Throwing error when insecure rng is used

- New option for v1 and v4: allowInsecureRng
  - Require to be `true` if `rng.insecure == true`

Closes #173

lib/rng-browser.js

@@ -23,12 +23,16 @@ if (getRandomValues) {
   // quality.
   var rnds = new Array(16);
 
-  module.exports = function mathRNG() {
+  function mathRNG() {
     for (var i = 0, r; i < 16; i++) {
       if ((i & 0x03) === 0) r = Math.random() * 0x100000000;
       rnds[i] = r >>> ((i & 0x03) << 3) & 0xff;
     }
 
     return rnds;
-  };
+  }
+  mathRNG.insecure = true;
+  mathRNG.insecureMessage = 'No CSPRNG provided. Provide a cryptographically secure rng' +
+                          ' via `rng` option or set `allowInsecureRNG` to true';
+  module.exports = mathRNG;
 }

test/test.js

@@ -64,6 +64,8 @@ test('mathRNG', function() {
   var rng = require('../lib/rng-browser');
   assert.equal(rng.name, 'mathRNG');
 
+  assert.equal(rng.insecure, true)
+
   var bytes = rng();
   assert.equal(bytes.length, 16);
 

v1.js

@@ -22,6 +22,9 @@ function v1(options, buf, offset) {
   var node = options.node || _nodeId;
   var clockseq = options.clockseq !== undefined ? options.clockseq : _clockseq;
 
+  if (rng.insecure && !options.allowInsecureRNG)
+    throw new Error(rng.insecureMessage);
+
   // node and clockseq need to be initialized to random values if they're not
   // specified.  We do this lazily to minimize issues related to insufficient
   // system entropy.  See #189

v4.js

@@ -12,6 +12,9 @@ function v4(options, buf, offset) {
 
   var rnds = options.random || (options.rng || rng)();
 
+  if (rng.insecure && !options.allowInsecureRNG)
+    throw new Error(rng.insecureMessage);
+
   // Per 4.4, set bits for version and `clock_seq_hi_and_reserved`
   rnds[6] = (rnds[6] & 0x0f) | 0x40;
   rnds[8] = (rnds[8] & 0x3f) | 0x80;