Fix negating a zero private key in JS, add more tests

Author: kripod

lib/elliptic/index.js

@@ -74,7 +74,7 @@ exports.privateKeyExport = function (privateKey, compressed) {
 
 exports.privateKeyNegate = function (privateKey) {
   var bn = new BN(privateKey)
-  return ecparams.n.sub(bn).toArrayLike(Buffer, 'be', 32)
+  return bn.isZero() ? Buffer.alloc(32) : ecparams.n.sub(bn).toArrayLike(Buffer, 'be', 32)
 }
 
 exports.privateKeyModInverse = function (privateKey) {

lib/js/index.js

@@ -20,7 +20,8 @@ exports.privateKeyExport = function (privateKey, compressed) {
 }
 
 exports.privateKeyNegate = function (privateKey) {
-  return BN.n.sub(BN.fromBuffer(privateKey)).toBuffer()
+  var bn = BN.fromBuffer(privateKey)
+  return bn.isZero() ? Buffer.alloc(32) : BN.n.sub(bn).toBuffer()
 }
 
 exports.privateKeyModInverse = function (privateKey) {

test/privatekey.js

@@ -129,6 +129,26 @@ module.exports = function (t, secp256k1) {
       t.end()
     })
 
+    t.test('private key is 0', function (t) {
+      var privateKey = util.BN_ZERO.toArrayLike(Buffer, 'be', 32)
+
+      var expected = Buffer.alloc(32)
+      var result = secp256k1.privateKeyNegate(privateKey)
+      t.same(result, expected)
+
+      t.end()
+    })
+
+    t.test('private key equal to N', function (t) {
+      var privateKey = util.ec.curve.n.toArrayLike(Buffer, 'be', 32)
+
+      var expected = Buffer.alloc(32)
+      var result = secp256k1.privateKeyNegate(privateKey)
+      t.same(result, expected)
+
+      t.end()
+    })
+
     util.repeat(t, 'random tests', util.env.repeat, function (t) {
       var privateKey = util.getPrivateKey()