# What Is Zero-Knowledge Proof (ZKP)?

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by Beatrice Mastropietro · 9 min read

Zero-knowledge proof (ZKP) is a cryptographic technique that allows one party to prove knowledge of a certain fact without revealing anything beyond what is known. This can be useful for authentication, data security, and privacy applications. In this guide, we will discuss the basics of ZKP and how it works.

In recent years, there has been a growing interest in the area of cryptography known as zero-knowledge proof (ZKP). A zero-knowledge proof is a method by which one party (the prover) can prove to another party (the verifier) that they know a value X without conveying any other information about X. The key feature of zero-knowledge proofs is that they are composed of two parts, the setup, and the proof.

The setup phase is where the prover creates a public and corresponding private key. The prover sends the public key to the verifier, who uses it to generate challenges. This challenge is a value that the prover must use to generate a response. The verifier checks the validity of the response. If it is correct, the prover knows the value of X without learning any other information about X.

Zero-knowledge proofs have numerous applications in cryptography, including digital signatures, identification schemes, and secure multiparty computation. Zero-knowledge proofs can also be used to prove the correctness of algorithms, such as proving that a given number is prime or that a given graph is 3-colorable.

## Zero-Knowledge Proof for Maintaining Privacy

As we have said, a zero-knowledge proof (ZKP) is a method by which one party (the prover) can prove to another party (the verifier) that they know a value X without conveying any information apart from the fact that they know the value X.

For example, suppose Alice wants to prove to Bob that she knows the secret password to a door without actually telling Bob what the password is. She can do this by giving Bob a series of challenges. For instance, he might ask her to tell him the fifth letter of the password or whether the sum of the first two letters is odd or even, and she can answer these correctly without revealing any other information about the password. This is an example of zero-knowledge proof.

Zero-knowledge proofs are used in various applications, including cryptographic protocols and privacy-preserving data analysis. They can be used to prove that a person is in possession of a certain piece of information without revealing what that information is. Zero-knowledge proofs can prove a person knows the secret key to a cryptographic system without revealing the key itself. They can allow two parties to compute a function on their private data without revealing anything.

ZKPs are a powerful tool for maintaining the privacy and have many potential applications in cryptography and data analysis. However, they are also relatively new, and there is still much active research into their feasibility and efficiency.

## Why Do We Need Zero-Knowledge Proofs?

In a world where data breaches are becoming more and more common, it’s important to be able to keep our information safe. One way to do this is to use a  zero-knowledge proof (ZKP).

For example, let’s say you wanted to prove to your friend that you know the answer to a certain math problem without telling them the answer. You could do this by using zero-knowledge proof.

First, you would need to come up with a way to encode the problem so that it can be turned into a mathematical formula. Next, you would solve the formula and create a proof you can share with your friend. Your friend can then use the proof to verify that you know the answer without learning what the answer is themselves.

One of the most well-known applications of zero-knowledge proofs is in the cryptocurrency Zcash. Zcash uses zero-knowledge proofs to allow users to send and receive funds without revealing their identity or transaction history.

Zero-knowledge proofs can also be used to create private transactions on public blockchains like Ethereum (ETH) and Bitcoin (BTC). These private transactions are often called “shielded transactions” or “ZK-SNARKS transactions”. Shielded transactions are useful for situations where you don’t want the details of your transaction to be public, such as when you’re buying something confidential or sending money to a friend.

## How Does Zero-Knowledge Proof Work?

There are a few different ways to go about implementing a zero-knowledge proof. One common approach is known as the Fiat-Shamir heuristic. This heuristic takes a computational problem, such as the knapsack problem, and transforms it into a Boolean circuit. The prover then sends the verifier a random assignment of true or false values to the input wires of the circuit. The verifier checks that the output of the circuit is consistent with the input values that the prover sent. If the output is consistent, then the verifier can be convinced that the prover knows a solution to the computational problem without learning anything about that solution.

Another common approach to implementing zero-knowledge proof is known as interactive proof systems. In an interactive proof system, the prover and verifier take turns sending messages to each other. Each message exchanged between the prover and verifier contains a small amount of information so that even if an eavesdropper intercepts all of the messages, they would not learn anything about the value of x. The advantage of this approach is that it is more flexible than the Fiat-Shamir heuristic and can be used to prove a wider range of statements.

Zero-knowledge proof is a powerful tool that can be used to improve the security of cryptographic protocols. For example, zero-knowledge proof can be used to create digital signatures that are impossible to forge. It can also be used to create anonymous communication systems in which the participants cannot be identified even if all of their messages are intercepted.

Zero-knowledge proof is also the basis for several proposed solutions to the problem of electronic voting, which is how to create a voting system that is both secure and private.

Electronic voting systems that use zero-knowledge proof would allow voters to verify that their vote was counted correctly without revealing how they voted. This would prevent voter fraud while also protecting the privacy of the voter.

## Types of ZKPs

There are two main types of zero-knowledge proofs: interactive and non-interactive. Interactive ZKPs require the prover and verifier to have some back-and-forth communication to complete the proof. In contrast, non-interactive ZKPs can be completed with a single message from the prover to the verifier.

• Interactive zero-knowledge proofs. The most common type of interactive zero-knowledge proof is the three-move protocol, which was first introduced by Shafi Goldwasser, Silvio Micali, and Charles Rackoff in 1985. In this protocol, the prover and verifier make moves, with each move consisting of the prover sending a message to the verifier and responding with a challenge. The challenge is a random value that the prover must use to generate their next message. If the prover can complete the three-move protocol, then they have proven that they know the value of the piece of information in question without conveying any other information about it.
• Non-interactive zero-knowledge proofs. A non-interactive zero-knowledge proof (NIZKP) is a type of ZKP that does not require any back-and-forth communication between the prover and verifier. Instead, the prover sends a single message to the verifier, which contains all of the information necessary for the verifier to verify that the prover knows the value of the piece of data in question.  NIZKPs are generally more efficient than interactive ZKPs, as they do not require multiple rounds of communication between the prover and verifier. However, NIZKPs are also more challenging to construct, as they must satisfy a number of additional security requirements.

## Challenges of ZKPs

Several challenges are faced when implementing ZKPs. They are as follows:

1) One needs to clearly understand what they want to prove and what information they are willing to share. The prover and the verifier need to agree on the statement that is being proved as well as the parameters of the proof.

2) It can be challenging to create a zero-knowledge proof that is both sound and complete can be challenging. Soundness means that if the statement being proven is true, then the verifier will accept the proof. Completeness means that if the proven statement is false, the verifier will reject the proof.

3) Zero-knowledge proofs can be very computationally intensive, both for the prover and the verifier. In some cases, it may not be feasible to create a zero-knowledge proof that is both sound and complete may not be feasible.

4) There is a trade-off between the soundness and completeness of a zero-knowledge proof and the size of the proof. Generally, the larger the proof, the more likely it is to be sound and complete. However, large proofs can be very inconvenient for the prover and verifier.

5) Zero-knowledge proofs are usually specific to a particular statement or class of statements. It can be very difficult to create a single zero-knowledge proof that can be used to prove multiple statements.

6) Zero-knowledge proofs can be vulnerable to side-channel attacks. Side-channel attacks are a type of attack that exploits the fact that the prover and verifier need to interact to create the proof. This interaction can leak information about the statement being proven, which can be used to mount an attack.

7) Zero-knowledge proofs are usually probabilistic, meaning there is a non-zero chance that the verifier will incorrectly accept a false proof. This probability can be made arbitrarily small, but it can never be exactly zero.

8) In some cases, it may be possible to construct a so-called “ malicious” zero-knowledge proof specifically designed to fool the verifier. Malicious zero-knowledge proofs are very difficult to create and detect. However, they highlight the fact that zero-knowledge proofs are not perfect.

Despite these challenges, zero-knowledge proofs are a powerful tool that can protect the privacy of both the prover and the verifier. Zero-knowledge proofs can be used to create digital signatures, verify the correctness of computations, and securely exchange information between two parties.

## Conclusion

In the end, a zero-knowledge proof (ZKP) is a powerful tool that allows one party to prove to another party that they know something without conveying any other information. This has numerous applications in the real world, from proving digital signatures to ensuring the privacy of confidential data. While some challenges are associated with implementing zero-knowledge-proof systems, overall, it is a promising technology with a high potential.

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## FAQ

### What is zero-knowledge proof?

Zero-knowledge proof (ZKP) is a type of cryptographic protocol in which one party (the prover) can prove to another party (the verifier) that they know a certain piece of information without conveying any other information about that data. The key feature of zero-knowledge proofs is that the verifier does not learn anything new during the protocol except that the prover knows what they claim to know.

### How does ZKP work?

There are a few different ways to go about implementing a zero-knowledge proof. One common approach is known as the Fiat-Shamir heuristic. This heuristic takes a computational problem, such as the knapsack problem, and transforms it into a Boolean circuit. The prover then sends the verifier a random assignment of true or false values to the input wires of the circuit. The verifier checks that the output of the circuit is consistent with the input values that the prover sent. If the output is consistent, then the verifier can be convinced that the prover knows a solution to the computational problem without learning anything about that solution.

### What is zero-knowledge security?

Zero-knowledge security is a type of security that allows one party to prove to another party that they know a piece of information without revealing any details about that piece of information. It is a way to ensure privacy and security while still allowing for verification.

### What are interactive ZKPs?

Interactive zero-knowledge proofs (ZKPs) are a type of cryptographic protocol in which one party (the prover) can prove to another party (the verifier) that they know a value x without conveying any other information about x. The key feature of ZKPs is that they are interactive, meaning that the prover and verifier must engage in back-and-forth communication to complete the proof.

### What are non-interactive ZKPs?

Non-interactive zero-knowledge proofs (NIZKPs) are a type of zero-knowledge proof that does not require any interaction between the prover and verifier. NIZKPs are considered more secure than traditional ZKPs because they cannot be faked or manipulated by either party. For a NIZKP to be successful, the prover must create a mathematical proof demonstrating their knowledge of the secret without revealing it. The prover then sends this proof to the verifier, who can confirm its validity without having to interact with the prover.

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