User identity verification with blockchains

This section takes a closer look at the user verification process and the Solidity code that lies behind it. Let us consider the following Solidity code:

pragma solidity ^0.4.17;
contract UserExample {
 
 mapping(address => bool) user_verified;
 mapping(address => bytes32) user_codes;
 
 mapping(bytes32 => address) to_sign;
 mapping(bytes32 => bool) signed;
 
 address owner;
 
 modifier onlyOwner() {
 require(msg.sender == owner);
 _;
 }
 
 function UserExample() public {
 owner = msg.sender;
 }
 
function addUser(address user, bytes32 hashed_verification) public onlyOwner {
 user_verified[user] = false;
 user_codes[user] = hashed_verification;
 }
 
 function verify(bytes32 verification_code) public {
 if (user_verified[msg.sender] == false && 
 sha256(verification_code) == user_codes[msg.sender]) {
 user_verified[msg.sender] = true;
 }
 }
}

As...