title: iOS in-app purchase's receipt string explained
body: <p>I try to understand the receipt information obtained from iTunes server, but could not find relevant documentation. </p>
  
  <p>Particularly, what's the difference among <code>unique_identifier</code>, <code>unique_vendor_identifier</code>, <code>original_transaction_id</code> (which is claimed, in a WWDC'12 session, to be a de facto customer id) and <code>[[[UIDevice currentDevice] identifierForVendor] UUIDString]</code>?</p>
  
  <pre><code>{"receipt":
      {"original_purchase_date_pst":"...",        
       "purchase_date_ms":"...", 
       "unique_identifier":"...", 
       "original_transaction_id":"...", 
       "bvrs":"...", 
       "transaction_id":"...", 
       "quantity":"...", 
       "unique_vendor_identifier":"...", 
       "item_id":"...", 
       "product_id":"...", 
       "purchase_date":"...", 
       "original_purchase_date":"...", 
       "purchase_date_pst":"...", 
       "bid":"...", 
       "original_purchase_date_ms":"..."},  
  "status":0}`
  </code></pre>
  
  <p>I wish to store this receipt information on my server to track subscription validity. So it is better to know which id I should use as a surrogate for user identity.</p>