Contract Source Code:

pragma solidity ^0.4.24;


/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
  address public owner;


  event OwnershipRenounced(address indexed previousOwner);
  event OwnershipTransferred(
    address indexed previousOwner,
    address indexed newOwner
  );


  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */
  constructor() public {
    owner = msg.sender;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }

  /**
   * @dev Allows the current owner to relinquish control of the contract.
   * @notice Renouncing to ownership will leave the contract without an owner.
   * It will not be possible to call the functions with the `onlyOwner`
   * modifier anymore.
   */
  function renounceOwnership() public onlyOwner {
    emit OwnershipRenounced(owner);
    owner = address(0);
  }

  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * @param _newOwner The address to transfer ownership to.
   */
  function transferOwnership(address _newOwner) public onlyOwner {
    _transferOwnership(_newOwner);
  }

  /**
   * @dev Transfers control of the contract to a newOwner.
   * @param _newOwner The address to transfer ownership to.
   */
  function _transferOwnership(address _newOwner) internal {
    require(_newOwner != address(0));
    emit OwnershipTransferred(owner, _newOwner);
    owner = _newOwner;
  }
}


contract DocumentStore is Ownable {
  string public name;
  string public version = "2.2.0";

  /// A mapping of the document hash to the block number that was issued
  mapping(bytes32 => uint) documentIssued;
  /// A mapping of the hash of the claim being revoked to the revocation block number
  mapping(bytes32 => uint) documentRevoked;

  event DocumentIssued(bytes32 indexed document);
  event DocumentRevoked(
    bytes32 indexed document
  );

  constructor(
    string _name
  ) public
  {
    name = _name;
  }

  function issue(
    bytes32 document
  ) public onlyOwner onlyNotIssued(document)
  {
    documentIssued[document] = block.number;
    emit DocumentIssued(document);
  }

  function getIssuedBlock(
    bytes32 document
  ) public onlyIssued(document) view returns (uint)
  {
    return documentIssued[document];
  }

  function isIssued(
    bytes32 document
  ) public view returns (bool)
  {
    return (documentIssued[document] != 0);
  }

  function isIssuedBefore(
    bytes32 document,
    uint blockNumber
  ) public view returns (bool)
  {
    return documentIssued[document] != 0 && documentIssued[document] <= blockNumber;
  }

  function revoke(
    bytes32 document
  ) public onlyOwner onlyNotRevoked(document) returns (bool)
  {
    documentRevoked[document] = block.number;
    emit DocumentRevoked(document);
  }

  function isRevoked(
    bytes32 document
  ) public view returns (bool)
  {
    return documentRevoked[document] != 0;
  }

  function isRevokedBefore(
    bytes32 document,
    uint blockNumber
  ) public view returns (bool)
  {
    return documentRevoked[document] <= blockNumber && documentRevoked[document] != 0;
  }

  modifier onlyIssued(bytes32 document) {
    require(isIssued(document), "Error: Only issued document hashes can be revoked");
    _;
  }

  modifier onlyNotIssued(bytes32 document) {
    require(!isIssued(document), "Error: Only hashes that have not been issued can be issued");
    _;
  }

  modifier onlyNotRevoked(bytes32 claim) {
    require(!isRevoked(claim), "Error: Hash has been revoked previously");
    _;
  }
}