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A zero-knowledge proof reveals the public claim being checked and any inputs the system designates as public. It can keep the private data—the “witness”—that makes the claim true out of the proof. But it does not automatically hide information exposed elsewhere by an app, contract, wallet, or network.
What a zero-knowledge proof tells the verifier
A zero-knowledge proof lets a prover convince a verifier that a defined statement is true without revealing the private information used to establish it. Ethereum.org describes the verifier’s takeaway as the truth of that specific statement—not total ignorance of what happened. In the usual terminology, the statement is the claim being checked, and the witness is the private information that makes the claim true. Ethereum.org’s overview of zero-knowledge proofs explains these roles and the properties a proof aims to provide.
For example, “this person meets the required age” could be the public statement. A birth date and credential data could form part of the witness. A suitable proof can let a verifier check the threshold without disclosing the exact date of birth. The verifier still learns that the threshold claim passed.
What can stay private—and what becomes public
The proof’s privacy depends on how its statement and inputs are defined. The application deliberately exposes some information so the verifier can check the claim; other inputs may remain private. A proof cannot hide a value that the application makes public or reveals through a separate channel.
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| Information | What the verifier or observer may learn |
|---|---|
| Public statement | The claim being checked, such as “the user is eligible.” |
| Public inputs | Values supplied openly to the verification process. Their contents depend on the design. |
| Private witness | Inputs used to establish the claim that the proof is designed to keep hidden. |
| Application outputs | Information exposed outside the proof, including contract calldata, emitted events, stored values, or transaction records. |
| Metadata | Information such as an address, timestamp, IP address, or repeated session that may identify or link activity. |
For Ethereum applications, Ethereum.org’s builder guide to privacy apps warns that public inputs, events, calldata, and on-chain storage remain visible. The privacy question is therefore not just what the circuit proves; it is also what the surrounding application publishes.
Examples: proving age or group membership
Proving an age threshold
A person could prove they meet an age requirement without presenting their full date of birth. The public result is the threshold claim; the birth date and supporting credential data can remain witness inputs. This only protects the date if no other field, log, or application channel exposes it.
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Proving group membership
A member could prove they belong to an eligible group without identifying which member they are. Ethereum.org gives World ID as an example of proving uniqueness: in its description, the revealed statement is that the person is unique. That is an implementation example, not a guarantee that every identity or membership system hides the individual.
These examples illustrate selective disclosure: reveal the fact needed for a decision while withholding unnecessary source data. What is disclosed still depends on the exact statement, public inputs, and application behavior.
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Why zero knowledge does not mean anonymity
The cryptographic proof may hide its witness while the way the proof is submitted reveals who is using it or links it to another action. Ethereum.org identifies risks such as reused IP addresses, RPC providers, sessions, wallets, and frontend traces. Logs, analytics, and submission patterns can also add clues.
- Address and transaction data: A public sender address or transaction record may associate a proof with an account.
- Timing and patterns: The time or sequence of submissions can help link activity, even when the proof’s private inputs stay hidden.
- Network and frontend traces: Repeated use of an IP address, RPC service, session, or website can connect otherwise separate actions.
- Contract behavior: Calldata, events, and stored values may reveal more than the private witness would.
Privacy is an end-to-end property of the proof design and its surrounding systems, not a blanket consequence of using a zero-knowledge proof.
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A validity proof does not necessarily hide transactions
A proof can show that a computation was performed correctly without hiding the data used in that computation. Ethereum.org’s explanation of zero-knowledge rollups distinguishes validity proofs from the additional zero-knowledge property: a rollup may use a validity proof to verify a batch while leaving transaction data public. The word “zero-knowledge” in a technology label is not enough to establish that its transaction details are private.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to assess a proof’s privacy
Before treating a system as private, identify the exact claim it proves and follow the data beyond the proof. These checks help distinguish hidden inputs from information the application still exposes:
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- Read the statement. What exact claim will the verifier accept?
- Identify public inputs. Which values are provided openly to verify the claim?
- Identify the witness. Which sensitive source values are intended to remain private?
- Inspect application outputs. Check whether calldata, events, contract storage, or transaction records reveal relevant details.
- Consider linkability. Ask whether addresses, timestamps, network services, sessions, or frontend logging could connect the proof to a person or another action.
- Check the system’s assumptions. Proof families can have different security assumptions. For example, Ethereum.org notes that a ZK-SNARK common reference string setup creates a security dependency; this should not be generalized to every proof system.
The standards page for ERC-5851 also provides a definition of a zero-knowledge proof. The central practical question remains the same: what does this particular verifier learn from the statement and the complete application flow?
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