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

@@ -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();
 

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() {