Change hashDrbg to take a drbgSeed for initialization.

This paves the way for having servers use the same seed for all
incoming connections, across multiple startup/shutdown cycles.  As
opposed to the current situation where each Obfs4Listener will
randomly generate it's seed at creation time.

Additionally, use 256 bit seeds (128 bit SipHash-2-4 key + 16 bytes of
initial material).

1 files changed, 7 insertions, 2 deletions

diff --git a/packet.go b/packet.go
index 75179cb..2528a53 100644
--- a/packet.go
+++ b/packet.go
@@ -173,8 +173,13 @@ func (c *Obfs4Conn) consumeFramedPackets(w io.Writer) (n int, err error) {
 			}
 		case packetTypePrngSeed:
 			// Only regenerate the distribution if we are the client.
-			if len(payload) >= distSeedLength && !c.isServer {
-				c.lenProbDist.reset(payload[:distSeedLength])
+			if len(payload) >= drbgSeedLength && !c.isServer {
+				var seed *drbgSeed
+				seed, err = drbgSeedFromBytes(payload[:drbgSeedLength])
+				if err != nil {
+					break
+				}
+				c.lenProbDist.reset(seed)
 			}
 		default:
 			// Ignore unrecognised packet types.