Treat the PrngSeed frame as part of the handshake.

Clients will now always add 87 bytes of padding to the clientRequest,
and Servers will always send the PRNG seed frame unpadded, and bundled
with the serverResponse.

Why 87 bytes?  The amount of data that the server sends is 87.

This fixes #5.

2 files changed, 23 insertions, 9 deletions

diff --git a/handshake_ntor.go b/handshake_ntor.go
index 80a9698..b8fd222 100644
--- a/handshake_ntor.go
+++ b/handshake_ntor.go
@@ -44,19 +44,23 @@ import (
 )
 
 const (
-	clientMinPadLength       = serverMinHandshakeLength - clientMinHandshakeLength
+	clientMinPadLength = (serverMinHandshakeLength + inlineSeedFrameLength) -
+		clientMinHandshakeLength
 	clientMaxPadLength       = framing.MaximumSegmentLength - clientMinHandshakeLength
 	clientMinHandshakeLength = ntor.RepresentativeLength + markLength + macLength
 	clientMaxHandshakeLength = framing.MaximumSegmentLength
 
-	serverMinPadLength       = 0
-	serverMaxPadLength       = framing.MaximumSegmentLength - serverMinHandshakeLength
+	serverMinPadLength = 0
+	serverMaxPadLength = framing.MaximumSegmentLength - (serverMinHandshakeLength +
+		inlineSeedFrameLength)
 	serverMinHandshakeLength = ntor.RepresentativeLength + ntor.AuthLength +
 		markLength + macLength
 	serverMaxHandshakeLength = framing.MaximumSegmentLength
 
 	markLength = sha256.Size / 2
 	macLength  = sha256.Size / 2
+
+	inlineSeedFrameLength = framing.FrameOverhead + packetOverhead + seedPacketPayloadLength
 )
 
 var ErrMarkNotFoundYet = errors.New("handshake: M_[C,S] not found yet")
@@ -121,7 +125,7 @@ func (hs *clientHandshake) generateHandshake() ([]byte, error) {
 
 	// The client handshake is X | P_C | M_C | MAC(X | P_C | M_C | E) where:
 	//  * X is the client's ephemeral Curve25519 public key representative.
-	//  * P_C is [0,clientMaxPadLength] bytes of random padding.
+	//  * P_C is [clientMinPadLength,clientMaxPadLength] bytes of random padding.
 	//  * M_C is HMAC-SHA256-128(serverIdentity | NodeID, X)
 	//  * MAC is HMAC-SHA256-128(serverIdentity | NodeID, X .... E)
 	//  * E is the string representation of the number of hours since the UNIX
@@ -314,7 +318,7 @@ func (hs *serverHandshake) generateHandshake() ([]byte, error) {
 	// The server handshake is Y | AUTH | P_S | M_S | MAC(Y | AUTH | P_S | M_S | E) where:
 	//  * Y is the server's ephemeral Curve25519 public key representative.
 	//  * AUTH is the ntor handshake AUTH value.
-	//  * P_S is [0,serverMaxPadLength] bytes of random padding.
+	//  * P_S is [serverMinPadLength,serverMaxPadLength] bytes of random padding.
 	//  * M_S is HMAC-SHA256-128(serverIdentity | NodeID, Y)
 	//  * MAC is HMAC-SHA256-128(serverIdentity | NodeID, Y .... E)
 	//  * E is the string representation of the number of hours since the UNIX
diff --git a/obfs4.go b/obfs4.go
index f13e14a..52e1b02 100644
--- a/obfs4.go
+++ b/obfs4.go
@@ -263,29 +263,39 @@ func (c *Obfs4Conn) serverHandshake(nodeID *ntor.NodeID, keypair *ntor.Keypair)
 		break
 	}
 
+	//
+	// Since the current and only implementation always sends a PRNG seed for
+	// the length obfuscation, this makes the amount of data received from the
+	// server inconsistent with the length sent from the client.
+	//
+	// Rebalance this by tweaking the client mimimum padding/server maximum
+	// padding, and sending the PRNG seed unpadded (As in, treat the PRNG seed
+	// as part of the server response).  See inlineSeedFrameLength in
+	// handshake_ntor.go.
+	//
+
 	// Generate/send the response.
 	var blob []byte
 	blob, err = hs.generateHandshake()
 	if err != nil {
 		return
 	}
-	_, err = c.conn.Write(blob)
+	var frameBuf bytes.Buffer
+	_, err = frameBuf.Write(blob)
 	if err != nil {
 		return
 	}
 	c.state = stateEstablished
 
 	// Send the PRNG seed as the first packet.
-	var frameBuf bytes.Buffer
 	err = c.producePacket(&frameBuf, packetTypePrngSeed, c.listener.seed.Bytes()[:], 0)
 	if err != nil {
 		return
 	}
-	err = c.padBurst(&frameBuf)
+	_, err = c.conn.Write(frameBuf.Bytes())
 	if err != nil {
 		return
 	}
-	_, err = c.conn.Write(frameBuf.Bytes())
 
 	return
 }