Setting Up a Decentralized Reputation System for Messaging

Decentralized messaging protocols like XMTP and Waku enable secure, private communication between blockchain wallets. However, they lack a native mechanism to assess the trustworthiness of unknown senders. A decentralized reputation system solves this by allowing users to assign and query verifiable, portable social scores. This guide explains how to build such a system using smart contracts for on-chain attestations and zero-knowledge proofs for privacy-preserving verification.

The core architecture involves three components: an attestation registry, a reputation aggregator, and a verification layer. The registry, built as an Ethereum Attestation Service (EAS) schema or a similar smart contract, records signed statements (attestations) about a user's behavior. The aggregator, which can be an off-chain indexer or a The Graph subgraph, calculates a composite reputation score from these attestations. The verification layer, potentially using zk-SNARKs via a circuit library like Circom, allows users to prove they have a minimum reputation score without revealing their full history.

Key design considerations include sybil-resistance, data privacy, and portability. To combat fake identities, the system can anchor attestations to Proof of Humanity or Gitcoin Passport credentials. Privacy is maintained by storing only hashes of private data on-chain and using zero-knowledge proofs for queries. Portability is achieved by storing reputation data in a user's ERC-725 or ERC-734 identity contract, making it usable across any dApp that integrates the standard.

This guide will walk through implementing a basic version using Solidity, Hardhat, and Next.js. We'll create an EAS schema for MessageQuality attestations, build an aggregator to calculate a score from spamReport and helpfulResponse counts, and demonstrate a simple client that queries a user's reputation before accepting a direct message. The final code will be available on GitHub for further experimentation.

A decentralized reputation system for messaging relies on three core components: a blockchain for immutable data storage, a decentralized identity (DID) framework for user sovereignty, and a decentralized storage solution for message payloads. You should be familiar with smart contract development, as the reputation logic and attestation rules will be encoded on-chain. For this guide, we'll use Ethereum and the Solidity language, but the principles apply to other EVM-compatible chains like Polygon or Arbitrum. Ensure you have a basic understanding of concepts like public/private key cryptography, gas fees, and transaction finality.

You will need a development environment with Node.js (v18 or later) and npm or yarn installed. We'll use Hardhat as our Ethereum development framework for compiling, testing, and deploying smart contracts. Install it globally with npm install --global hardhat. For decentralized identity, we will reference the W3C Decentralized Identifiers (DIDs) specification and use Verifiable Credentials for attestations. Familiarity with libraries like did-resolver and veramo is beneficial. For off-chain message storage, we'll integrate with IPFS (InterPlanetary File System) via a service like Pinata or by running a local node.

Set up a wallet for development and testing. MetaMask is the most common browser extension wallet. Create a new wallet specifically for development and fund it with test ETH from a faucet (e.g., the Sepolia or Goerli faucet). Securely store your wallet's mnemonic seed phrase and never commit it to version control. You will also need an Alchemy or Infura account to access an Ethereum node RPC endpoint without running your own. These services provide free tier API keys essential for deploying contracts to testnets.

The architecture for a decentralized reputation system is built on a three-layer model: the data availability layer, the reputation logic layer, and the client application layer. The data layer, typically a blockchain like Ethereum or an L2 such as Base, stores the canonical, immutable record of attestations—signed statements about a user's behavior. These attestations are often implemented using standards like EIP-712 for structured signing and stored via scalable attestation protocols like Ethereum Attestation Service (EAS) or Verax. This ensures data is tamper-proof and publicly verifiable.

The reputation logic layer is where the raw attestation data is processed into a usable reputation score. This layer consists of off-chain indexers (e.g., using The Graph) that query the blockchain for relevant attestations and reputation algorithms that apply weights and rules. For example, an algorithm might assign higher value to attestations from well-reputed attesters or decay scores over time. This computation happens off-chain for efficiency, with the resulting scores optionally anchored back on-chain via a Merkle root for verification. Smart contracts on this layer manage the rules for who can attest and how scores are calculated.

The data flow begins when a user sends a message in a client app. The client queries the indexer for the recipient's reputation score, which is derived from their attestation history. If the score meets a threshold, the message is sent. After a successful interaction, either party can submit a new attestation (e.g., "successful trade") by signing an EIP-712 structured message. This signed payload is sent to a relayer service, which pays the gas fee to submit it as a transaction to the data availability layer, making it permanent. The indexer then picks up this new attestation and updates the reputation scores.

Key technical considerations include sybil resistance and privacy. To prevent fake identities, the system can integrate with proof-of-personhood protocols like Worldcoin or require a stake in a smart contract. For privacy, zero-knowledge proofs (ZKPs) can be used via zk-SNARKs or zk-STARKs to prove a reputation score meets a threshold without revealing the underlying attestations. Frameworks like Semaphore or Sismo offer tools for such private reputation proofs. The client application layer must securely handle signing requests and interact with wallets like MetaMask for attestation creation.

To implement a basic version, you would deploy an EAS schema on Sepolia testnet, create an off-chain indexer using The Graph's subgraph to track attestations, and write a simple algorithm in JavaScript to calculate a score. The frontend client, built with a library like viem and wagmi, would request signatures for attestations and query the subgraph for scores. This architecture creates a robust, user-controlled reputation system that can scale across various messaging platforms, from decentralized social apps to marketplace DMs.

The system you've implemented uses on-chain attestations to create a portable, user-controlled reputation layer. Key components include a ReputationRegistry contract for managing attestations, a MessageVerifier for checking sender scores, and a client-side SDK for integration. This architecture ensures that reputation is composable—a user's score from one dApp can be used to verify their trustworthiness in another, breaking down data silos. The use of EIP-712 typed signatures provides a secure, gas-efficient method for off-chain attestation issuance.

To extend this system, consider implementing more sophisticated scoring algorithms. The current example uses a simple average, but real-world systems often employ weighted averages, time-decay functions (where older attestations matter less), or Sybil-resistance mechanisms like proof-of-humanity or social graph analysis. You could also explore integrating with oracles like Chainlink to pull in off-chain data (e.g., GitHub contributions, domain ownership) to create composite reputation scores that reflect a user's broader digital identity.

For production deployment, security and gas optimization are critical next steps. Conduct a thorough audit of the smart contract logic, particularly the revokeAttestation function and any admin controls. Use a proxy upgrade pattern (like ERC-1967) to allow for future improvements without losing state. Consider storing attestation data on a Layer 2 solution like Arbitrum or Optimism, or an EVM-compatible appchain like Polygon zkEVM, to drastically reduce transaction costs for users issuing and querying reputation.

The final step is integration into a real messaging client. For a web app, wrap the SDK functions in React hooks or Vue composables. For a mobile dApp, use a library like WalletConnect to handle signatures. Monitor key metrics post-launch: the number of active attestors, the distribution of scores, and the gas costs for key operations. This data will inform necessary protocol adjustments. The Ethereum Attestation Service (EAS) and Verax are excellent references for more advanced schemas and attestation registry designs.