TL;DR — A soulbound (non-transferable) ERC-721 credential system for issuing verifiable on-chain badges, with revoke and re-issue controlled by the issuing authority. Deployed to Sepolia, 15 passing Foundry tests.
Problem
Certificates and badges only mean something if they can’t be bought. A standard NFT credential can be sold or transferred, destroying its value as proof of achievement.
Approach
ProofPass extends OpenZeppelin’s ERC-721 and overrides the transfer entry points to revert, so tokens can be minted and burned but never moved between wallets. The owner issues a badge with a type label, can revoke it (burn), and — because revoking clears the holder’s slot — can re-issue to the same address later, covering re-certification and lost wallets. One badge per address at a time, checked in O(1).
Security
Non-transferability lives at the two public transfer entry points, both overridden to revert — minting and burning route through OpenZeppelin’s internal _update, which the overrides don’t touch, so the owner’s issue/revoke paths keep working. issueBadge and revokeBadge are onlyOwner; issueBadge reverts early if the recipient already holds a badge.
function transferFrom(address, address, uint256) public pure override {
revert TransferNotAllowed();
}
function safeTransferFrom(address, address, uint256, bytes memory) public pure override {
revert TransferNotAllowed();
}Minting still works because _safeMint calls the internal _update directly, never transferFrom.