Content Security Policy (csp) nonce: how long or complex should be a nonce

Question

I have a site which uses nonce. Everything works well. But how long or complex should be the nonce.

My little nonce maker is just this:

let generateNonce = (length = 32) => {
  const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
  let nonce = '';
  for (let i = 0; i < length; i++) 
    nonce += chars.charAt(Math.floor(Math.random() * chars.length));
  return nonce;
};

A call of generateNonce() returns something like hERnT30lr0G3Hw4b5eQCjuC423a3PcBl.

32 characters of numbers, lower and upper case letters. Is this complex enough or even too long?

See the CSP spec section at https://w3c.github.io/webappsec-csp/#security-nonces, which says nonce values *“should be at least 128 bits long (before encoding), & should be generated via a cryptographically secure random number generator”*. Your `hERnT30lr0G3Hw4b5eQCjuC423a3PcBl` has more than 128 bits, so that”s fine. But Math.random() isn’t cryptographically secure. See the explanations at https://stackoverflow.com/a/5651854/441757 & https://security.stackexchange.com/q/181580/86150. Use Crypto.getRandomValues() https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues instead. — sideshowbarker, Sep 07 '19 at 00:28
@sideshowbarker thank you. Very informative and useful links. Could you post it as an answer please. — tom, Sep 07 '19 at 07:33

sideshowbarker · Accepted Answer · 2021-01-14T18:21:47.653

See the CSP spec section at https://w3c.github.io/webappsec-csp/#security-nonces, which says:

[nonce values] should be at least 128 bits long (before encoding), and should be generated via a cryptographically secure random number generator

The question’s hERnT30lr0G3Hw4b5eQCjuC423a3PcBl value is more than 128 bits, so that’s OK.

But Math.random() isn’t cryptographically secure; https://stackoverflow.com/a/5651854/441757 and https://security.stackexchange.com/q/181580/86150. Use Crypto.getRandomValues instead.

score 1 · Answer 2 · answered Sep 07 '19 at 09:34

With the help of @sideshowbarker the nonce generator could be like this (nodejs)

// require nodes native crypto module
const crypto = require('crypto');

// create 128 bit nonce synchronously
const nonce = crypto.randomBytes(16).toString('hex');

output = 1e31b6130c5be9ef4cbab7eb38df5491

crypto.randomBytes(size[, callback])

Generates cryptographically strong pseudo-random data. The size argument is a number indicating the number of bytes to generate.

Content Security Policy (csp) nonce: how long or complex should be a nonce

2 Answers2