ZK-Email

ZK-Email is a cryptographic protocol that enables a user to generate a zero-knowledge proof (ZKP) attesting to the content of an email they have received, without revealing the email's body, sender, or other sensitive metadata. It works by cryptographically verifying the DKIM (DomainKeys Identified Mail) signature, a standard email authentication method, to prove that a specific message was legitimately sent from a domain. The user's client parses the raw email, and a ZK circuit generates a proof that certain statements about the email (e.g., "this email contains a specific transaction hash" or "the sender's domain is @example.com") are true. This proof is succinct and can be verified by anyone without access to the underlying data.

The core innovation lies in its use of existing email infrastructure as a decentralized data source and identity layer. By leveraging DKIM, ZK-Email allows users to prove facts derived from their communications with trusted entities like banks, exchanges, or employers. This bridges the gap between the traditional, verified world of email and the on-chain world of decentralized applications (dApps). For instance, a user could prove they received a confirmation for a bank transfer above a certain amount to access a DeFi loan, or verify they hold an account with a specific institution for a credentialing service, all while maintaining complete privacy over their financial details.

Key technical components include a circuit compiler that transforms email parsing logic into an arithmetic circuit, and a DKIM verifier within the ZK-SNARK proving system. Developers define a template (e.g., looking for a specific keyword or regex pattern in the subject or body), and the circuit generates a proof that an email matching that template was validly signed. This makes it a powerful primitive for privacy-preserving attestations, moving beyond simple social logins to granular, provable claims. Projects are exploring its use for wallet recovery, credit delegation, anonymous airdrops based on proven membership, and DAO governance where voting rights are gated by verified real-world actions.

ZK-Email works by cryptographically verifying the DKIM signature attached to an email. This signature, created by the sender's mail server using a private key, is a standard email authentication method. The protocol parses the raw email, extracts the DKIM header and body, and uses it as a public input to a zero-knowledge circuit. The circuit's logic proves that a valid signature exists for an email matching specific, user-defined criteria—such as containing a certain sender, subject line, or transaction amount—without exposing the email's full contents.

The core technical process involves three main steps. First, the user provides their raw email file (.eml). Second, a ZK prover (like circom or halo2) executes a circuit that hashes the email's relevant components and verifies the DKIM-SHA256 cryptographic signature against the sender's public key (published in DNS). Finally, the circuit generates a succinct zk-SNARK proof attesting to the statement "I possess a validly signed email from sender@domain.com confirming a deposit of $X." This proof is tiny and can be verified by anyone in milliseconds.

This mechanism enables powerful applications by turning email into a verifiable data source for blockchains. For example, a user can prove they received a bank statement or a flight confirmation to trigger an on-chain action, a process known as proof of email. The system's security inherits from the robustness of DKIM and the soundness of the underlying zk-SNARK protocol. Importantly, the user never needs to share private email credentials; they only need access to the email file itself, which can be exported from any standard client.

ZK-Email is a cryptographic primitive that uses zero-knowledge proofs (ZKPs) to cryptographically verify the content and sender of an email without revealing the underlying message data. It achieves this by generating a proof that a valid DomainKeys Identified Mail (DKIM) signature exists for a specific email header and body, proving the email's authenticity and that it contains certain predefined data, such as a transaction authorization code. This allows sensitive information within emails to be used as trustless inputs for on-chain actions, like claiming an airdrop or executing a transaction, while preserving user privacy.

The core technical dependency is the DKIM protocol, an email authentication standard that allows a receiving mail server to verify an email was sent and authorized by the owner of a specific domain. A DKIM signature is a cryptographic signature added to an email's headers, created using the sender's private key. The corresponding public key is published in the sender's domain DNS records. A ZK-Email circuit does not verify the DKIM signature directly on-chain; instead, it proves knowledge of a valid DKIM signature for a given email, along with the fact that the email's content matches certain constraints, all within a zero-knowledge Succinct Non-interactive Argument of Knowledge (zk-SNARK).

Constructing a zero-knowledge circuit for email verification involves several complex steps. The circuit must: parse the raw email to separate headers from the body, extract the DKIM signature and its associated headers, implement the precise canonicalization algorithms specified in the DKIM standard, and perform the RSA signature verification—all within the constraints of an arithmetic circuit. This requires translating string parsing, hash functions (like SHA-256), and RSA public key cryptography into a format compatible with finite field arithmetic, making it one of the more complex real-world applications of zk-SNARKs.

A primary use case is permissionless attestation, where an email from a trusted entity (e.g., a government domain for KYC, a university for credentials, or a exchange for transaction confirmations) serves as a private proof of eligibility. For example, a user could prove they received a specific email from rewards@protocol.org containing a unique code, enabling them to claim tokens in a privacy-preserving airdrop without revealing their email address or the full contents of the message to the blockchain or the claiming contract.

The development stack for ZK-Email typically involves specialized tools like the zk-email-verify circuit, which is often written in Circom, a domain-specific language for defining zk-SNARK circuits. Developers must provide the raw email and the sender's public RSA key as private inputs to the circuit. The public output is a cryptographic commitment to the verified data (e.g., a hash of the relevant email content). This proof can then be verified on-chain by a smart contract using a verifier contract, which checks the zk-SNARK proof against the public parameters, enabling trustless execution based on verified email data.