https://www.energycharter.org/typo3/sysext/rsaauth/resources/jsbn/prng4.js

1// prng4.js - uses Arcfour as a PRNG
2
3function Arcfour() {
4  this.i = 0;
5  this.j = 0;
6  this.S = new Array();
7}
8
9// Initialize arcfour context from key, an array of ints, each from [0..255]
10function ARC4init(key) {
11  var i, j, t;
12  for(i = 0; i < 256; ++i)
13    this.S[i] = i;
14  j = 0;
15  for(i = 0; i < 256; ++i) {
16    j = (j + this.S[i] + key[i % key.length]) & 255;
17    t = this.S[i];
18    this.S[i] = this.S[j];
19    this.S[j] = t;
20  }
21  this.i = 0;
22  this.j = 0;
23}
24
25function ARC4next() {
26  var t;
27  this.i = (this.i + 1) & 255;
28  this.j = (this.j + this.S[this.i]) & 255;
29  t = this.S[this.i];
30  this.S[this.i] = this.S[this.j];
31  this.S[this.j] = t;
32  return this.S[(t + this.S[this.i]) & 255];
33}
34
35Arcfour.prototype.init = ARC4init;
36Arcfour.prototype.next = ARC4next;
37
38// Plug in your RNG constructor here
39function prng_newstate() {
40  return new Arcfour();
41}
42
43// Pool size must be a multiple of 4 and greater than 32.
44// An array of bytes the size of the pool will be passed to init()
45var rng_psize = 256;