title: What's the Hash in HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.<extension>\UserChoice?
body: <p>With Windows 8, the user's choice for which application to open for a given document type seems to be kept in</p>
  
  <pre><code>HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\&lt;extension&gt;\UserChoice
  </code></pre>
  
  <p>For PDFs on my machine, this contains:</p>
  
  <pre><code>[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf\UserChoice]
  "Hash"="xh8KhPWlZL0="
  "ProgId"="AcroExch.Document"
  </code></pre>
  
  <p>Yet on another machine the hash is different. What's being hashed, and is there any way to create a .reg file which can be applied to another machine to set this preference?</p>