From f11ed4d20b3f18dd21f15d8454051bd7c88a918d Mon Sep 17 00:00:00 2001
From: "Ryan X. Charles" <ryan@bitpay.com>
Date: Fri, 15 Aug 2014 15:09:28 -0400
Subject: [PATCH] bug: should mod bn addition

when adding two private keys to get a new private key, you should mod the
result with N so that it is always less than N.
---
 lib/expmt/stealth.js | 3 ++-
 test/test.stealth.js | 5 +++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/expmt/stealth.js b/lib/expmt/stealth.js
index 87d0431..20fe407 100644
--- a/lib/expmt/stealth.js
+++ b/lib/expmt/stealth.js
@@ -1,6 +1,7 @@
 var Key = require('../key');
 var Privkey = require('../privkey');
 var Pubkey = require('../pubkey');
+var Point = require('../point');
 var Hash = require('../hash');
 var KDF = require('../kdf');
 var base58check = require('../base58check');
@@ -76,7 +77,7 @@ Stealth.prototype.getReceivePubkeyAsSender = function(senderKey) {
 
 Stealth.prototype.getReceiveKey = function(senderPubkey) {
   var sharedKey = this.getSharedKeyAsReceiver(senderPubkey);
-  var privkey = Privkey(this.payloadKey.privkey.bn.add(sharedKey.privkey.bn));
+  var privkey = Privkey(this.payloadKey.privkey.bn.add(sharedKey.privkey.bn).mod(Point.getN()));
   var key = Key(privkey);
   key.privkey2pubkey();
 
diff --git a/test/test.stealth.js b/test/test.stealth.js
index 7ebbf7d..6bacacd 100644
--- a/test/test.stealth.js
+++ b/test/test.stealth.js
@@ -141,6 +141,11 @@ describe('Stealth', function() {
       key.pubkey.toString().should.equal(pubkey.toString());
     });
 
+    it('should return private key with length 32 or less', function() {
+      var key = stealth.getReceiveKey(senderKey.pubkey);
+      key.privkey.bn.toBuffer().length.should.be.below(33);
+    });
+
   });
 
   describe('#isForMe', function() {