Proving Vaccination Status Privately on-Chain: A Healthcare Blueprint

Current vaccination records are centralized, forgeable, and reveal your entire medical history when verified. This creates a single point of failure and violates data minimization principles.

• Data Breach Risk: Centralized health databases are prime targets, exposing millions of records.
• Oversharing: Showing a physical card reveals your DOB, patient ID, and other inoculations.
• No User Control: Individuals cannot selectively disclose or revoke access to their credentials.

ZKPs allow a user to generate a cryptographic proof that they have a valid credential (e.g., "vaccinated") without revealing the credential itself or any identifying information. This is the core privacy layer.

• Selective Disclosure: Prove you are "over 18 and vaccinated" without revealing your birth date or vaccine lot number.
• On-Chain Verifiability: A smart contract can verify the ZKP in ~500ms for less than $0.01 in gas.
• Interoperability Foundation: Enables trust-minimized bridges to systems like Worldcoin's Proof of Personhood or enterprise SSO.

DIDs give users a self-sovereign identifier, while Verifiable Credentials are the tamper-proof attestations (like a digital vaccination card) issued by a trusted entity (e.g., a hospital).

• User Sovereignty: The DID is stored in your wallet; you own and control it, not the issuer.
• Tamper-Proof Issuance: Credentials are cryptographically signed by the issuer's DID, making forgeries impossible.
• Revocable Trust: Issuers can update a revocation registry on-chain without compromising user privacy.

ZKPs like zk-SNARKs (Zcash, Aztec) can prove vaccination status without revealing the underlying record. However, they introduce new attack surfaces and usability cliffs.\n- Key Risk 1: Trusted Setup Ceremonies for circuits become single points of failure and require continuous social consensus.\n- Key Risk 2: Proving times and costs scale with logic complexity, creating a ~5-30 second latency and >$0.50 cost barrier for mass adoption.

The credential's validity depends on a trusted issuer (e.g., CDC, hospital). This creates a centralized oracle dependency akin to Chainlink's early challenges.\n- Key Risk 1: A compromised or malicious issuer key can mint unlimited false credentials, poisoning the entire system.\n- Key Risk 2: Data availability and update finality from legacy health IT systems (HL7/FHIR) creates lags and reconciliation nightmares.

Even with a ZKP, the act of presenting a proof to a specific verifier (e.g., an airline smart contract) creates immutable, linkable metadata.\n- Key Risk 1: Pattern analysis can deanonymize users, revealing travel habits, employment status, and social connections.\n- Key Risk 2: Immutable on-chain denylists or policy changes can lead to permanent exclusion, a harder problem than credential revocation.

W3C Verifiable Credentials, IATA Travel Pass, and EU Digital Green Certificates all solve the same problem differently. On-chain, this fragments into competing smart contract ecosystems.\n- Key Risk 1: Nations or institutions adopt proprietary standards, creating walled gardens that defeat the purpose of a global ledger.\n- Key Risk 2: Protocol bridges between these standards (like Across or LayerZero for assets) become critical, yet fragile, interoperability layers.

The EU's "Right to Be Forgotten" (GDPR Article 17) is fundamentally incompatible with immutable blockchain storage. A ZKP receipt is still a persistent data record.\n- Key Risk 1: Legal liability shifts to dApp developers and node operators, not just the issuing authority.\n- Key Risk 2: Solutions like timelock encryption (e.g., Drand network) or state expiry add massive complexity and may not satisfy regulators.

Preventing one person from minting multiple credentials requires linking to a unique identity. This forces a choice: integrate a centralized KYC provider (e.g., Civic) or a decentralized primitive like Proof-of-Personhood (Worldcoin, BrightID).\n- Key Risk 1: Centralized KYC re-introduces the exact surveillance risks the system aims to avoid.\n- Key Risk 2: Decentralized personhood proofs have low adoption (~2M Worldcoin IDs) and are untested at global, critical scale.

Storing raw vaccination records on a public ledger like Ethereum is a compliance nightmare and creates permanent, linkable identity graphs.

• PII Exposure: Public test results or vaccine IDs become immutable, searchable data.
• Regulatory Non-Compliance: Violates GDPR/ HIPAA by design, making protocols legally toxic.
• Front-Running Risk: Public minting of health status can be exploited by MEV bots.

Prove credential validity without revealing the underlying data. The user cryptographically proves a statement (e.g., "I am vaccinated") to a verifier.

• Privacy-Preserving: Verifier sees only the proof, not the credential details or user identity.
• Interoperable: Proofs can be verified by any smart contract (DeFi, event access) or off-chain service.
• Compliant by Design: Core data remains off-chain, owned by the user, aligning with data minimization principles.

Adopt the W3C VC standard. A trusted issuer (clinic) signs a VC, which the holder stores in a wallet. DIDs provide a privacy-preserving identifier.

• Standardized Schema: Enables cross-platform verification, unlike proprietary solutions.
• User-Centric: Holder controls credential storage and presentation, preventing vendor lock-in.
• Revocable: Issuers can update revocation registries (e.g., on-chain) without exposing user data.

Leverage existing infrastructure instead of building ZK circuits from scratch.

• Polygon ID: Iden3 protocol for private identity, with ~2 second proof generation on mobile.
• Sismo: ZK badges for aggregated, reusable attestations, used by ~200k+ users.
• zkPass: Uses MPC-TLS for verifying data from any HTTPS website (e.g., health portal) privately.

The smart contract only needs to verify a ZK proof and check a revocation registry. Keep logic minimal.

• Cost: Verification gas can be < 200k gas for optimized Groth16 SNARKs.
• Stateless: The contract holds no user data, only the public verification key and issuer DID.
• Composable: Returns a simple true/false, enabling integration with DeFi, DAO governance, or POAP minting.

Using an oracle to fetch and attest off-chain data (e.g., "API returns true") merely shifts trust and leaks query patterns.

• Trust Assumption: Replaces decentralized verification with a single signature from the oracle operator.
• Metadata Leakage: The oracle sees who is querying for verification and when.
• Architectural Regret: This is a dead-end for building truly decentralized and private credential systems.