Chapter 5: Token Standards and Contract Design

5.1 Core Standards Comparison

5.2 Critical Compliance Note

ERC-20 alone does not enforce legal restrictions. If transfers are permitted only to eligible investors, eligibility must be checked before every transfer. ERC-3643 is designed precisely for this permissioned architecture and embeds identity registry and compliance into the transfer cycle.

// Educational example — NOT production-ready.
pragma solidity ^0.8.24;

contract SimpleAssetToken {
    mapping(address => uint256) public balanceOf;
    mapping(address => bool) public eligible;
    uint256 public totalSupply;
    address public immutable issuer;

    modifier onlyEligible(address account) {
        require(eligible[account], "NOT_ELIGIBLE");
        _;
    }

    function transfer(address to, uint256 amount)
        external onlyEligible(msg.sender) onlyEligible(to) returns (bool)
    {
        require(balanceOf[msg.sender] >= amount, "INSUFFICIENT");
        balanceOf[msg.sender] -= amount;
        balanceOf[to] += amount;
        return true;
    }
}