title: Safety_Question - undecidable to prove a system is safe
body: <p>This question is about the "safety question" of lecture 2 of  iTunes U security course at 
  <a href="https://itunes.apple.com/us/itunes-u/computer-science-foundations/id389259109" rel="nofollow">https://itunes.apple.com/us/itunes-u/computer-science-foundations/id389259109</a></p>
  
  <p>and @37:00 there is a proof that  the problem of proving a system is safe is undecidable. The argument is that the problem is undecidable because  a protection system exactly simulates a TM, and if we defined a "halting right"  then it follows that the question of whether the TM ever  leaks the "halting right" is undecidable .</p>
  
  <p>What is a concrete example of a "halting right"?</p>