title: Securing an "open" web service without HTTPS or private shared key
body: <p>I am producing a web service which will allow any third party "device" to communicate with it. Each device has a reasonably unusual string to identify itself and uses the web service to store data against this id. However, this allows someone who wishes to game the service to scan through and guess device ids and store malicious data against them.</p>
  
  <p>The device itself using this web service is relatively "dumb" and doesn't offer a suitable interface for data entry, so a password or any form of entry on the client is not available.</p>
  
  <p>As this web service is open for anyone who wishes to produce a device of this nature to use, I can't increase security with the use a private key as this would be publicly defined in a specification. Also due to the simplistic nature of the device and it's IP/HTTP stack, HTTPS is unsuitable for this implementation.</p>
  
  <p>To the best of my knowledge I can't see a way of using a privately shared key in this operation. To this extent, I believe it be impossible to secure a system of this nature, but I am wondering if some other methods which I've yet to find may help me secure this system somewhat?</p>