[Pkg-javascript-commits] [node-miller-rabin] 01/08: lib: sample bases from the range [2, n-1)

[Bastien Roucariès]

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rouca pushed a commit to branch master
in repository node-miller-rabin.

commit a84666c2521f33709ee6c553abbe112a8dd90680
Author: Jared Deckard <jared.deckard at gmail.com>
Date:   Tue Sep 26 11:27:07 2017 -0500

    lib: sample bases from the range [2, n-1)
---
 lib/mr.js | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/lib/mr.js b/lib/mr.js
index a9e935b..60d2a8e 100644
--- a/lib/mr.js
+++ b/lib/mr.js
@@ -10,20 +10,24 @@ MillerRabin.create = function create(rand) {
   return new MillerRabin(rand);
 };
 
-MillerRabin.prototype._rand = function _rand(n) {
+MillerRabin.prototype._randbelow = function _randbelow(n) {
   var len = n.bitLength();
-  var buf = this.rand.generate(Math.ceil(len / 8));
+  var min_bytes = Math.ceil(len / 8);
 
-  // Set low bits
-  buf[0] |= 3;
+  // Generage random bytes until a number less than n is found.
+  // This ensures that 0..n-1 have an equal probability of being selected.
+  do
+    var a = new bn(this.rand.generate(min_bytes));
+  while (a.cmp(n) >= 0);
 
-  // Mask high bits
-  var mask = len & 0x7;
-  if (mask !== 0)
-    buf[buf.length - 1] >>= 7 - mask;
+  return a;
+};
 
-  return new bn(buf);
-}
+MillerRabin.prototype._randrange = function _randrange(start, stop) {
+  // Generate a random number greater than or equal to start and less than stop.
+  var size = stop.sub(start);
+  return start.add(this._randbelow(size));
+};
 
 MillerRabin.prototype.test = function test(n, k, cb) {
   var len = n.bitLength();
@@ -35,7 +39,6 @@ MillerRabin.prototype.test = function test(n, k, cb) {
 
   // Find d and s, (n - 1) = (2 ^ s) * d;
   var n1 = n.subn(1);
-  var n2 = n1.subn(1);
   for (var s = 0; !n1.testn(s); s++) {}
   var d = n.shrn(s);
 
@@ -43,7 +46,7 @@ MillerRabin.prototype.test = function test(n, k, cb) {
 
   var prime = true;
   for (; k > 0; k--) {
-    var a = this._rand(n2);
+    var a = this._randrange(new bn(2), n1);
     if (cb)
       cb(a);
 
@@ -77,14 +80,13 @@ MillerRabin.prototype.getDivisor = function getDivisor(n, k) {
 
   // Find d and s, (n - 1) = (2 ^ s) * d;
   var n1 = n.subn(1);
-  var n2 = n1.subn(1);
   for (var s = 0; !n1.testn(s); s++) {}
   var d = n.shrn(s);
 
   var rn1 = n1.toRed(red);
 
   for (; k > 0; k--) {
-    var a = this._rand(n2);
+    var a = this._randrange(new bn(2), n1);
 
     var g = n.gcd(a);
     if (g.cmpn(1) !== 0)

-- 
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