Unlinkability of Transactions Other than identity anonymity, it is also necessary that the users related transactions should not be linked. If they are linkable, it would become easy to infer user information such as the frequency of the transactions, type of the transactions, account balance, etc. Such linkable transactions data about the user is risky. An adversary can use it to guess the real identity of the user. 4. INTRODUCTION TO ZERO-KNOWLEDGE PROOF

Public blockchain platforms cannot address data and user privacy, so there is an unavoidable need to use ZKP approaches. As in a blockchain network, all transaction data is transmitted to all participants, the transaction details of the two parties are visible to all the network users. It can result in the violation of confidentiality and privacy . In this scenario, ZKP can assure that others only learn that a correct transaction occurred without revealing any transactionrelated information. No information about the sender, receiver, and amount or type of asset transferred is visible. From nuclear disarmament to public blockchain networks for providing anonymous and secure transactions, ZKP is a brilliant sample of a cryptographic invention. The basis of zero-knowledge proof is to prove that somebody knows some fact by just revealing it is easy. The challenge is to justify the ownership of the known fact without revealing any extra information.

General structure, properties, types and challenges of ZKP are discussed as follows: 1) General Structure of Zero-Knowledge Proof A ZKP general structure consists of three consecutive actions between two parties Prover A and Verifier B as shown in figure 2. These actions are discussed as follows: Witness: Prover A decides a set of questions that always could be answered correctly. A chooses any question from the set randomly, calculates a Proof, and sends the Proof to B. Challenge: From the set shared by A, B selects a question and sends it to A to reply with its answer. Response: After calculating the answer, A sends it to B. It helps B to verify that A knows the secret indeed. The verifier can repeat the procedure multiple times so that the probability of A making guesses becomes very less. 2) Properties of Zero-knowledge Proof All Zero-Knowledge proofs include three essential properties. These properties are as follows:

The prover could always Completeness: demonstrate to the verifier that he or she has the necessary input. Soundness: The prover cannot persuade the verifier that he or she has the requested input unless the prover actually has it. Zero-knowledge: The verifier cannot know any information other than the proof that the prover knows the secret. Personal data of the other party stay anonymous.