Tweak the obfs4 handshake code.

 * Fixed where the code wasn't ensuring that the MAC_[C,S] was present.
 * Optimized the server side to only look at the tail of the (possibly
   incomplete handshakeRequest).

handshake_ntor.go

@@ -166,15 +166,15 @@ func (hs *clientHandshake) parseServerHandshake(resp []byte) (int, []byte, error
 		hs.serverAuth = new(ntor.Auth)
 		copy(hs.serverAuth.Bytes()[:], resp[ntor.RepresentativeLength:])
 
-		// Derive the mark
+		// Derive the mark.
 		hs.mac.Reset()
 		hs.mac.Write(hs.serverRepresentative.Bytes()[:])
 		hs.serverMark = hs.mac.Sum(nil)[:markLength]
 	}
 
 	// Attempt to find the mark + MAC.
-	pos := findMark(hs.serverMark, resp, ntor.RepresentativeLength+ntor.AuthLength+serverMinPadLength,
-		serverMaxHandshakeLength)
+	pos := findMarkMac(hs.serverMark, resp, ntor.RepresentativeLength+ntor.AuthLength+serverMinPadLength,
+		serverMaxHandshakeLength, false)
 	if pos == -1 {
 		if len(resp) >= serverMaxHandshakeLength {
 			return 0, nil, ErrInvalidHandshake
@@ -240,15 +240,15 @@ func (hs *serverHandshake) parseClientHandshake(resp []byte) ([]byte, error) {
 		hs.clientRepresentative = new(ntor.Representative)
 		copy(hs.clientRepresentative.Bytes()[:], resp[0:ntor.RepresentativeLength])
 
-		// Derive the mark
+		// Derive the mark.
 		hs.mac.Reset()
 		hs.mac.Write(hs.clientRepresentative.Bytes()[:])
 		hs.clientMark = hs.mac.Sum(nil)[:markLength]
 	}
 
 	// Attempt to find the mark + MAC.
-	pos := findMark(hs.clientMark, resp, ntor.RepresentativeLength+clientMinPadLength,
-		serverMaxHandshakeLength)
+	pos := findMarkMac(hs.clientMark, resp, ntor.RepresentativeLength+clientMinPadLength,
+		clientMaxHandshakeLength, true)
 	if pos == -1 {
 		if len(resp) >= clientMaxHandshakeLength {
 			return nil, ErrInvalidHandshake
@@ -351,23 +351,51 @@ func getEpochHour() int64 {
 	return time.Now().Unix() / 3600
 }
 
-func findMark(mark, buf []byte, startPos, maxPos int) int {
+func findMarkMac(mark, buf []byte, startPos, maxPos int, fromTail bool) (pos int) {
+	if len(mark) != markLength {
+		panic(fmt.Sprintf("BUG: Invalid mark length: %d", len(mark)))
+	}
+
 	endPos := len(buf)
+	if startPos > len(buf) {
+		return -1
+	}
 	if endPos > maxPos {
 		endPos = maxPos
 	}
-	if startPos > len(buf) {
+	if endPos - startPos < markLength + macLength {
 		return -1
 	}
 
+	if fromTail {
+		// The server can optimize the search process by only examining the
+		// tail of the buffer.  The client can't send valid data past M_C |
+		// MAC_C as it does not have the server's public key yet.
+		pos = endPos - (markLength + macLength)
+		if !hmac.Equal(buf[pos:pos+markLength], mark) {
+			return -1
+		}
+
+		return
+	}
+
+	// The client has to actually do a substring search since the server can
+	// and will send payload trailing the response.
+	//
 	// XXX: bytes.Index() uses a naive search, which kind of sucks.
-	pos := bytes.Index(buf[startPos:endPos], mark)
+	pos = bytes.Index(buf[startPos:endPos], mark)
 	if pos == -1 {
 		return -1
 	}
 
+	// Ensure that there is enough trailing data for the MAC.
+	if startPos + pos + markLength + macLength > endPos {
+		return -1
+	}
+
 	// Return the index relative to the start of the slice.
-	return pos + startPos
+	pos += startPos
+	return
 }
 
 func makePad(min, max int) ([]byte, error) {