// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract ContractOnlyReceipt {
    mapping(bytes32 => uint256) private nullifierBlock;

    event Receipt(bytes32 indexed attestationHash, address indexed agent, bytes32 indexed nullifier);

    function attest(bytes32 attestationHash, bytes32 nullifier) external {
        uint256 codeSize;
        address sender = msg.sender;
        assembly {
            codeSize := extcodesize(sender)
        }
        require(codeSize > 0, 'contract only');
        require(nullifierBlock[nullifier] == 0, 'nullifier used');

        nullifierBlock[nullifier] = block.number;

        emit Receipt(attestationHash, sender, nullifier);
    }
}