How do computers generate random numbers?

29 July 2025

Ah, randomness! It’s everywhere in nature—think dice rolls, quantum physics, or even your cat’s indecision. But when it comes to computers, randomness doesn’t come naturally. Why? Because computers are logical machines, designed to follow precise instructions. So, when we ask for a “random” number, they can’t just pluck one from thin air. Instead, they rely on something called a pseudo-random number generator (PRNG)—essentially, randomness with a script.

How does a pseudo-random number generator work?

Here’s how computers fake randomness step by step:

1. The magic seed
   The process begins with a “seed” number. This seed could be almost anything—like the exact millisecond from the system clock, the temperature of your CPU, or even the quirky motion of a lava lamp (a trick famously used by Cloudflare for added unpredictability).
2. Math happens
   Once the seed is set, it’s run through a complex mathematical formula designed to churn out seemingly random results. Picture a blender spinning at full speed, tossing numbers into a chaotic whirl.
3. Voilà! fake randomness
   Out comes a number that looks completely random. However, if someone knows the original seed and the formula, they can predict the outcome—like a magician pulling the same rabbit from their hat every time.

Can computers create true randomness?

When it comes to security, like encrypting sensitive data, fake randomness isn’t enough. For truly unpredictable results, computers turn to nature for help. They measure chaotic phenomena like radioactive decay, electrical noise, or even the small, unpredictable quirks of daily life. This kind of randomness, called “true randomness,” is far more secure and impossible to predict.

So, while computers don’t naturally do random, they’ve mastered the art of faking it with clever algorithms. But when we need something truly unpredictable, we can rely on the chaos of the natural world. Or, as a simpler alternative, just watch a cat trying to decide whether to go outside.

Want to learn more?

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