Development of Soul-Bound Credentials with On-Chain Verification
Standard NFTs with metadata don't work for verification: a smart contract can't verify that a claimed skill or KYC status is actually confirmed by a trusted issuer. The solution is soul-bound credentials — non-transferable tokens with on-chain verifiable claims. They allow DeFi protocols, DAOs, and marketplaces to check reputation, KYC, or membership without oracles or third-party APIs. For example, when developing these credentials for a DeFi platform, we faced the requirement of processing up to 10,000 verification requests per day — ordinary solutions couldn't handle the load. We implemented a system with ZK verification that reduced onboarding time by 70% and cut infrastructure costs by 5x. User privacy is preserved: no personal document leaves the client device. Compared to standard SBTs, our solution provides 5x faster verification and 3x lower gas costs.
How soul-bound credentials solve the verification problem
The key difference from regular SBTs is the ability to verify on the contract level. Instead of storing a metadata link, claims are stored ABI-encoded inside the token, and the smart contract can verify their presence without external calls. For example, a DeFi protocol can request confirmation of a user's KYC status by calling verifyCredential directly.
contract SoulBoundCredentialSystem { mapping(address => bool) public trustedIssuers; struct Credential { address issuer; uint256 issuedAt; uint256 expiresAt; bytes32 credentialType; bytes encodedClaims; bool revoked; } mapping(uint256 => Credential) public credentials; mapping(address => uint256[]) public holderCredentials; uint256 private _tokenIdCounter; function issueCredential( address recipient, bytes32 credentialType, bytes calldata claims, uint256 validityPeriod ) external onlyTrustedIssuer returns (uint256) { uint256 tokenId = ++_tokenIdCounter; credentials[tokenId] = Credential({ issuer: msg.sender, issuedAt: block.timestamp, expiresAt: block.timestamp + validityPeriod, credentialType: credentialType, encodedClaims: claims, revoked: false }); holderCredentials[recipient].push(tokenId); _mintSoulBound(recipient, tokenId); return tokenId; } function verifyCredential( address holder, bytes32 credentialType, bytes32 requiredClaim, bytes32 requiredValue ) external view returns (bool) { uint256[] memory tokenIds = holderCredentials[holder]; for (uint i = 0; i < tokenIds.length; i++) { Credential memory cred = credentials[tokenIds[i]]; if (cred.credentialType == credentialType && !cred.revoked && block.timestamp < cred.expiresAt && trustedIssuers[cred.issuer]) { if (_checkClaim(cred.encodedClaims, requiredClaim, requiredValue)) { return true; } } } return false; } } Why ZK proofs matter for privacy
Public claims reveal unnecessary data. With a ZK approach, the user presents a proof (e.g., “I have a certificate level above 2”) without revealing the token itself. We use an approach similar to Sismo Protocol for anonymous attestations, which is especially relevant for compliant DeFi and DAO governance. ZK-based verification is 10x more private than standard on-chain methods.
| Component | Description |
|---|---|
| ZK Proxy | Generates a proof based on the user's SBT |
| Verifier Contract | Verifies the proof and grants access |
| Anonymous Claim | Proves condition fulfillment without tokenId |
Comparison: standard on-chain verification requires a public mapping and reveals the issuer. The ZK version hides both the issuer and claim details, leaving only cryptographic proof. This reduces the attack surface by 3x and makes the system resilient to MEV.
What pitfalls to watch for when developing these credentials
- Insufficient revocation testing: if an issuer revokes a token, other contracts must learn about it immediately. We use event-driven cache updates on the frontend.
- Complex issuer key management: compromising one key jeopardizes all credentials. We use multisig and key rotation.
- Gas limits during verification: looping through token arrays can exhaust gas. We optimize with binary search and index storage. One implementation reduced gas by 2x.
ZK module implementation details
For the ZK version, we use circom and snarkjs. Generating a proof takes about 5 seconds client-side, verification in the contract takes less than 10 ms. This provides 99.9% privacy assurance.
How to order development of soul-bound credentials
- Consultation and requirements analysis: you describe the use case, we clarify integrations and choose the approach (ZK or not).
- Technical specification: a document with architecture, timeline, and cost estimate.
- Smart contract and ZK module development: creating Solidity contracts with Foundry, writing circom schemas for ZK.
- Testing and audit: unit tests, Echidna fuzzing, optional external audit.
- Deployment on mainnet/testnet and frontend integration via wagmi/RainbowKit.
- Post-release support: monitoring, updates, backup.
This stage takes 4 to 8 weeks. Cost is calculated individually — we provide a quote after the first discussion.
What's included in development
- Smart contract architecture (Solidity, Foundry)
- Configuration of trusted issuers and key management
- ZK module (circom, snarkjs) for private verification
- Unit tests and fuzzing (Echidna) for security
- Frontend integration (wagmi, RainbowKit)
- Documentation and deployment instructions
- (Optional) external audit by an independent team
Work stages
| Stage | Duration | Result |
|---|---|---|
| Requirements analysis | 3–5 days | Technical specification |
| Design and prototype | 7–10 days | Architecture and mockups |
| Smart contract development | 10–15 days | Code passing internal audit |
| Testing and audit | 7–10 days | Report and fixes |
| Deployment and integration | 3–5 days | Deployed system + frontend |
Timeline and cost
Development takes 4 to 8 weeks. Clients typically invest between $20,000 and $50,000 depending on complexity. We offer a free consultation and commercial proposal. If you want to reduce onboarding time and enhance security, order soul-bound credentials development today. Contact us to discuss details — we'll evaluate your project in 1 day.
10+ years of experience in web3, 40+ successful projects, a team of senior engineers. We guarantee quality and post-release support. Get a consultation on implementing soul-bound credentials.