mixpool: Reject KEs submitted too early

mixing/mixpool/mixpool.go

@@ -28,6 +28,7 @@ import (
 
 const minconf = 1
 const feeRate = 0.0001e8
+const clientTimeoutDuration = 30 * time.Second
 
 type idPubKey = [33]byte
 
@@ -1394,6 +1395,12 @@ func (p *Pool) checkAcceptKE(ke *wire.MsgMixKeyExchange) error {
 		return ruleError(ErrPeerPositionOutOfBounds)
 	}
 
+	keEpoch := time.Unix(int64(ke.Epoch), 0)
+	if keEpoch.Add(clientTimeoutDuration).Before(time.Now()) {
+		err := fmt.Errorf("KE received too early for stated epoch")
+		return ruleError(err)
+	}
+
 	return nil
 }