1 // https://wiki.vuze.com/w/Message_Stream_Encryption
20 "github.com/bradfitz/iter"
22 "github.com/anacrolix/torrent/util"
28 cryptoMethodPlaintext = 1
33 // Prime P according to the spec, and G, the generator.
35 // The rand.Int max arg for use in newPadLen()
37 // For use in initer's hashes
41 // Verification constant "VC" which is all zeroes in the bittorrent
46 // Tracks counts of received crypto_provides
47 cryptoProvidesCount = expvar.NewMap("mseCryptoProvides")
51 p.SetString("0xFFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A63A36210000000000090563", 0)
53 newPadLenMax.SetInt64(maxPadLen + 1)
56 func hash(parts ...[]byte) []byte {
58 for _, p := range parts {
70 func newEncrypt(initer bool, s []byte, skey []byte) (c *rc4.Cipher) {
71 c, err := rc4.NewCipher(hash([]byte(func() string {
81 var burnSrc, burnDst [1024]byte
82 c.XORKeyStream(burnDst[:], burnSrc[:])
86 type cipherReader struct {
91 func (me *cipherReader) Read(b []byte) (n int, err error) {
92 be := make([]byte, len(b))
93 n, err = me.r.Read(be)
94 me.c.XORKeyStream(b[:n], be[:n])
98 func newCipherReader(c *rc4.Cipher, r io.Reader) io.Reader {
99 return &cipherReader{c, r}
102 type cipherWriter struct {
107 func (me *cipherWriter) Write(b []byte) (n int, err error) {
108 be := make([]byte, len(b))
109 me.c.XORKeyStream(be, b)
110 n, err = me.w.Write(be)
112 // The cipher will have advanced beyond the callers stream position.
113 // We can't use the cipher anymore.
119 func readY(r io.Reader) (y big.Int, err error) {
121 _, err = io.ReadFull(r, b[:])
129 func newX() big.Int {
131 X.SetBytes(func() []byte {
133 _, err := rand.Read(b[:])
142 func paddedLeft(b []byte, _len int) []byte {
146 ret := make([]byte, _len)
147 if n := copy(ret[_len-len(b):], b); n != len(b) {
153 // Calculate, and send Y, our public key.
154 func (h *handshake) postY(x *big.Int) error {
157 return h.postWrite(paddedLeft(y.Bytes(), 96))
160 func (h *handshake) establishS() (err error) {
164 _, err = io.ReadFull(h.conn, b[:])
171 util.CopyExact(&h.s, paddedLeft(S.Bytes(), 96))
175 func newPadLen() int64 {
176 i, err := rand.Int(rand.Reader, &newPadLenMax)
181 if ret < 0 || ret > maxPadLen {
187 type handshake struct {
193 ia []byte // Initial payload. Only used by the initiator.
206 func (h *handshake) finishWriting() {
209 h.writeCond.Broadcast()
220 func (h *handshake) writer() {
224 h.writerCond.Broadcast()
230 if len(h.writes) != 0 {
240 h.writes = h.writes[1:]
242 _, err := h.conn.Write(b)
252 func (h *handshake) postWrite(b []byte) error {
254 defer h.writeMu.Unlock()
255 if h.writeErr != nil {
258 h.writes = append(h.writes, b)
263 func xor(dst, src []byte) (ret []byte) {
268 ret = make([]byte, 0, max)
269 for i := range iter.N(max) {
270 ret = append(ret, dst[i]^src[i])
275 func marshal(w io.Writer, data ...interface{}) (err error) {
276 for _, data := range data {
277 err = binary.Write(w, binary.BigEndian, data)
285 func unmarshal(r io.Reader, data ...interface{}) (err error) {
286 for _, data := range data {
287 err = binary.Read(r, binary.BigEndian, data)
295 // Looking for b at the end of a.
296 func suffixMatchLen(a, b []byte) int {
300 // i is how much of b to try to match
301 for i := len(b); i > 0; i-- {
302 // j is how many chars we've compared
305 if b[i-1-j] != a[len(a)-1-j] {
315 func readUntil(r io.Reader, b []byte) error {
316 b1 := make([]byte, len(b))
319 _, err := io.ReadFull(r, b1[i:])
323 i = suffixMatchLen(b1, b)
327 if copy(b1, b1[len(b1)-i:]) != i {
334 type readWriter struct {
339 func (h *handshake) newEncrypt(initer bool) *rc4.Cipher {
340 return newEncrypt(initer, h.s[:], h.skey)
343 func (h *handshake) initerSteps() (ret io.ReadWriter, err error) {
344 h.postWrite(hash(req1, h.s[:]))
345 h.postWrite(xor(hash(req2, h.skey), hash(req3, h.s[:])))
346 buf := &bytes.Buffer{}
347 padLen := uint16(newPadLen())
348 err = marshal(buf, vc[:], uint32(cryptoMethodRC4), padLen, zeroPad[:padLen], uint16(len(h.ia)), h.ia)
352 e := h.newEncrypt(true)
353 be := make([]byte, buf.Len())
354 e.XORKeyStream(be, buf.Bytes())
356 bC := h.newEncrypt(false)
358 bC.XORKeyStream(eVC[:], vc[:])
359 // Read until the all zero VC. At this point we've only read the 96 byte
360 // public key, Y. There is potentially 512 byte padding, between us and
361 // the 8 byte verification constant.
362 err = readUntil(io.LimitReader(h.conn, 520), eVC[:])
365 err = errors.New("failed to synchronize on VC")
367 err = fmt.Errorf("error reading until VC: %s", err)
371 r := &cipherReader{bC, h.conn}
373 err = unmarshal(r, &method, &padLen)
377 if method != cryptoMethodRC4 {
378 err = fmt.Errorf("receiver chose unsupported method: %x", method)
381 _, err = io.CopyN(ioutil.Discard, r, int64(padLen))
385 ret = readWriter{r, &cipherWriter{e, h.conn}}
389 var ErrNoSecretKeyMatch = errors.New("no skey matched")
391 func (h *handshake) receiverSteps() (ret io.ReadWriter, err error) {
392 // There is up to 512 bytes of padding, then the 20 byte hash.
393 err = readUntil(io.LimitReader(h.conn, 532), hash(req1, h.s[:]))
396 err = errors.New("failed to synchronize on S hash")
401 _, err = io.ReadFull(h.conn, b[:])
405 err = ErrNoSecretKeyMatch
406 for _, skey := range h.skeys {
407 if bytes.Equal(xor(hash(req2, skey), hash(req3, h.s[:])), b[:]) {
416 r := newCipherReader(newEncrypt(true, h.s[:], h.skey), h.conn)
423 err = unmarshal(r, vc[:], &method, &padLen)
427 cryptoProvidesCount.Add(strconv.FormatUint(uint64(method), 16), 1)
428 if method&cryptoMethodRC4 == 0 {
429 err = errors.New("no supported crypto methods were provided")
432 _, err = io.CopyN(ioutil.Discard, r, int64(padLen))
439 h.ia = make([]byte, lenIA)
442 buf := &bytes.Buffer{}
443 w := cipherWriter{h.newEncrypt(false), buf}
444 padLen = uint16(newPadLen())
445 err = marshal(&w, &vc, uint32(cryptoMethodRC4), padLen, zeroPad[:padLen])
449 err = h.postWrite(buf.Bytes())
453 ret = readWriter{io.MultiReader(bytes.NewReader(h.ia), r), &cipherWriter{w.c, h.conn}}
457 func (h *handshake) Do() (ret io.ReadWriter, err error) {
458 h.writeCond.L = &h.writeMu
459 h.writerCond.L = &h.writerMu
469 err = fmt.Errorf("error while establishing secret: %s", err)
472 pad := make([]byte, newPadLen())
473 io.ReadFull(rand.Reader, pad)
474 err = h.postWrite(pad)
479 ret, err = h.initerSteps()
481 ret, err = h.receiverSteps()
486 func InitiateHandshake(rw io.ReadWriter, skey []byte, initialPayload []byte) (ret io.ReadWriter, err error) {
495 func ReceiveHandshake(rw io.ReadWriter, skeys [][]byte) (ret io.ReadWriter, err error) {