Change the server failure close delay/discard length to be consistent.

It will vary per bridge as it is based off the DRBG, but ever attempt
at poking at any given bridge will exhibit consistent behavior.

obfs4.go

@@ -44,10 +44,8 @@ const (
 	headerLength      = framing.FrameOverhead + packetOverhead
 	connectionTimeout = time.Duration(30) * time.Second
 
-	minCloseThreshold = 0
-	maxCloseThreshold = framing.MaximumSegmentLength * 5
-	minCloseInterval  = 0
-	maxCloseInterval  = 60
+	maxCloseDelayBytes = framing.MaximumSegmentLength * 5
+	maxCloseDelay  = 60
 )
 
 type connState int
@@ -77,6 +75,9 @@ type Obfs4Conn struct {
 
 	// Server side state.
 	listener *Obfs4Listener
+	startTime time.Time
+	closeDelayBytes int
+	closeDelay int
 }
 
 func (c *Obfs4Conn) padBurst(burst *bytes.Buffer) (err error) {
@@ -116,11 +117,13 @@ func (c *Obfs4Conn) closeAfterDelay() {
 	// I-it's not like I w-wanna handshake with you or anything.  B-b-baka!
 	defer c.conn.Close()
 
-	delaySecs := randRange(minCloseInterval, maxCloseInterval)
-	toDiscard := randRange(minCloseThreshold, maxCloseThreshold)
+	delay := time.Duration(c.closeDelay) * time.Second
+	deadline := c.startTime.Add(delay)
+	if time.Now().After(deadline) {
+		return
+	}
 
-	delay := time.Duration(delaySecs) * time.Second
-	err := c.conn.SetReadDeadline(time.Now().Add(delay))
+	err := c.conn.SetReadDeadline(deadline)
 	if err != nil {
 		return
 	}
@@ -129,7 +132,7 @@ func (c *Obfs4Conn) closeAfterDelay() {
 	// interval passes or a certain size has been reached.
 	discarded := 0
 	var buf [framing.MaximumSegmentLength]byte
-	for discarded < int(toDiscard) {
+	for discarded < int(c.closeDelayBytes) {
 		n, err := c.conn.Read(buf[:])
 		if err != nil {
 			return
@@ -324,9 +327,6 @@ func (c *Obfs4Conn) ServerHandshake() error {
 	err := c.serverHandshake(c.listener.nodeID, c.listener.keyPair)
 	c.listener = nil
 	if err != nil {
-		// XXX: Maybe make the timeout period deterministic, since random
-		// hangup intervals are also suspicious.  An ok value would be someting
-		// like the Nginx client_header_timeout (60s).
 		c.closeAfterDelay()
 	}
 
@@ -544,6 +544,9 @@ func (l *Obfs4Listener) Accept() (net.Conn, error) {
 		c.Close()
 		return nil, err
 	}
+	cObfs.startTime = time.Now()
+	cObfs.closeDelayBytes = cObfs.lenProbDist.rng.Intn(maxCloseDelayBytes)
+	cObfs.closeDelay = cObfs.lenProbDist.rng.Intn(maxCloseDelay)
 
 	return cObfs, nil
 }

weighted_dist.go

@@ -136,6 +136,8 @@ type wDist struct {
 	maxValue int
 	values   []int
 	buckets  []float64
+
+	rng *rand.Rand
 }
 
 // newWDist creates a weighted distribution of values ranging from min to max
@@ -174,15 +176,15 @@ func (w *wDist) sample() int {
 func (w *wDist) reset(seed *DrbgSeed) {
 	// Initialize the deterministic random number generator.
 	drbg := newHashDrbg(seed)
-	dRng := rand.New(drbg)
+	w.rng = rand.New(drbg)
 
 	nBuckets := (w.maxValue + 1) - w.minValue
-	w.values = dRng.Perm(nBuckets)
+	w.values = w.rng.Perm(nBuckets)
 
 	w.buckets = make([]float64, nBuckets)
 	var totalProb float64
 	for i, _ := range w.buckets {
-		prob := dRng.Float64() * (1.0 - totalProb)
+		prob := w.rng.Float64() * (1.0 - totalProb)
 		w.buckets[i] = prob
 		totalProb += prob
 	}