From ec735edde187a43693197f6fa3667ceade751a3a Mon Sep 17 00:00:00 2001
From: Fedor Indutny <fedor@indutny.com>
Date: Fri, 22 Nov 2019 11:01:25 -0800
Subject: [PATCH] utils: leak less information in `getNAF()`

---
 lib/elliptic/curve/base.js | 10 ++++++----
 lib/elliptic/utils.js      | 19 +++++++++----------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/lib/elliptic/curve/base.js b/lib/elliptic/curve/base.js
index 3ce313a..83ffe9f 100644
--- a/lib/elliptic/curve/base.js
+++ b/lib/elliptic/curve/base.js
@@ -28,6 +28,8 @@ function BaseCurve(type, conf) {
   this._wnafT3 = new Array(4);
   this._wnafT4 = new Array(4);
 
+  this._bitLength = this.n ? this.n.bitLength() : 0;
+
   // Generalized Greg Maxwell's trick
   var adjustCount = this.n && this.p.div(this.n);
   if (!adjustCount || adjustCount.cmpn(100) > 0) {
@@ -51,7 +53,7 @@ BaseCurve.prototype._fixedNafMul = function _fixedNafMul(p, k) {
   assert(p.precomputed);
   var doubles = p._getDoubles();
 
-  var naf = getNAF(k, 1);
+  var naf = getNAF(k, 1, this._bitLength);
   var I = (1 << (doubles.step + 1)) - (doubles.step % 2 === 0 ? 2 : 1);
   I /= 3;
 
@@ -88,7 +90,7 @@ BaseCurve.prototype._wnafMul = function _wnafMul(p, k) {
   var wnd = nafPoints.points;
 
   // Get NAF form
-  var naf = getNAF(k, w);
+  var naf = getNAF(k, w, this._bitLength);
 
   // Add `this`*(N+1) for every w-NAF index
   var acc = this.jpoint(null, null, null);
@@ -144,8 +146,8 @@ BaseCurve.prototype._wnafMulAdd = function _wnafMulAdd(defW,
     var a = i - 1;
     var b = i;
     if (wndWidth[a] !== 1 || wndWidth[b] !== 1) {
-      naf[a] = getNAF(coeffs[a], wndWidth[a]);
-      naf[b] = getNAF(coeffs[b], wndWidth[b]);
+      naf[a] = getNAF(coeffs[a], wndWidth[a], this._bitLength);
+      naf[b] = getNAF(coeffs[b], wndWidth[b], this._bitLength);
       max = Math.max(naf[a].length, max);
       max = Math.max(naf[b].length, max);
       continue;
diff --git a/lib/elliptic/utils.js b/lib/elliptic/utils.js
index 9827381..f6d5616 100644
--- a/lib/elliptic/utils.js
+++ b/lib/elliptic/utils.js
@@ -12,14 +12,17 @@ utils.toHex = minUtils.toHex;
 utils.encode = minUtils.encode;
 
 // Represent num in a w-NAF form
-function getNAF(num, w) {
-  var naf = [];
+function getNAF(num, w, bits) {
+  var naf = new Array(Math.max(num.bitLength(), bits) + 1);
+  naf.fill(0);
+
   var ws = 1 << (w + 1);
   var k = num.clone();
-  while (k.cmpn(1) >= 0) {
+
+  for (var i = 0; i < naf.length; i++) {
     var z;
+    var mod = k.andln(ws - 1);
     if (k.isOdd()) {
-      var mod = k.andln(ws - 1);
       if (mod > (ws >> 1) - 1)
         z = (ws >> 1) - mod;
       else
@@ -28,13 +31,9 @@ function getNAF(num, w) {
     } else {
       z = 0;
     }
-    naf.push(z);
 
-    // Optimization, shift by word if possible
-    var shift = (k.cmpn(0) !== 0 && k.andln(ws - 1) === 0) ? (w + 1) : 1;
-    for (var i = 1; i < shift; i++)
-      naf.push(0);
-    k.iushrn(shift);
+    naf[i] = z;
+    k.iushrn(1);
   }
 
   return naf;