title: How to determine the exact number of users of a web application?
body: <p>The company I am working at offers a web based calculation tool which has to be paid monthly (a fixed price for a license).</p>
  
  <p>Normally, users go to our website and authenticate themselves with their credentials and then can use the application. When they cancel their subscription they are not able to use the tool anymore, obviously.</p>
  
  <p>Now another company called us because they want to provide our application for their own clients. We have already fixed that they have to pay a license fee for every of their clients. But there is also a restriction: their users should not have to log in on any of our websites (only on the website of our client). But the web application is hosted on our server and is loaded as an iframe.</p>
  
  <p>Now there is that problem that we are not sure whether our client tells us the correct number of people who use our application wherefore we would like to verify that in some way.</p>
  
  <p>One of my ideas is the following:</p>
  
  <ul>
  <li>Our client has to call an API for every users who would like to use our application in order to submit some information like name or an unique ID of that user</li>
  <li>When the user would like to access our application, an ID parameter is appended to the iFrame URL</li>
  </ul>
  
  <p>I think that this is not a very good solution because our client could use the same ID for every access and pretend that only one users uses the application. By saving the ip address and id of the accesses it is possible to determine fraud in some cases because ip address will not change frequently.</p>
  
  <p>We even do not have to know WHICH user accesses the application but only the NUMBER of users per month.</p>
  
  <p>I am interested if there is a cryptographic solution where it is hard to cheat. Something like an authentication method which does not require any interaction of the user.</p>