRC4
33 type CryptoMethod uint32
36 // Prime P according to the spec, and G, the generator.
38 // The rand.Int max arg for use in newPadLen()
40 // For use in initer's hashes
44 // Verification constant "VC" which is all zeroes in the bittorrent
49 // Tracks counts of received crypto_provides
50 cryptoProvidesCount = expvar.NewMap("mseCryptoProvides")
54 p.SetString("0xFFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A63A36210000000000090563", 0)
56 newPadLenMax.SetInt64(maxPadLen + 1)
59 func hash(parts ...[]byte) []byte {
61 for _, p := range parts {
73 func newEncrypt(initer bool, s []byte, skey []byte) (c *rc4.Cipher) {
74 c, err := rc4.NewCipher(hash([]byte(func() string {
84 var burnSrc, burnDst [1024]byte
85 c.XORKeyStream(burnDst[:], burnSrc[:])
89 type cipherReader struct {
96 func (cr *cipherReader) Read(b []byte) (n int, err error) {
99 if len(cr.be) >= len(b) {
105 be = make([]byte, len(b))
107 n, err = cr.r.Read(be[:len(b)])
108 cr.c.XORKeyStream(b[:n], be[:n])
110 if len(be) > len(cr.be) {
117 func newCipherReader(c *rc4.Cipher, r io.Reader) io.Reader {
118 return &cipherReader{c: c, r: r}
121 type cipherWriter struct {
127 func (cr *cipherWriter) Write(b []byte) (n int, err error) {
128 be := func() []byte {
129 if len(cr.b) < len(b) {
130 return make([]byte, len(b))
137 cr.c.XORKeyStream(be[:], b)
138 n, err = cr.w.Write(be[:len(b)])
140 // The cipher will have advanced beyond the callers stream position.
141 // We can't use the cipher anymore.
144 if len(be) > len(cr.b) {
150 func newX() big.Int {
152 X.SetBytes(func() []byte {
154 _, err := rand.Read(b[:])
163 func paddedLeft(b []byte, _len int) []byte {
167 ret := make([]byte, _len)
168 if n := copy(ret[_len-len(b):], b); n != len(b) {
174 // Calculate, and send Y, our public key.
175 func (h *handshake) postY(x *big.Int) error {
178 return h.postWrite(paddedLeft(y.Bytes(), 96))
181 func (h *handshake) establishS() error {
185 _, err := io.ReadFull(h.conn, b[:])
187 return fmt.Errorf("error reading Y: %s", err)
193 copy(h.s[96-len(sBytes):96], sBytes)
197 func newPadLen() int64 {
198 i, err := rand.Int(rand.Reader, &newPadLenMax)
203 if ret < 0 || ret > maxPadLen {
209 // Manages state for both initiating and receiving handshakes.
210 type handshake struct {
213 initer bool // Whether we're initiating or receiving.
214 skeys SecretKeyIter // Skeys we'll accept if receiving.
215 skey []byte // Skey we're initiating with.
216 ia []byte // Initial payload. Only used by the initiator.
217 // Return the bit for the crypto method the receiver wants to use.
218 chooseMethod CryptoSelector
219 // Sent to the receiver.
220 cryptoProvides CryptoMethod
233 func (h *handshake) finishWriting() {
236 h.writeCond.Broadcast()
246 func (h *handshake) writer() {
250 h.writerCond.Broadcast()
256 if len(h.writes) != 0 {
266 h.writes = h.writes[1:]
268 _, err := h.conn.Write(b)
278 func (h *handshake) postWrite(b []byte) error {
280 defer h.writeMu.Unlock()
281 if h.writeErr != nil {
284 h.writes = append(h.writes, b)
289 func xor(dst, src []byte) (ret []byte) {
294 ret = make([]byte, 0, max)
295 for i := range iter.N(max) {
296 ret = append(ret, dst[i]^src[i])
301 func marshal(w io.Writer, data ...interface{}) (err error) {
302 for _, data := range data {
303 err = binary.Write(w, binary.BigEndian, data)
311 func unmarshal(r io.Reader, data ...interface{}) (err error) {
312 for _, data := range data {
313 err = binary.Read(r, binary.BigEndian, data)
321 // Looking for b at the end of a.
322 func suffixMatchLen(a, b []byte) int {
326 // i is how much of b to try to match
327 for i := len(b); i > 0; i-- {
328 // j is how many chars we've compared
331 if b[i-1-j] != a[len(a)-1-j] {
341 // Reads from r until b has been seen. Keeps the minimum amount of data in
343 func readUntil(r io.Reader, b []byte) error {
344 b1 := make([]byte, len(b))
347 _, err := io.ReadFull(r, b1[i:])
351 i = suffixMatchLen(b1, b)
355 if copy(b1, b1[len(b1)-i:]) != i {
362 type readWriter struct {
367 func (h *handshake) newEncrypt(initer bool) *rc4.Cipher {
368 return newEncrypt(initer, h.s[:], h.skey)
371 func (h *handshake) initerSteps() (ret io.ReadWriter, selected CryptoMethod, err error) {
372 h.postWrite(hash(req1, h.s[:]))
373 h.postWrite(xor(hash(req2, h.skey), hash(req3, h.s[:])))
374 buf := &bytes.Buffer{}
375 padLen := uint16(newPadLen())
376 if len(h.ia) > math.MaxUint16 {
377 err = errors.New("initial payload too large")
380 err = marshal(buf, vc[:], h.cryptoProvides, padLen, zeroPad[:padLen], uint16(len(h.ia)), h.ia)
384 e := h.newEncrypt(true)
385 be := make([]byte, buf.Len())
386 e.XORKeyStream(be, buf.Bytes())
388 bC := h.newEncrypt(false)
390 bC.XORKeyStream(eVC[:], vc[:])
391 // Read until the all zero VC. At this point we've only read the 96 byte
392 // public key, Y. There is potentially 512 byte padding, between us and
393 // the 8 byte verification constant.
394 err = readUntil(io.LimitReader(h.conn, 520), eVC[:])
397 err = errors.New("failed to synchronize on VC")
399 err = fmt.Errorf("error reading until VC: %s", err)
403 r := newCipherReader(bC, h.conn)
404 var method CryptoMethod
405 err = unmarshal(r, &method, &padLen)
409 _, err = io.CopyN(ioutil.Discard, r, int64(padLen))
413 selected = method & h.cryptoProvides
415 case CryptoMethodRC4:
416 ret = readWriter{r, &cipherWriter{e, h.conn, nil}}
417 case CryptoMethodPlaintext:
420 err = fmt.Errorf("receiver chose unsupported method: %x", method)
425 var ErrNoSecretKeyMatch = errors.New("no skey matched")
427 func (h *handshake) receiverSteps() (ret io.ReadWriter, chosen CryptoMethod, err error) {
428 // There is up to 512 bytes of padding, then the 20 byte hash.
429 err = readUntil(io.LimitReader(h.conn, 532), hash(req1, h.s[:]))
432 err = errors.New("failed to synchronize on S hash")
437 _, err = io.ReadFull(h.conn, b[:])
441 err = ErrNoSecretKeyMatch
442 h.skeys(func(skey []byte) bool {
443 if bytes.Equal(xor(hash(req2, skey), hash(req3, h.s[:])), b[:]) {
453 r := newCipherReader(newEncrypt(true, h.s[:], h.skey), h.conn)
456 provides CryptoMethod
460 err = unmarshal(r, vc[:], &provides, &padLen)
464 cryptoProvidesCount.Add(strconv.FormatUint(uint64(provides), 16), 1)
465 chosen = h.chooseMethod(provides)
466 _, err = io.CopyN(ioutil.Discard, r, int64(padLen))
473 h.ia = make([]byte, lenIA)
476 buf := &bytes.Buffer{}
477 w := cipherWriter{h.newEncrypt(false), buf, nil}
478 padLen = uint16(newPadLen())
479 err = marshal(&w, &vc, uint32(chosen), padLen, zeroPad[:padLen])
483 err = h.postWrite(buf.Bytes())
488 case CryptoMethodRC4:
490 io.MultiReader(bytes.NewReader(h.ia), r),
491 &cipherWriter{w.c, h.conn, nil},
493 case CryptoMethodPlaintext:
495 io.MultiReader(bytes.NewReader(h.ia), h.conn),
499 err = errors.New("chosen crypto method is not supported")
504 func (h *handshake) Do() (ret io.ReadWriter, method CryptoMethod, err error) {
505 h.writeCond.L = &h.writeMu
506 h.writerCond.L = &h.writerMu
516 err = fmt.Errorf("error while establishing secret: %s", err)
519 pad := make([]byte, newPadLen())
520 io.ReadFull(rand.Reader, pad)
521 err = h.postWrite(pad)
526 ret, method, err = h.initerSteps()
528 ret, method, err = h.receiverSteps()
533 func InitiateHandshake(rw io.ReadWriter, skey []byte, initialPayload []byte, cryptoProvides CryptoMethod) (ret io.ReadWriter, method CryptoMethod, err error) {
539 cryptoProvides: cryptoProvides,
541 defer perf.ScopeTimerErr(&err)()
545 type HandshakeResult struct {
552 func ReceiveHandshake(rw io.ReadWriter, skeys SecretKeyIter, selectCrypto CryptoSelector) (io.ReadWriter, CryptoMethod, error) {
553 res := ReceiveHandshakeEx(rw, skeys, selectCrypto)
554 return res.ReadWriter, res.CryptoMethod, res.error
557 func ReceiveHandshakeEx(rw io.ReadWriter, skeys SecretKeyIter, selectCrypto CryptoSelector) (ret HandshakeResult) {
562 chooseMethod: selectCrypto,
564 ret.ReadWriter, ret.CryptoMethod, ret.error = h.Do()
565 ret.SecretKey = h.skey
569 // A function that given a function, calls it with secret keys until it
570 // returns false or exhausted.
571 type SecretKeyIter func(callback func(skey []byte) (more bool))
573 func DefaultCryptoSelector(provided CryptoMethod) CryptoMethod {
574 // We prefer plaintext for performance reasons.
575 if provided&CryptoMethodPlaintext != 0 {
576 return CryptoMethodPlaintext
578 return CryptoMethodRC4
581 type CryptoSelector func(CryptoMethod) CryptoMethod