How to get a cryptographically strong number in the range [0,1)?

Question

I've been trying to create a function that will return a random decimal number between 0 (inclusive) and 1 (exclusive) via the Window.crypto.getRandomValues() function. Currently, my code is as follows:

var rand = function(){
    var ra = window.crypto.getRandomValues(new Uint32Array(1))[0];
    function dec(n,m){
        return (n>=0&&n<=1)?n:dec(n/m,m);
    }
    return dec(ra,8);
}

My problem is that this isn't uniformly distributed; it tends to be between .2 and .7, and rarely falls under .2 and over .7. Is there a better way to use the .getRandomValues() method to obtain a number like that of the Math.random() function?

I am not using the .random() method because I wish to have secure numbers for what I am trying to perform.

Answer 1

Why not simplify things

function cryptoRandom() {
    return window.crypto.getRandomValues(new Uint32Array(1))[0] / 0x100000000;
}

What's going on here?

• Assume window.crypto.getRandomValues will giving us truely random bits
• Ask for 32 bits (unsigned integer i)
• The minium value of i is 0x00000000 (or 0)
• The maximum value of i is 0xFFFFFFFF (or 4294967295)
• Transform these points onto the range you want; (i - min) / (max + 1)