Merge pull request #650 from Kr0emer/fix/bug-006-modpow-negative-exponent

fix(jsbn2): handle negative exponents in BigInteger.modPow

Author: kjur

ext/jsbn2.js

@@ -411,6 +411,9 @@ Barrett.prototype.sqrTo = barrettSqrTo;
 
 // (public) this^e % m (HAC 14.85)
 function bnModPow(e,m) {
+  if(e.signum() < 0) {
+    return this.modInverse(m).modPow(e.negate(), m);
+  }
   var i = e.bitLength(), k, r = nbv(1), z;
   if(i <= 0) return r;
   else if(i < 18) k = 1;
@@ -670,4 +673,3 @@ BigInteger.prototype.sqrt = function() {
 	div = y;
     }
 }
-

test/qunit-do-crypto.html

@@ -465,6 +465,23 @@
 equal(result, true, "SHA1withRSA(x1234) sign/verify key2prv/key2pub");
 });
 
+test("BigInteger.modPow handles negative exponent -1", function() {
+  var a = new BigInteger("7", 10);
+  var m = new BigInteger("19", 10);
+  var got = a.modPow(new BigInteger("-1", 10), m);
+  var expected = a.modInverse(m);
+  equal(got.toString(10), expected.toString(10), "a^-1 mod m");
+});
+
+test("BigInteger.modPow handles negative exponent -k", function() {
+  var a = new BigInteger("3", 10);
+  var m = new BigInteger("23", 10);
+  var e = new BigInteger("-3", 10);
+  var got = a.modPow(e, m);
+  var expected = a.modInverse(m).modPow(e.negate(), m);
+  equal(got.toString(10), expected.toString(10), "a^-k mod m");
+});
+
 });
 -->
 </script>