From c94a7b1eff718355417ea49ba98dcd3ff021ca84 Mon Sep 17 00:00:00 2001
From: Moxie Marlinspike <moxie@thoughtcrime.org>
Date: Tue, 22 Jul 2014 17:48:02 -0700
Subject: [PATCH] Make sure "previous counter" is never negative.

---
 .../main/java/org/whispersystems/libaxolotl/SessionCipher.java  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libaxolotl/src/main/java/org/whispersystems/libaxolotl/SessionCipher.java b/libaxolotl/src/main/java/org/whispersystems/libaxolotl/SessionCipher.java
index 2a70a80951..edf2c7c4db 100644
--- a/libaxolotl/src/main/java/org/whispersystems/libaxolotl/SessionCipher.java
+++ b/libaxolotl/src/main/java/org/whispersystems/libaxolotl/SessionCipher.java
@@ -261,7 +261,7 @@ public class SessionCipher {
 
         sessionState.setRootKey(senderChain.first());
         sessionState.addReceiverChain(theirEphemeral, receiverChain.second());
-        sessionState.setPreviousCounter(sessionState.getSenderChainKey().getIndex()-1);
+        sessionState.setPreviousCounter(Math.max(sessionState.getSenderChainKey().getIndex()-1, 0));
         sessionState.setSenderChain(ourNewEphemeral, senderChain.second());
 
         return receiverChain.second();