title: validation in p2p communication
body: <p>Suppose that in my "network" of users each node can create only one hashed code (generated with the node's id).</p>
  
  <p>Now, node A sends to node B this hashed code. node A is no longer the owner, but B is. 
  still, it's just a string, so node A decide to trick the system and sends also to node C the same code. how can node C know that A made an illegal operation?</p>
  
  <p>can C validate the operation only by communicating with node A? (without server or communication with other nodes)</p>
  
  <p>thanks</p>