diff -r 98ae49ffc262 -r 62985f147c85 hws/hw07.tex
--- a/hws/hw07.tex	Tue Sep 26 12:03:24 2017 +0100
+++ b/hws/hw07.tex	Tue Sep 26 12:10:41 2017 +0100
@@ -3,63 +3,37 @@
 
 \begin{document}
 
-\section*{Homework 6}
+
+% For Alice to cheat, she has to get her transaction into the blockchain.
+% For this she has to solve proof-of-work puzzles faster than anybody
+% else. Is it possible for her to precompute several blocks that would
+% validate a fraudulent transaction by her? Give a short explanation
+% for your reasoning.  
+
+\section*{Homework 7}
+
+\HEADER
 
 \begin{enumerate}
-\item What are good uses of anonymity services like Tor?
-
-\item What is meant by the notion \emph{forward privacy}?
-
-\item What is a \emph{re-identification attack}?
+\item How can the hardness of the proof-of-work puzzles in
+      Bitcoins be adjusted? What is parameter that determines
+      how the hardness is adjusted?
 
-\item Imagine you have a completely `innocent' email message,
-      like birthday wishes to your grandmother. Why should you
-      still encrypt this message and your grandmother take the
-      effort to decrypt it? 
-
-      (Hint: The answer has nothing to do with preserving the
-      privacy of your grandmother and nothing to do with
-      keeping her birthday wishes supersecret. Also nothing to
-      do with you and grandmother testing the latest
-      encryption technology, nor just for the sake of it.)
+\item What is the main data that is stored in Bitcoin's
+      blockchain? 
+      
+\item What is is the purpose of the proof-of-work puzzle in
+      Bitcoins?      
 
-\item One part of achieving privacy (but not the only one) is to
-  properly encrypt your conversations on the Internet.  But this is
-  fiercely resisted by some spy agencies.  These agencies (and some
-  politicians for that matter) argue that, for example, ISIL's
-  recruiters broadcast messages on, say, Twitter, and get people to
-  follow them. Then they move potential recruits to Twitter Direct
-  Messaging to evaluate if they are a legitimate recruit. If yes, they
-  move them to an encrypted mobile-messaging app. The spy agencies
-  argue that although they can follow the conversations on Twitter,
-  they ``go dark'' on the encrypted message app. To counter this
-  ``going-dark problem'', the spy agencies push for the implementation
-  of back-doors in iMessage and Facebook and Skype and everything else
-  UK or US-made, which they can use eavesdrop on conversations without
-  the conversants' knowledge or consent.\medskip
-   
-      What is the fallacy in the spy agencies going-dark argument?
-      (Hint: Think what would happen if the spy agencies and certain
-      politicians get their wish.)
-       
-\item DNA data is very sensitive and can easily violate the privacy of
-  (living) people. To get around this, two scientists from Denmark
-  proposed to create a \emph{necrogenomic database} which would record
-  the DNA data of all Danish citizens and residents at the time of
-  their \emph{death}. By matching these to information about illnesses
-  and ailments in life, helpful evidence could be gathered about the
-  genetic origins of diseases.  The idea is that the privacy of dead
-  people cannot be violated.
+\item The department has large labs full of computers that are
+      pretty much idle over night. Why is it a bad idea to let
+      them mine for Bitcoins?
 
-      What is the fallacy behind this reasoning?
+\item Is it possible that Bitcoins can get lost (be
+      irretrievable)?
 
-\item A few years ago a Google executive tried to allay worries about
-  Google pooring over all your emails on Gmail. He said something
-  along the lines: you are watched by an algorithm; this is like being
-  naked in front of your dog. What is wrong with this argument?
-
-\item \POSTSCRIPT  
-\end{enumerate} 
+\item \POSTSCRIPT      
+\end{enumerate}
 \end{document}
 
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