code: function randBigInt_(b, n, s) {
    var i, a;
    for (i=0; i<b.length; i++)
      b[i]=0;
    a=Math.floor((n-1)/bpe)+1; //# array elements to hold the BigInt
    for (i=0; i<a; i++) {
      b[i]=Math.floor(Math.random()*(1<<(bpe-1)));
    }
    b[a-1] &= (2<<((n-1)%bpe))-1;
    if (s==1)
      b[a-1] |= (1<<((n-1)%bpe));
  }
docs: Set b to an n-bit random BigInt.  If s=1, then the most significant of those n bits is set to 1. Array b must be big enough to hold the result. Must have n>=1