What Is a Zero-Knowledge Proof?

While the inherent transparency of blockchains provides an advantage in many situations, there are also a number of smart contract use cases that require privacy due to various business or legal reasons, such as using proprietary data as inputs to trigger a smart contract’s execution. An increasingly common way privacy is achieved on public blockchain networks is through zero-knowledge proofs (ZKPs)—a method for one party to cryptographically prove to another that they possess knowledge about a piece of information without revealing the actual underlying information. In the context of blockchain networks, the only information revealed on-chain by a ZKP is that some piece of hidden information is valid and known by the prover with a high degree of certainty.

Zero Knowledge vs. Zero Trust

“Zero knowledge” refers to the specific cryptographic method of zero-knowledge proofs, while “zero trust” is a general cyber security model used by organizations to protect their data, premises, and other resources.

The zero-trust framework assumes that every person and device, both internal and external to the network, could be a threat due to malicious behavior or simple incompetence. To mitigate threats, zero-trust systems require users and devices to be authenticated, authorized, and continuously validated before access to resources is granted.

Zero-knowledge proofs can be used as part of a zero-trust framework. For example, zero-knowledge authentication solutions can allow employees to access their organization’s network, without having to reveal personal details.

How Do Zero-Knowledge Proofs Work

At a high level, a zero-knowledge proof works by having the verifier ask the prover to perform a series of actions that can only be performed accurately if the prover knows the underlying information. If the prover is only guessing as to the result of these actions, then they will eventually be proven wrong by the verifier’s test with a high degree of probability.

Zero-knowledge proofs were first described in a 1985 MIT paper from Shafi Goldwasser and Silvio Micali called “The Knowledge Complexity of Interactive Proof-Systems”. In this paper, the authors demonstrate that it is possible for a prover to convince a verifier that a specific statement about a data point is true without disclosing any additional information about the data. ZKPs can either be interactive—where a prover convinces a specific verifier but needs to repeat this process for each individual verifier—or non-interactive—where a prover generates a proof that can be verified by anyone using the same proof.

• Completeness: If a statement is true, then an honest verifier can be convinced by an honest prover that they possess knowledge about the correct input.
• Soundness: If a statement is false, then no dishonest prover can unilaterally convince an honest verifier that they possess knowledge about the correct input.
• Zero-knowledge: If the state is true, then the verifier learns nothing more from the prover other than the statement is true.