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Secure Random Numbers in Node.js

By Lane Wagner on Jul 3, 2019

Quick answer: use crypto.randomBytes() for cryptographically secure randomness in Node.js.

const { randomBytes } = await import('node:crypto');

const buf = randomBytes(256);
console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);

crypto.randomBytes() is a cryptographically secure random number generator based on openssl. Depending on the operating system of the user, randomBytes will use /dev/urandom (Unix) or CryptoGenRandom (Windows).

While still pseudo-random sources, the important thing is that they are not guessable by an attacker. In other words, after using crypto.randomBytes() to generate a secret key for AES-256 encryption, no one will be able to guess the key.

Randomness is a hard problem in Node.js

Randomness is a hard problem for computers. Most functions that generate randomness in Node.js are not considered cryptographically secure. As a result, it’s possible for attackers to take a good guess at which number will be generated. In the case of guessing a private key, insecure randomness can be actually be catastrophic.

How to generate insecure random numbers

Math.random() a JavaScript built-in function that returns a pseudo-random number between 0 and 1. At first this sounds fairly pointless, but by always generating a number between 0 and 1, the user of the function can scale that random result up to whatever size they need.

Insecure example

Generate an insecure random number between 0 and 10 in Node.js:

const betweenOneAndTen = Math.floor(Math.random() * 10)

Insecure example #2

Generate an insecure random number between 10 and 100 in Node.js:

const min = 10
const max = 100
const betweenOneAndTen = Math.floor(Math.random() * (max - min)) + min + 1

Why is Math.Random() insecure?

Many non-secure sources of entropy, like Math.Random(), do something similar to the following:

function getRandom(timestamp, maxNumber){
  // Take the deterministic hash of the timestamp
  const hashedTime = sha256(timestamp)
  // Reduce the hash to within the range [0, maxNumber)
  return hashedTime % maxNumber

This function (while ignoring some implementation details of modulus math by such a large number) will return random numbers that are based on a timestamp input, which is called the seed. If I pass in different timestamps, the corresponding outputs would appear random. This is an example of a weak pseudo-random number generator.

A weak pseudo-random number generator works perfectly fine if you’re trying to:

  • Create sample data for an application
  • Write a video game engine
  • etc …

However, weak pseudo-randomness can be catastrophically dangerous if you’re trying to:

  • Generate Bitcoin keys
  • Generate passwords or salts
  • etc …

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Should I always use crypto.randomBytes()?

No. There are dangers if you implement your random number generator on top of a low-level API like random bytes. Because it returns bytes and not numbers, it’s up to you to convert the bytes into numbers. If you make a mistake, it can result in a vulnerability in your system.

In short, use crypto.randomBytes() whenever you need raw bytes. If you need a number within a range, for example, a random number between 0 and 9, then use a non-biased function that uses crypto.randomBytes() as the source of entropy. For example: node-random-number-csprng

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