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294 lines
15 KiB
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294 lines
15 KiB
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<main>
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<h1><a class="header" href="#about" id="about">About</a></h1>
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<p><a href="https://github.com/appliedzkp/semaphore">Semaphore</a> is a zero-knowledge gadget
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which allows Ethereum users to prove their membership of a set which they had
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previously joined without revealing their original identity. At the same time,
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it allows users to signal their endorsement of an arbitrary string. It is
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designed to be a simple and generic privacy layer for Ethereum dApps. Use cases
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include private voting, whistleblowing, mixers, and anonymous authentication.
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Finally, it provides a simple built-in mechanism to prevent double-signalling
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or double-spending.</p>
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<p>This gadget comprises of smart contracts and
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<a href="https://z.cash/technology/zksnarks/">zero-knowledge</a> components which work in
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tandem. The Semaphore smart contract handles state, permissions, and proof
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verification on-chain. The zero-knowledge components work off-chain to allow
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the user to generate proofs, which allow the smart contract to update its state
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if these proofs are valid.</p>
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<p>Semaphore is designed for smart contract and dApp developers, not end users.
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Developers should abstract its features away in order to provide user-friendly
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privacy.</p>
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<p>Try a simple demo <a href="https://weijiekoh.github.io/semaphore-ui/">here</a> or read a
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high-level description of Semaphore
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<a href="https://medium.com/coinmonks/to-mixers-and-beyond-presenting-semaphore-a-privacy-gadget-built-on-ethereum-4c8b00857c9b">here</a>.</p>
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<h2><a class="header" href="#basic-features" id="basic-features">Basic features</a></h2>
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<p>In sum, Semaphore provides the ability to:</p>
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<ol>
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<li>
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<p>Register an identity in a smart contract, and then:</p>
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</li>
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<li>
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<p>Broadcast a signal:</p>
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<ul>
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<li>
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<p>Anonymously prove that their identity is in the set of registered
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identities, and at the same time:</p>
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</li>
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<li>
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<p>Publicly store an arbitrary string in the contract, if and only if that
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string is unique to the user and the contract’s current external
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nullifier, which is a unique value akin to a topic. This means that
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double-signalling the same message under the same external nullifier is
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not possible.</p>
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</li>
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</ul>
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</li>
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</ol>
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<h3><a class="header" href="#about-external-nullifiers" id="about-external-nullifiers">About external nullifiers</a></h3>
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<p>Think of an external nullifier as a voting booth where each user may only cast
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one vote. If they try to cast a second vote a the same booth, that vote is
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invalid.</p>
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<p>An external nullifier is any 29-byte value. Semaphore always starts with one
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external nullifier, which is set upon contract deployment. The owner of the
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Semaphore contract may add more external nullifiers, deactivate, or reactivate
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existing ones.</p>
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<p>The first time a particular user broadcasts a signal to an active external
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nullifier <code>n</code>, and if the user's proof of membership of the set of registered
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users is valid, the transaction will succeed. The second time she does so to
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the same <code>n</code>, however, her transaction will fail.</p>
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<p>Additionally, all signals broadcast transactions to a deactivated external
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nullifier will fail.</p>
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<p>Each client application must use the above features of Semaphore in a unique
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way to achieve its privacy goals. A mixer, for instance, would use one external
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nullifier as such:</p>
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<table><thead><tr><th>Signal</th><th>External nullifier</th></tr></thead><tbody>
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<tr><td>The hash of the recipient's address, relayer's address, and the relayer's fee</td><td>The mixer contract's address</td></tr>
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</tbody></table>
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<p>This allows anonymous withdrawals of funds (via a transaction relayer, who is
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rewarded with a fee), and prevents double-spending as there is only one
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external nullifier.</p>
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<p>An anonymous voting app would be configured differently:</p>
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<table><thead><tr><th>Signal</th><th>External nullifier</th></tr></thead><tbody>
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<tr><td>The hash of the respondent's answer</td><td>The hash of the question</td></tr>
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</tbody></table>
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<p>This allows any user to vote with an arbitary response (e.g. yes, no, or maybe)
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to any question. The user, however, can only vote once per question.</p>
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<h2><a class="header" href="#about-the-code" id="about-the-code">About the code</a></h2>
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<p>This repository contains the code for Semaphore's contracts written in
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Soliidty, and zk-SNARK circuits written in
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<a href="https://github.com/iden3/circom">circom</a>. It also contains Typescript code to
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execute tests.</p>
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<p>The code has been audited by ABDK Consulting. Their suggested security and
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efficiency fixes have been applied.</p>
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<p>A multi-party computation to produce the zk-SNARK proving and verification keys
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for Semaphore will begin in the near future.</p>
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