
diff -r c4fa7e8a2ffa -r 8f2749152f1e handouts/ho05.tex
--- a/handouts/ho05.tex	Wed Oct 29 18:02:32 2014 +0000
+++ b/handouts/ho05.tex	Wed Oct 29 19:36:15 2014 +0000
@@ -511,22 +511,47 @@
  
 \subsubsection*{Averting Person-in-the-Middle Attacks}
 
-The idea of public-private key encryption is that one can
-make public the key $P^{pub}$ which people can use to
-encrypt messages for me. and I can use my key $P^{priv}$
-to be the only one that can decrypt them. While this sounds
-all good, it relies that people can associate me, for example,
-with my public key. That i snot so trivial as it sounds. 
-For example, if I would be the government, Obama for example, 
-and find out who are the trouble makers, I would publish an
-innocent looking webpage and say I am the New York Times, for 
-example, publish a public key, and then just wait for incoming 
-messages. 
+The idea of public-private key encryption is that one can make
+public the key $K^{pub}$ which people can use to encrypt
+messages for me. and I can use my key $K^{priv}$ to be the
+only one that can decrypt them. While this sounds all good, it
+relies that people can associate me, for example, with my
+public key. That i snot so trivial as it sounds. For example,
+if I would be the government, say Cameron, and try to find out
+who are the trouble makers in the country, I would publish an
+innocent looking webpage and say I am The Guardian newspaper
+(or alternatively The Sun for all the juicy stories), publish
+a public key on it, and then just wait for incoming messages. 
 
-
+This problem is supposed to be solved by using certificates.
+The purpose of certification organisations is that they verify
+that a public key, say $K^{pub}_{Bob}$, really belongs to Bob.
+This is also the mechanism underlying the HTTPS protocol. The
+problem is that this system is essentially completely
+broken\ldots{}but this is a story for another time. Suffice
+to say for now that one of the main certification
+organisations, VeriSign, has limited its liability to \$100 in
+case it issues a false certificate. This is really a joke and
+really the wrong incentive for the certification organisations
+to clean up their mess.
 
-\bigskip\bigskip
-Keyfobs - protocol
+The problem we want to study closer here is that
+public-private key encryption is susceptible to
+person-in-the-middle attack. Consider the following protocol
+where $A$ and $B$ attempt to exchange secret messages using 
+public-private keys. 
+
+\begin{itemize}
+\item $A$ sends public key  to $B$
+\item $B$ sends public key  to $A$
+\item $A$ sends message encrypted with $B$'s public 
+key,\\ $B$ decrypts it with its private key
+\item $B$ sends message encrypted with $A$'s public 
+key,\\ $A$ decrypts it with its private key
+\end{itemize}
+  
+
+\bigskip\bigskip Keyfobs - protocol
 
 \subsubsection*{Further Reading}
 




