Simplified Document Autenticity

sda.sol

pragma solidity ^0.4.19;

contract SimplifiedDocAuthenticity {
    
  mapping (string => uint) private authenticity;

  function storeAuthenticity(string docSha256) public {
    if (checkAuthenticity(docSha256) == 0) {
        authenticity[docSha256] = now;
    }   
  }

  function checkAuthenticity(string docSha256) public constant returns (uint) {
    return authenticity[docSha256];
  }
}