seedrandom.js

Seeded random number generator for Javascript.

version 2.2.

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.

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.

More advanced examples:

Math.seedrandom("hello.");           // Use "hello." as the seed.
document.write(Math.random());       // Always 0.9282578795792454
document.write(Math.random());       // Always 0.3752569768646784
var rng1 = Math.random;              // Remember the current prng.

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

Math.random = rng1;                  // Continue "hello." prng sequence.
document.write(Math.random());       // Always 0.7316977468919549

Math.seedrandom(autoseed);           // Restart at the previous seed.
document.write(Math.random());       // 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.

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.

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, 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 about 3-10x 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
seedrandom() without crypto   - avg about 12 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 (BSD):

Copyright 2013 David Bau, all rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3. Neither the name of this module nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.