seedrandom.js

Seeded random number generator for Javascript.

version 2.3.4
Author: David Bau
Date: 2014 Mar 9

Defines a method Math.seedrandom() that, when called, substitutes an explicitly seeded RC4-based algorithm for Math.random(). Also supports automatic seeding from local or network sources of entropy. Can be used as a node.js or AMD module. Can be called with "new" to create a local PRNG without changing Math.random.

Basic usage:

<script src=http://davidbau.com/encode/seedrandom.min.js></script>

Math.seedrandom('yay.');  // Sets Math.random to a function that is
                          // initialized using the given explicit seed.

Math.seedrandom();        // Sets Math.random to a function that is
                          // seeded using the current time, dom state,
                          // and other accumulated local entropy.
                          // The generated seed string is returned.

Math.seedrandom('yowza.', true);
                          // Seeds using the given explicit seed mixed
                          // together with accumulated entropy.

<script src="https://jsonlib.appspot.com/urandom?callback=Math.seedrandom">
</script>                 <!-- Seeds using urandom bits from a server. -->

Math.seedrandom("hello.");           // Behavior is the same everywhere:
document.write(Math.random());       // Always 0.9282578795792454
document.write(Math.random());       // Always 0.3752569768646784

Math.seedrandom can be used as a constructor to return a seeded PRNG that is independent of Math.random:

var myrng = new Math.seedrandom('yay.');
var n = myrng();          // Using "new" creates a local prng without
                          // altering Math.random.

When used as a module, seedrandom is a function that returns a seeded PRNG instance without altering Math.random:

// With node.js (after "npm install seedrandom"):
var seedrandom = require('seedrandom');
var rng = seedrandom('hello.');
console.log(rng());                  // always 0.9282578795792454

// With require.js or other AMD loader:
require(['seedrandom'], function(seedrandom) {
  var rng = seedrandom('hello.');
  console.log(rng());                // always 0.9282578795792454
});

More examples:

var seed = Math.seedrandom();        // Use prng with an automatic seed.
document.write(Math.random());       // Pretty much unpredictable x.

var rng = new Math.seedrandom(seed); // A new prng with the same seed.
document.write(rng());               // Repeat the 'unpredictable' x.

function reseed(event, count) {      // Define a custom entropy collector.
  var t = [];
  function w(e) {
    t.push([e.pageX, e.pageY, +new Date]);
    if (t.length < count) { return; }
    document.removeEventListener(event, w);
    Math.seedrandom(t, true);        // Mix in any previous entropy.
  }
  document.addEventListener(event, w);
}
reseed('mousemove', 100);            // Reseed after 100 mouse moves.

The callback third arg can be used to get both the prng and the seed. The following returns both an autoseeded prng and the seed as an object, without mutating Math.random:

var obj = Math.seedrandom(null, false, function(prng, seed) {
  return { random: prng, seed: seed };
});

Version notes:

The random number sequence is the same as version 1.0 for string seeds.

• Version 2.0 changed the sequence for non-string seeds.
• Version 2.1 speeds seeding and uses window.crypto to autoseed if present.
• Version 2.2 alters non-crypto autoseeding to sweep up entropy from plugins.
• Version 2.3 adds support for "new", module loading, and a null seed arg.
• Version 2.3.1 adds a build environment, module packaging, and tests.
• Version 2.3.3 fixes bugs on IE8, and switches to MIT license.
• Version 2.3.4 fixes documentation to contain the MIT license.

The standard ARC4 key scheduler cycles short keys, which means that seedrandom('ab') is equivalent to seedrandom('abab') and 'ababab'. Therefore it is a good idea to add a terminator to avoid trivial equivalences on short string seeds, e.g., Math.seedrandom(str + '\0'). Starting with version 2.0, a terminator is added automatically for non-string seeds, so seeding with the number 111 is the same as seeding with '111\0'.

When seedrandom() is called with zero args or a null seed, it uses a seed drawn from the browser crypto object if present. If there is no crypto support, seedrandom() uses the current time, the native rng, and a walk of several DOM objects to collect a few bits of entropy.

Each time the one- or two-argument forms of seedrandom are called, entropy from the passed seed is accumulated in a pool to help generate future seeds for the zero- and two-argument forms of seedrandom.

On speed - This javascript implementation of Math.random() is several times slower than the built-in Math.random() because it is not native code, but that is typically fast enough. Some details (timings on Chrome 25 on a 2010 vintage macbook):

seeded Math.random() - avg less than 0.0002 milliseconds per call seedrandom('explicit.') - avg less than 0.2 milliseconds per call seedrandom('explicit.', true) - avg less than 0.2 milliseconds per call seedrandom() with crypto - avg less than 0.2 milliseconds per call

Autoseeding without crypto is somewhat slower, about 20-30 milliseconds on a 2012 windows 7 1.5ghz i5 laptop, as seen on Firefox 19, IE 10, and Opera. Seeded rng calls themselves are fast across these browsers, with slowest numbers on Opera at about 0.0005 ms per seeded Math.random().

LICENSE (MIT):

Copyright (c)2014 David Bau.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.