To What Extent Is Zero-Knowledge Proof 'Zero'? Does the Prover Really Reveal Nothing?

Hey friend, that question really hit the nail on the head! Many people hear "zero-knowledge" and think it's like magic, achieving absolute, complete "knowing nothing." But real-world technology is always a bit more complex than idealized concepts.

Let me use a classic story and some plain language to explain just how "zero" this "zero" really is.

First, a Classic Story: Ali Baba's Cave

Imagine a circular cave with one entrance and a magic door deep inside that connects two paths, requiring a password to open.

The Prover (Xiao Hong): She claims to know the door's password.
The Verifier (Xiao Ming): He wants to verify if Xiao Hong is telling the truth, but he doesn't want to know the password itself.

Here's what they can do:

Xiao Ming waits outside the cave, not watching which path (left or right) Xiao Hong takes in.
Xiao Hong randomly chooses a path and enters the cave.
After a while, Xiao Ming goes to the entrance and randomly shouts: "Come out from the left!"
Scenario One: If Xiao Hong truly knows the password, no matter which path she entered from, she can open the magic door in the middle and come out from the specified "left" path.
Scenario Two: If Xiao Hong is a fraud and doesn't know the password, she only has a 50% chance of randomly choosing the path Xiao Ming demands. If Xiao Ming shouts "Come out from the left!" and she happened to enter from the right, she gets stuck inside and can't come out.

This process done just once gives Xiao Hong a 50% chance of bluffing. But if they repeat this process 20 times, and Xiao Hong comes out the correct path every single time, the probability of her bluffing becomes minuscule (about one in a million). At this point, Xiao Ming can be extremely confident: Xiao Hong does indeed know the password.

The most crucial point is that throughout this entire process, Xiao Ming only sees Xiao Hong emerge from the correct path. He learns absolutely nothing about any character of the password. This is the core idea of "zero-knowledge."

So, Does the Prover Really Reveal Nothing?

The answer is: "Zero" refers to not revealing the "core secret" itself, but it's not absolute "zero information."

Just like in the story above, although Xiao Ming doesn't learn the password, he does learn at least the following things:

The fact that "Xiao Hong knows the password" is revealed. This is the purpose of the proof, right? If you want to prove you're wealthy, the effect you achieve is making others believe "you are wealthy." This conclusion itself is information.
Metadata might be revealed. Who (Xiao Hong) initiated a proof with whom (Xiao Ming), when, and where. On a blockchain, this could mean your wallet address initiated a private transaction. While others might not know the amount or counterparty, they know "this address was active at a certain time."
The existence of the proof. The act of performing a "zero-knowledge proof" operation is itself a piece of information.

So, we can understand the degree of "zero" like this:

The "zero" in zero-knowledge proof means it does not reveal any [additional knowledge] unrelated to the "fact being proven" itself.

For example, imagine you want to prove to a bank that your annual income exceeds 1 million to apply for a premium credit card.

Traditional Way: You show the bank your bank statements and tax records. The bank not only learns that you earn over 1 million annually but also learns the source of every income, specific amounts, spending habits, etc. A lot of extra information is leaked.
Zero-Knowledge Proof Way: You only give the bank a "proof." By verifying this proof, the bank can be 100% certain your annual income indeed exceeds 1 million, but they cannot deduce from this proof whether your specific income is 1.01 million or 10 million, nor can they learn where your money comes from.

See? The fact "income exceeds 1 million" is proven, but all the details (the core secret) about this fact are perfectly hidden. This is the true meaning of "zero."

How Reliable is this "Zero" in Blockchain and the Real World?

In the world of cryptography, "zero-knowledge" has stricter distinctions:

Perfect Zero-Knowledge: The theoretical ideal. Like Ali Baba's Cave, even if the verifier has unlimited computational power (e.g., a super-god computer), they absolutely cannot extract any information about the secret from the proof. Such proofs are usually less efficient.
Computational Zero-Knowledge: This is the scheme adopted by the vast majority of real-world applications (like zk-SNARKs on blockchain). It means that for an attacker with limited computational power (e.g., all computers on Earth combined), it's impossible to crack the secret from the proof within a reasonable timeframe (e.g., before the end of the universe). While theoretically crackable, it's practically equivalent to impossible.

So, when you use privacy coins like Zcash or privacy protocols like Tornado Cash on Ethereum, the proofs you generate belong to "computational zero-knowledge." They prove to the network that your transaction is valid (e.g., you didn't double-spend, your account has sufficient balance), but hide the sender, receiver, and amount.

For us ordinary users, this "computational zero" is secure enough; you can practically consider it "zero."

To Summarize

The "zero" in zero-knowledge proof is very close to zero, but not an absolute vacuum.
It "zeros out" the core secret itself, like your password, your specific wealth amount, your personal identity information, etc.
It does not "zero out" the conclusion that "the statement is true", nor some unavoidable metadata (like who is proving).
In practical applications, the "computational zero-knowledge" we rely on is rock-solid in security, sufficient to protect our privacy.

Hope this explanation helps you better understand this awesome technology! It's like a perfect "magic trick" that reveals the outcome while perfectly concealing the method.