Signing a document on the blockchain 

This section will teach you how to sign documents such as contracts on the blockchain. Let us consider the following code to understand how this is done:

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 inviteUser(bytes32 documentHash, address user) public onlyOwner {
 to_sign[documentHash] = user;
 }

 function signDocument(bytes32 documentHash) public {
 if (msg.sender != to_sign[documentHash]) {
 revert();
 }
 signed[documentHash] = true;
 }
function addUser(address user, bytes32 hashed_verification) public onlyOwner {
 user_verified[user] = false;
 user_codes[user] = hashed_verification;
 }

 function verify(bytes32 verification_code) public...