Secure random values

This article was originally published at https://gist.github.com/joepie91/7105003c3b26e65efcea63f3db82dfba.

Not all random values are created equal - for security-related code, you need a specific kind of random value.

A summary of this article, if you don't want to read the entire thing:

• Don't use Math.random(). There are extremely few cases where Math.random() is the right answer. Don't use it, unless you've read this entire article, and determined that it's necessary for your case.
• Don't use crypto.getRandomBytes directly. While it's a CSPRNG, it's easy to bias the result when 'transforming' it, such that the output becomes more predictable.
• If you want to generate random tokens or API keys: Use uuid, specifically the uuid.v4() method. Avoid node-uuid - it's not the same package, and doesn't produce reliably secure random values.
• If you want to generate random numbers in a range: Use random-number-csprng.

You should seriously consider reading the entire article, though - it's not that long :)

Types of "random"

There exist roughly three types of "random":

• Truly random: Exactly as the name describes. True randomness, to which no pattern or algorithm applies. It's debatable whether this really exists.
• Unpredictable: Not truly random, but impossible for an attacker to predict. This is what you need for security-related code - it doesn't matter how the data is generated, as long as it can't be guessed.
• Irregular: This is what most people think of when they think of "random". An example is a game with a background of a star field, where each star is drawn in a "random" position on the screen. This isn't truly random, and it isn't even unpredictable - it just doesn't look like there's a pattern to it, visually.

Irregular data is fast to generate, but utterly worthless for security purposes - even if it doesn't seem like there's a pattern, there is almost always a way for an attacker to predict what the values are going to be. The only realistic usecase for irregular data is things that are represented visually, such as game elements or randomly generated phrases on a joke site.

Unpredictable data is a bit slower to generate, but still fast enough for most cases, and it's sufficiently hard to guess that it will be attacker-resistant. Unpredictable data is provided by what's called a CSPRNG.

Types of RNGs (Random Number Generators)

• CSPRNG: A Cryptographically Secure Pseudo-Random Number Generator. This is what produces unpredictable data that you need for security purposes.
• PRNG: A Pseudo-Random Number Generator. This is a broader category that includes CSPRNGs and generators that just return irregular values - in other words, you cannot rely on a PRNG to provide you with unpredictable values.
• RNG: A Random Number Generator. The meaning of this term depends on the context. Most people use it as an even broader category that includes PRNGs and truly random number generators.

Every random value that you need for security-related purposes (ie. anything where there exists the possibility of an "attacker"), should be generated using a CSPRNG. This includes verification tokens, reset tokens, lottery numbers, API keys, generated passwords, encryption keys, and so on, and so on.

Bias

In Node.js, the most widely available CSPRNG is the crypto.randomBytes function, but you shouldn't use this directly, as it's easy to mess up and "bias" your random values - that is, making it more likely that a specific value or set of values is picked.

A common example of this mistake is using the % modulo operator when you have less than 256 possibilities (since a single byte has 256 possible values). Doing so actually makes lower values more likely to be picked than higher values.

For example, let's say that you have 36 possible random values - 0-9 plus every lowercase letter in a-z. A naive implementation might look something like this:

let randomCharacter = randomByte % 36;

That code is broken and insecure. With the code above, you essentially create the following ranges (all inclusive):

• 0-35 stays 0-35.
• 36-71 becomes 0-35.
• 72-107 becomes 0-35.
• 108-143 becomes 0-35.
• 144-179 becomes 0-35.
• 180-215 becomes 0-35.
• 216-251 becomes 0-35.
• 252-255 becomes 0-3.

If you look at the above list of ranges you'll notice that while there are 7 possible values for each randomCharacter between 4 and 35 (inclusive), there are 8 possible values for each randomCharacter between 0 and 3 (inclusive). This means that while there's a 2.64% chance of getting a value between 4 and 35 (inclusive), there's a 3.02% chance of getting a value between 0 and 3 (inclusive).

This kind of difference may look small, but it's an easy and effective way for an attacker to reduce the amount of guesses they need when bruteforcing something. And this is only one way in which you can make your random values insecure, despite them originally coming from a secure random source.

So, how do I obtain random values securely?

In Node.js:

• If you need a sequence of random bytes: Use crypto.randomBytes.
• If you need individual random numbers in a certain range: use crypto.randomInt.
• If you need a random string: You have two good options here, depending on your needs.
  1. Use a v4 UUID. Safe ways to generate this are crypto.randomUUID, and the uuid library (only the v4 variant!).
  2. Use a nanoid, using the nanoid library. This also allows specifying a custom alphabet to use for your random string.

Both of these use a CSPRNG, and 'transform' the bytes in an unbiased (ie. secure) way.

In the browser:

• When using the Node.js options, your bundler should automatically select equivalently safe browser implementations for all of these.
• If not using a bundler:
  • If you need a sequence of random bytes: Use crypto.getRandomValues with a Uint8Array. Other array types will get you numbers in different ranges.
  • If you need a random string: You have two good options here, depending on your needs.
    1. Use a v4 UUID, with the crypto.randomUUID method.
    2. Use a nanoid, using the standalone build of the nanoid library. This also allows specifying a custom alphabet to use for your random string.

However, it is strongly recommended that you use a bundler, in general.