obfs4: Alter tear down behavior to be less distinctive

The old behavior closed the connection on handshake failure after:
 * The first N bytes (random on a per-server basis).
 * The first M seconds (random on a per-server basis).

Whichever came first.  As Sergey Frolov kindly points out, depending on
which conditions cause termination, the server will send either a FIN or
a RST.  This change will remove the "amount read" based termination
threshold, so that connections that cause failed handshakes will discard
all data received until the teardown time is reached.

Thanks to Sergey Frolov for bringing this issue to my attention.

ChangeLog

@@ -1,5 +1,7 @@
 Changes in version 0.0.11 - UNRELEASED:
  - Update my e-mail address.
+ - Change the obfs4 behavior for handling handshake failure to be more
+   uniform.  Thanks to Sergey Frolov for assistance.
 
 Changes in version 0.0.10 - 2019-04-12:
  - Disable behavior distinctive to crypto/tls when using utls.

transports/obfs4/obfs4.go

@@ -34,6 +34,8 @@ import (
 	"crypto/sha256"
 	"flag"
 	"fmt"
+	"io"
+	"io/ioutil"
 	"math/rand"
 	"net"
 	"strconv"
@@ -67,9 +69,8 @@ const (
 	serverHandshakeTimeout = time.Duration(30) * time.Second
 	replayTTL              = time.Duration(3) * time.Hour
 
-	maxIATDelay        = 100
-	maxCloseDelayBytes = maxHandshakeLength
-	maxCloseDelay      = 60
+	maxIATDelay   = 100
+	maxCloseDelay = 60
 )
 
 const (
@@ -138,7 +139,7 @@ func (t *Transport) ServerFactory(stateDir string, args *pt.Args) (base.ServerFa
 	}
 	rng := rand.New(drbg)
 
-	sf := &obfs4ServerFactory{t, &ptArgs, st.nodeID, st.identityKey, st.drbgSeed, iatSeed, st.iatMode, filter, rng.Intn(maxCloseDelayBytes), rng.Intn(maxCloseDelay)}
+	sf := &obfs4ServerFactory{t, &ptArgs, st.nodeID, st.identityKey, st.drbgSeed, iatSeed, st.iatMode, filter, rng.Intn(maxCloseDelay)}
 	return sf, nil
 }
 
@@ -233,8 +234,7 @@ type obfs4ServerFactory struct {
 	iatMode      int
 	replayFilter *replayfilter.ReplayFilter
 
-	closeDelayBytes int
-	closeDelay      int
+	closeDelay int
 }
 
 func (sf *obfs4ServerFactory) Transport() base.Transport {
@@ -592,17 +592,9 @@ func (conn *obfs4Conn) closeAfterDelay(sf *obfs4ServerFactory, startTime time.Ti
 		return
 	}
 
-	// Consume and discard data on this connection until either the specified
-	// interval passes or a certain size has been reached.
-	discarded := 0
-	var buf [framing.MaximumSegmentLength]byte
-	for discarded < int(sf.closeDelayBytes) {
-		n, err := conn.Conn.Read(buf[:])
-		if err != nil {
-			return
-		}
-		discarded += n
-	}
+	// Consume and discard data on this connection until the specified interval
+	// passes.
+	_, _ = io.Copy(ioutil.Discard, conn.Conn)
 }
 
 func (conn *obfs4Conn) padBurst(burst *bytes.Buffer, toPadTo int) (err error) {