title: Generate unique ID per user
body: <p>I am developing a cross platform game for which I needs to generate <strong>unique identifier (User ID)</strong> for each user. I known some platform (Android or iOS) specific approaches to get device related identifiers but I am looking for a solution independent of the device identifiers.</p>
  
  <p><strong>User ID Requirements:</strong></p>
  
  <ol>
  <li>Independent of the device's platform</li>
  <li>Offline implementation (no communication with any servers)</li>
  <li>Without sign-up process</li>
  </ol>
  
  <p>I have implemented one approach to create User IDs where I store the system time when the game was launched for the first time on the device.</p>
  
  <p><strong>I have following questions:</strong></p>
  
  <ol>
  <li>Are there any other approaches to generate User IDs (which will meet the above requirements)?</li>
  <li>What are the common approaches to create unique identifiers with taking any information from the user?</li>
  <li>Are there any third party plug-ins to implement User IDs?</li>
  </ol>
  
  <p>I would appreciate any suggestions and thoughts on this topic.</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>There are lot of responses to use <strong>UUID/GUID</strong>. Generally, this approach looks fine but I am looking for a solution which can generate same User ID even if the user reinstall the game. </p>