How to replace Math.random with crypto in JavaScript

JavaScript has a method for generating random numbers from 0 to less than 1: Math.random(). If you’ve ever been working on an app that selects a random item from a list, chances are that you’ve done something like this:

const randomItem = list[Math.floor(Math.random() * list.length)];

This will probably work for 99% of the use cases of a random number. However, numbers generated from Math.random are not cryptographically secure. That means that the “random” values are guessable by a computer and therefore not suitable for use in cryptography.

Thankfully, we have other methods of generating random numbers in the browser: crypto. This global object has a getRandomValues method that we can use to recreate the functionality above. The method needs to be called with an typed array, so we’ll create one with the Uint8Array constructor with a length of 1:

const typedArray = new Uint8Array(1);

Now we pass this to getRandomValues and select the first (and only) item from the returned array.

const typedArray = new Uint8Array(1)
const randomValue = crypto.getRandomValues(typedArray)[0]
// E.g. 122

Since we used an Uint8Array, all of the numbers generated will from 0 to 255. To convert that into the same float returned from Math.random, we need to divide the value by the total number of possible numbers: 256 or 2⁸.

const typedArray = new Uint8Array(1)
const randomValue = crypto.getRandomValues(typedArray)[0]
const randomFloat = randomValue / Math.pow(2, 8)
// E.g. 0.4765625

With these steps in place, we can create a function that is a drop-in replacement for Math.random called cryptoRandom.

function cryptoRandom() {
  const typedArray = new Uint8Array(1);
  const randomValue = crypto.getRandomValues(typedArray)[0];
  const randomFloat = randomValue / Math.pow(2, 8);
  return randomFloat;
}

A few things to note:

1. You’ll (probably) never need this. Reach for Math.random unless you have a very specific reason for cryptographic security.
2. The floats from this method are shorter. Math.random pretty consistently returns floats that are 10+ decimal places long. If you are depending on a long tail of numbers, then consider switching to Uint32Array and dividing by Math.pow(2, 32).
3. I’m not a security expert. All of the information in this article was from a quick Google search; make sure to consult people who know what they’re talking about before copying and pasting code from a random website.

I hope that helps and as always: let me know if you have any corrections to the article.

Happy randomizing!