Change the weighted distribution algorithm be uniform.

The old way was biasted towards the earlier values.  Thanks to asn for
pointing this out and suggesting an alternative.

As an additional tweak, do not reuse the drbg seed when calculating the
IAT distribution, but instead run the seed through SHA256 first, for
extra tinfoil goodness.

1 files changed, 7 insertions, 1 deletions

diff --git a/packet.go b/packet.go
index 78f6697..61ed981 100644
--- a/packet.go
+++ b/packet.go
@@ -28,6 +28,7 @@
 package obfs4
 
 import (
+	"crypto/sha256"
 	"encoding/binary"
 	"fmt"
 	"io"
@@ -182,7 +183,12 @@ func (c *Obfs4Conn) consumeFramedPackets(w io.Writer) (n int, err error) {
 				}
 				c.lenProbDist.reset(seed)
 				if c.iatProbDist != nil {
-					c.iatProbDist.reset(seed)
+					iatSeedSrc := sha256.Sum256(seed.Bytes()[:])
+					iatSeed, err := DrbgSeedFromBytes(iatSeedSrc[:])
+					if err != nil {
+						break
+					}
+					c.iatProbDist.reset(iatSeed)
 				}
 			}
 		default: