Building the DApp

Let's write the smart contracts for digitalizing fiat currency and storing cell phone numbers linked to bank accounts. Here is the smart contract for digitalizing fiat currency:

pragma solidity ^0.4.18;

contract USD {
    address centralBank;

    mapping (address => uint256) balances;
    uint256 totalDestroyed;
    uint256 totalIssued;

    event usdIssued(uint256 amount, address to);
    event usdDestroyed(uint256 amount, address from);
    event usdTransferred(uint256 amount, address from, address to,
      string description);

    function USD() {
        centralBank = msg.sender;
    }

    function issueUSD(uint256 amount, address to) {
        if(msg.sender == centralBank) {
            balances[to] += amount; 
            totalIssued += amount;
            usdIssued(amount, to);
        }
    }

    function destroyUSD(uint256 amount) {
        balances[msg.sender] -= amount;
        totalDestroyed += amount;
        usdDestroyed(amount, msg.sender);
    }...