Zero-knowledge proof facts for kids

A zero-knowledge proof is a clever way for one person to prove they know a secret to another person, without actually telling them the secret. Think of it like this: you can show someone you know a secret password without ever saying the password out loud! This idea is used in cryptography, which is the study of secure communication and secret codes.

What Are Zero-Knowledge Proofs?

A zero-knowledge proof involves two main people:

• Peggy (the prover): This is the person who wants to prove they know the secret.
• Victor (the verifier): This is the person who needs to be convinced that Peggy knows the secret.

The goal is for Peggy to convince Victor that she knows something, but without giving away any extra information about that secret. Victor should only learn that Peggy knows it, nothing more.

The Magic Cave Story

Let's imagine a fun story to understand how zero-knowledge proofs work.

There's a special cave shaped like a circle. It has an entrance on one side and a magic door blocking the other side. To open this magic door, you need a secret word.

Peggy has found out the secret word for the door. Victor wants to buy the secret word from her, but he won't pay until he's sure she really knows it. Peggy doesn't want to tell him the word until she gets paid. So, they come up with a smart plan!

How Peggy Proves Her Knowledge

First, Victor waits outside the cave. Peggy goes inside. The cave has two paths from the entrance, let's call them Path A and Path B. Peggy chooses one path, either A or B, and walks down it. Victor doesn't know which path she took.

Then, Victor enters the cave. He stands at the entrance and shouts out the name of a path, either A or B, chosen randomly. He wants Peggy to come back out using that specific path.

If Peggy truly knows the magic word, this is easy for her. If she went down Path A and Victor called for Path B, she can open the magic door, walk through to the other side, and come out through Path B. If she went down Path A and Victor called for Path A, she just walks back out that way. She can always come out the path Victor asks for because she can open the door.

Why This Proof Works

Now, imagine Peggy did NOT know the magic word. She could only come out the path Victor asked for if she happened to pick the same path Victor called out. For example, if she went down Path A and Victor called for Path A, she could come back out. But if Victor called for Path B, she'd be stuck because she couldn't open the magic door to switch paths.

Since Victor chooses the path randomly (A or B), Peggy would have a 50% chance of guessing correctly if she didn't know the word.

To make sure Peggy really knows the word, they repeat this trick many times. If they do it 20 times in a row, and Peggy successfully comes out the correct path every single time, her chance of just guessing correctly every single time becomes extremely, extremely small. It's like flipping a coin 20 times and getting heads every single time!

Because Peggy reliably appears at the exit Victor names, Victor can be very sure that she knows the secret word, without her ever having to say the word itself! This is the power of a zero-knowledge proof.

See also

In Spanish: Prueba de conocimiento cero para niños

Images for kids

• 

  Peggy (the prover) and Victor (the verifier).

• 

  Peggy enters the cave, choosing a path.

• 

  Victor calls out a path, and Peggy exits through it.